NASA Astrophysics Data System (ADS)
Majumder, Moumita; Dawes, Richard; Wang, Xiao-Gang; Carrington, Tucker; Li, Jun; Guo, Hua; Manzhos, Sergei
2014-06-01
New potential energy surfaces for methane were constructed, represented as analytic fits to about 100,000 individual high-level ab initio data. Explicitly-correlated multireference data (MRCI-F12(AE)/CVQZ-F12) were computed using Molpro [1] and fit using multiple strategies. Fits with small to negligible errors were obtained using adaptations of the permutation-invariant-polynomials (PIP) approach [2,3] based on neural-networks (PIP-NN) [4,5] and the interpolative moving least squares (IMLS) fitting method [6] (PIP-IMLS). The PESs were used in full-dimensional vibrational calculations with an exact kinetic energy operator by representing the Hamiltonian in a basis of products of contracted bend and stretch functions and using a symmetry adapted Lanczos method to obtain eigenvalues and eigenvectors. Very close agreement with experiment was produced from the purely ab initio PESs. References 1- H.-J. Werner, P. J. Knowles, G. Knizia, 2012.1 ed. 2012, MOLPRO, a package of ab initio programs. see http://www.molpro.net. 2- Z. Xie and J. M. Bowman, J. Chem. Theory Comput 6, 26, 2010. 3- B. J. Braams and J. M. Bowman, Int. Rev. Phys. Chem. 28, 577, 2009. 4- J. Li, B. Jiang and Hua Guo, J. Chem. Phys. 139, 204103 (2013). 5- S Manzhos, X Wang, R Dawes and T Carrington, JPC A 110, 5295 (2006). 6- R. Dawes, X-G Wang, A.W. Jasper and T. Carrington Jr., J. Chem. Phys. 133, 134304 (2010).
An accurate potential energy curve for helium based on ab initio calculations
NASA Astrophysics Data System (ADS)
Janzen, A. R.; Aziz, R. A.
1997-07-01
Korona, Williams, Bukowski, Jeziorski, and Szalewicz [J. Chem. Phys. 106, 1 (1997)] constructed a completely ab initio potential for He2 by fitting their calculations using infinite order symmetry adapted perturbation theory at intermediate range, existing Green's function Monte Carlo calculations at short range and accurate dispersion coefficients at long range to a modified Tang-Toennies potential form. The potential with retardation added to the dipole-dipole dispersion is found to predict accurately a large set of microscopic and macroscopic experimental data. The potential with a significantly larger well depth than other recent potentials is judged to be the most accurate characterization of the helium interaction yet proposed.
NASA Astrophysics Data System (ADS)
Zhang, Xiao-Hu; Li, Ming; Wang, Yan-Ting; Ouyang, Zhong-Can
2014-02-01
Formation and dissociation mechanisms of C—C+ base pairs in acidic and alkaline environments are investigated, employing ab initio quantum chemical calculations. Our calculations suggest that, in an acidic environment, a cytosine monomer is first protonated and then dimerized with an unprotonated cytosine monomer to form a C—C+ base pair; in an alkaline environment, a protonated cytosine dimer is first unprotonated and then dissociated into two cytosine monomers. In addition, the force for detaching a C—C+ base pair was found to be inversely proportional to the distance between the two cytosine monomers. These results provide a microscopic mechanism to qualitatively explain the experimentally observed reversible formation and dissociation of i-motifs.
Ab Initio Calculation of the Hoyle State
Epelbaum, Evgeny; Krebs, Hermann; Lee, Dean; Meissner, Ulf-G.
2011-05-13
The Hoyle state plays a crucial role in the helium burning of stars heavier than our Sun and in the production of carbon and other elements necessary for life. This excited state of the carbon-12 nucleus was postulated by Hoyle as a necessary ingredient for the fusion of three alpha particles to produce carbon at stellar temperatures. Although the Hoyle state was seen experimentally more than a half century ago nuclear theorists have not yet uncovered the nature of this state from first principles. In this Letter we report the first ab initio calculation of the low-lying states of carbon-12 using supercomputer lattice simulations and a theoretical framework known as effective field theory. In addition to the ground state and excited spin-2 state, we find a resonance at -85(3) MeV with all of the properties of the Hoyle state and in agreement with the experimentally observed energy.
Ab Initio Calculations Of Light-Ion Reactions
Navratil, P; Quaglioni, S; Roth, R; Horiuchi, W
2012-03-12
The exact treatment of nuclei starting from the constituent nucleons and the fundamental interactions among them has been a long-standing goal in nuclear physics. In addition to the complex nature of nuclear forces, one faces the quantum-mechanical many-nucleon problem governed by an interplay between bound and continuum states. In recent years, significant progress has been made in ab initio nuclear structure and reaction calculations based on input from QCD employing Hamiltonians constructed within chiral effective field theory. In this contribution, we present one of such promising techniques capable of describing simultaneously both bound and scattering states in light nuclei. By combining the resonating-group method (RGM) with the ab initio no-core shell model (NCSM), we complement a microscopic cluster approach with the use of realistic interactions and a microscopic and consistent description of the clusters. We discuss applications to light nuclei scattering, radiative capture and fusion reactions.
Ab initio calculations for industrial materials engineering: successes and challenges.
Wimmer, Erich; Najafabadi, Reza; Young, George A; Ballard, Jake D; Angeliu, Thomas M; Vollmer, James; Chambers, James J; Niimi, Hiroaki; Shaw, Judy B; Freeman, Clive; Christensen, Mikael; Wolf, Walter; Saxe, Paul
2010-09-29
Computational materials science based on ab initio calculations has become an important partner to experiment. This is demonstrated here for the effect of impurities and alloying elements on the strength of a Zr twist grain boundary, the dissociative adsorption and diffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grain boundary and in bulk Ni, and the dependence of the work function of a TiN-HfO(2) junction on the replacement of N by O atoms. In all of these cases, computations provide atomic-scale understanding as well as quantitative materials property data of value to industrial research and development. There are two key challenges in applying ab initio calculations, namely a higher accuracy in the electronic energy and the efficient exploration of large parts of the configurational space. While progress in these areas is fueled by advances in computer hardware, innovative theoretical concepts combined with systematic large-scale computations will be needed to realize the full potential of ab initio calculations for industrial applications.
Ab initio calculations of nitramine dimers
NASA Astrophysics Data System (ADS)
Koh-Fallet, Sharon; Schweigert, Igor
2015-06-01
Elevated temperatures and pressures are typically thought to have opposing effects on the reaction channels of nitramine decomposition. These high temperatures promote reactions with loose transition structures (positive activation entropies and volumes), such as N-N bond homolysis. Elevated pressures promote reactions with tight transition structures (negative activation entropies and volumes), such as intramolecular and intermolecular H transfer. However, no quantitative data exists regarding the range of temperatures and pressures at which these effects become pronounced. We are pursuing ab initio calculations of the corresponding unimolecular and bimolecular transition structures with the objective of estimating the relevant thermochemical parameters and quantifying the effects of elevated temperature and pressures on the corresponding rate constants. Here, we present density functional theory and complete active space calculations of gas-phase molecular dimers of nitramines as an intermediate step toward modeling transition structures directly in the condensed phase. This work was supported by the Naval Research Laboratory via the American Society for Engineering and Education and by the Office of Naval Research, both directly and through the Naval Research Laboratory.
Summation of Parquet diagrams as an ab initio method in nuclear structure calculations
Bergli, Elise; Hjorth-Jensen, Morten
2011-05-15
Research Highlights: > We present a Green's function based approach for doing ab initio nuclear structure calculations. > In particular the sum the subset of so-called Parquet diagrams. > Applying the theory to a simple but realistic model, results in good agreement with other ab initio methods. > This opens up for ab initio calculations for medium-heavy nuclei. - Abstract: In this work we discuss the summation of the Parquet class of diagrams within Green's function theory as a possible framework for ab initio nuclear structure calculations. The theory is presented and some numerical details are discussed, in particular the approximations employed. We apply the Parquet method to a simple model, and compare our results with those from an exact solution. The main conclusion is that even at the level of approximation presented here, the results shows good agreement with other comparable ab initio approaches.
NASA Astrophysics Data System (ADS)
Polyansky, Oleg L.; Ovsyannikov, Roman I.; Kyuberis, Aleksandra A.; Lodi, Lorenzo; Tennyson, Jonathan; Yachmenev, Andrey; Yurchenko, Sergei N.; Zobov, Nikolai F.
2016-09-01
An ab initio potential energy surface (PES) for gas-phase ammonia NH3 has been computed using the methodology pioneered for water (Polyansky et al., 2013). Multireference configuration interaction calculations are performed at about 50 000 points using the aug-cc-pCVQZ and aug-cc-pCV5Z basis sets and basis set extrapolation. Relativistic and adiabatic surfaces are also computed. The points are fitted to a suitable analytical form, producing the most accurate ab initio PES for this molecule available. The rotation-vibration energy levels are computed using nuclear motion program TROVE in both linearised and curvilinear coordinates. Better convergence is obtained using curvilinear coordinates. Our results are used to assign the visible spectrum of 14NH3 recorded by Coy and Lehmann (1986). Rotation-vibration energy levels for the isotopologues NH2D, NHD2, ND3 and 15NH3 are also given. An ab initio value for the dissociation energy D0 of 14NH3 is also presented.
NASA Astrophysics Data System (ADS)
Knyazev, D. V.; Levashov, P. R.
2015-11-01
This work covers an ab initio calculation of transport and optical properties of plastics of the effective composition CH2 at density 0.954 g/cm3 in the temperature range from 5 kK up to 100 kK. The calculation is based on the quantum molecular dynamics, density functional theory and the Kubo-Greenwood formula. The temperature dependence of the static electrical conductivity σ1DC (T) has an unusual shape: σ1DC(T) grows rapidly for 5 kK ≤ T ≤ 10 kK and is almost constant for 20 kK ≤ T ≤ 60 kK. The additional analysis based on the investigation of the electron density of states (DOS) was performed. The rapid growth of σ1DC(T) at 5 kK ≤ T ≤ 10 kK is connected with the increase of DOS at the electron energy equal to the chemical potential ɛ = μ. The frequency dependence of the dynamic electrical conductivity σ1(ω) at 5 kK has the distinct non-Drude shape with the peak at ω ≈ 10 eV. This behavior of σ1(ω) was explained by the dip at the electron DOS.
Heats of Segregation of BCC Binaries from ab Initio and Quantum Approximate Calculations
NASA Technical Reports Server (NTRS)
Good, Brian S.
2004-01-01
We compare dilute-limit heats of segregation for selected BCC transition metal binaries computed using ab initio and quantum approximate energy methods. Ab initio calculations are carried out using the CASTEP plane-wave pseudopotential computer code, while quantum approximate results are computed using the Bozzolo-Ferrante-Smith (BFS) method with the most recent LMTO-based parameters. Quantum approximate segregation energies are computed with and without atomistic relaxation, while the ab initio calculations are performed without relaxation. Results are discussed within the context of a segregation model driven by strain and bond-breaking effects. We compare our results with full-potential quantum calculations and with available experimental results.
Ab initio Calculations of Electronic Fingerprints of DNA bases on Graphene
NASA Astrophysics Data System (ADS)
Ahmed, Towfiq; Rehr, John J.; Kilina, Svetlana; Das, Tanmoy; Haraldsen, Jason T.; Balatsky, Alexander V.
2012-02-01
We have carried out first principles DFT calculations of the electronic local density of states (LDOS) of DNA nucleotide bases (A,C,G,T) adsorbed on graphene using LDA with ultra-soft pseudo-potentials. We have also calculated the longitudinal transmission currents T(E) through graphene nano-pores as an individual DNA base passes through it, using a non-equilibrium Green's function (NEGF) formalism. We observe several dominant base-dependent features in the LDOS and T(E) in an energy range within a few eV of the Fermi level. These features can serve as electronic fingerprints for the identification of individual bases from dI/dV measurements in scanning tunneling spectroscopy (STS) and nano-pore experiments. Thus these electronic signatures can provide an alternative approach to DNA sequencing.
Li, Y Q; Zhang, P Y; Han, K L
2015-03-28
A global many-body expansion potential energy surface is reported for the electronic ground state of CH2 (+) by fitting high level ab initio energies calculated at the multireference configuration interaction level with the aug-cc-pV6Z basis set. The topographical features of the new global potential energy surface are examined in detail and found to be in good agreement with those calculated directly from the raw ab initio energies, as well as previous calculations available in the literature. In turn, in order to validate the potential energy surface, a test theoretical study of the reaction CH(+)(X(1)Σ(+))+H((2)S)→C(+)((2)P)+H2(X(1)Σg (+)) has been carried out with the method of time dependent wavepacket on the title potential energy surface. The total integral cross sections and the rate coefficients have been calculated; the results determined that the new potential energy surface can both be recommended for dynamics studies of any type and as building blocks for constructing the potential energy surfaces of larger C(+)/H containing systems.
Liu, Miao; Rong, Ziqin; Malik, Rahul; Canepa, Pieremanuele; Jain, Anubhav; Ceder, Gerbrand; Persson, Kristin A.
2014-12-16
In this study, batteries that shuttle multivalent ions such as Mg2+ and Ca2+ ions are promising candidates for achieving higher energy density than available with current Li-ion technology. Finding electrode materials that reversibly store and release these multivalent cations is considered a major challenge for enabling such multivalent battery technology. In this paper, we use recent advances in high-throughput first-principles calculations to systematically evaluate the performance of compounds with the spinel structure as multivalent intercalation cathode materials, spanning a matrix of five different intercalating ions and seven transition metal redox active cations. We estimate the insertion voltage, capacity, thermodynamic stabilitymore » of charged and discharged states, as well as the intercalating ion mobility and use these properties to evaluate promising directions. Our calculations indicate that the Mn2O4 spinel phase based on Mg and Ca are feasible cathode materials. In general, we find that multivalent cathodes exhibit lower voltages compared to Li cathodes; the voltages of Ca spinels are ~0.2 V higher than those of Mg compounds (versus their corresponding metals), and the voltages of Mg compounds are ~1.4 V higher than Zn compounds; consequently, Ca and Mg spinels exhibit the highest energy densities amongst all the multivalent cation species. The activation barrier for the Al³⁺ ion migration in the Mn₂O₄ spinel is very high (~1400 meV for Al3+ in the dilute limit); thus, the use of an Al based Mn spinel intercalation cathode is unlikely. Amongst the choice of transition metals, Mn-based spinel structures rank highest when balancing all the considered properties.« less
NASA Technical Reports Server (NTRS)
Smith, Grant D.; Jaffe, R. L.; Yoon, D. Y.; Arnold, James O. (Technical Monitor)
1994-01-01
Conformational energy contours of perfluoroalkanes, determined from ab initio calculations, confirm the well-known spitting of trans states into two minima at plus or minus 17 degrees but also show that the gauche states split as well, with minima at plus or minus 124 degrees and plus or minus 84 in order to relieve steric crowding. The directions of such split distortions from the perfectly staggered states are strongly coupled for adjacent pairs of bonds in a manner identical to the intradyad pair for poly (isobutylene) chains. These conformational characteristics are fully represented by a six-state rotational isomeric state (RIS) model for PTFE comprised of t(+), t(-), g(sup +)+, g(sup +)-, g(sup -) + and g(sup -)-states, located at the split energy minima. The resultant 6 x 6 statistical weight matrix is described by first-order interaction parameters for the g+(+) (ca. 0.6 kcal/mol) and g+- (ca. 2.0 kcal/mol) states, and second order parameters for the g(sup +)+g(sup +)+ (ca 0.6 kcal/mol) and g(sup +)+g(sup -)+ (ca. 1.0 kcal/mol) states. This six-state RIS model, without adjustment of the geometric or energy parameters as determined from the ab initio calculations, predicts the unperturbed chain dimensions and the fraction of gauche bonds as a function of temperature for PTFE in good agreement with available experimental values.
Douillard, J M; Henry, M
2003-07-15
A very simple route to calculation of the surface energy of solids is proposed because this value is very difficult to determine experimentally. The first step is the calculation of the attractive part of the electrostatic energy of crystals. The partial charges used in this calculation are obtained by using electronegativity equalization and scales of electronegativity and hardness deduced from physical characteristics of the atom. The lattice energies of the infinite crystal and of semi-infinite layers are then compared. The difference is related to the energy of cohesion and then to the surface energy. Very good results are obtained with ice, if one compares with the surface energy of liquid water, which is generally considered a good approximation of the surface energy of ice.
González-Lavado, Eloisa; Corchado, Jose C; Espinosa-Garcia, Joaquin
2014-02-14
Based exclusively on high-level ab initio calculations, a new full-dimensional analytical potential energy surface (PES-2014) for the gas-phase reaction of hydrogen abstraction from methane by an oxygen atom is developed. The ab initio information employed in the fit includes properties (equilibrium geometries, relative energies, and vibrational frequencies) of the reactants, products, saddle point, points on the reaction path, and points on the reaction swath, taking especial caution respecting the location and characterization of the intermediate complexes in the entrance and exit channels. By comparing with the reference results we show that the resulting PES-2014 reproduces reasonably well the whole set of ab initio data used in the fitting, obtained at the CCSD(T) = FULL/aug-cc-pVQZ//CCSD(T) = FC/cc-pVTZ single point level, which represents a severe test of the new surface. As a first application, on this analytical surface we perform an extensive dynamics study using quasi-classical trajectory calculations, comparing the results with recent experimental and theoretical data. The excitation function increases with energy (concave-up) reproducing experimental and theoretical information, although our values are somewhat larger. The OH rotovibrational distribution is cold in agreement with experiment. Finally, our results reproduce experimental backward scattering distribution, associated to a rebound mechanism. These results lend confidence to the accuracy of the new surface, which substantially improves the results obtained with our previous surface (PES-2000) for the same system.
Ab initio calculations and crystal symmetry considerations for novel FeSe-based superconductors
NASA Astrophysics Data System (ADS)
Mazin, Igor
2013-03-01
Density functional calculations disagree with the ARPES measurements on both K0.3Fe2Se2 superconducting phase and FeSe/SrTiO3 monolayers. Yet they can still be dramatically useful for the reason that they respect full crystallographic symmetry and take good account of electron-ion interaction. Using just symmetry analysis, it is shown that nodeless d-wave superconductivity is not an option in these systems, and a microscopic framework is derived that leads to a novel s-wave sign-reversal state, qualitatively different from the already familiar s+/- state in pnictides and bulk binary selenides. Regarding the FeSe monolayer, bonding and charge transfer between the film and the substrate is analyzed and it is shown that the former is weak and the latter negligible, which sets important restrictions on possible mechanisms of doping and superconductivity in these monolayers. In particular, the role of the so-called ``Se etching,'' necessary for superconductivity in FeSe monolayers, is analyzed in terms of electronic structure and bonding with the substrate.
Porphyrin-based polymeric nanostructures for light harvesting applications: Ab initio calculations
NASA Astrophysics Data System (ADS)
Orellana, Walter
The capture and conversion of solar energy into electricity is one of the most important challenges to the sustainable development of mankind. Among the large variety of materials available for this purpose, porphyrins concentrate great attention due to their well-known absorption properties in the visible range. However, extended materials like polymers with similar absorption properties are highly desirable. In this work, we investigate the stability, electronic and optical properties of polymeric nanostructures based on free-base porphyrins and phthalocyanines (H2P, H2Pc), within the framework of the time-dependent density functional perturbation theory. The aim of this work is the stability, electronic, and optical characterization of polymeric sheets and nanotubes obtained from H2P and H2Pc monomers. Our results show that H2P and H2Pc sheets exhibit absorption bands between 350 and 400 nm, slightly different that the isolated molecules. However, the H2P and H2Pc nanotubes exhibit a wide absorption in the visible and near-UV range, with larger peaks at 600 and 700 nm, respectively, suggesting good characteristic for light harvesting. The stability and absorption properties of similar structures obtained from ZnP and ZnPc molecules is also discussed. Departamento de Ciencias Físicas, República 220, 037-0134 Santiago, Chile.
Zhu, Tong; Zhang, John Z H; He, Xiao
2014-09-14
In this work, protein side chain (1)H chemical shifts are used as probes to detect and correct side-chain packing errors in protein's NMR structures through structural refinement. By applying the automated fragmentation quantum mechanics/molecular mechanics (AF-QM/MM) method for ab initio calculation of chemical shifts, incorrect side chain packing was detected in the NMR structures of the Pin1 WW domain. The NMR structure is then refined by using molecular dynamics simulation and the polarized protein-specific charge (PPC) model. The computationally refined structure of the Pin1 WW domain is in excellent agreement with the corresponding X-ray structure. In particular, the use of the PPC model yields a more accurate structure than that using the standard (nonpolarizable) force field. For comparison, some of the widely used empirical models for chemical shift calculations are unable to correctly describe the relationship between the particular proton chemical shift and protein structures. The AF-QM/MM method can be used as a powerful tool for protein NMR structure validation and structural flaw detection.
Wong, Kin-Yiu; Xu, Yuqing; York, Darrin M
2014-06-30
Detailed understandings of the reaction mechanisms of RNA catalysis in various environments can have profound importance for many applications, ranging from the design of new biotechnologies to the unraveling of the evolutionary origin of life. An integral step in the nucleolytic RNA catalysis is self-cleavage of RNA strands by 2'-O-transphosphorylation. Key to elucidating a reaction mechanism is determining the molecular structure and bonding characteristics of transition state. A direct and powerful probe of transition state is measuring isotope effects on biochemical reactions, particularly if we can reproduce isotope effect values from quantum calculations. This article significantly extends the scope of our previous joint experimental and theoretical work in examining isotope effects on enzymatic and nonenzymatic 2'-O-transphosphorylation reaction models that mimic reactions catalyzed by RNA enzymes (ribozymes), and protein enzymes such as ribonuclease A (RNase A). Native reactions are studied, as well as reactions with thio substitutions representing chemical modifications often used in experiments to probe mechanism. Here, we report and compare results from eight levels of electronic-structure calculations for constructing the potential energy surfaces in kinetic and equilibrium isotope effects (KIE and EIE) computations, including a "gold-standard" coupled-cluster level of theory [CCSD(T)]. In addition to the widely used Bigeleisen equation for estimating KIE and EIE values, internuclear anharmonicity and quantum tunneling effects were also computed using our recently developed ab initio path-integral method, that is, automated integration-free path-integral method. The results of this work establish an important set of benchmarks that serve to guide calculations of KIE and EIE for RNA catalysis.
Wong, Kin-Yiu; Yuqing, Xu; York, Darrin M.
2014-01-01
Detailed understandings of the reaction mechanisms of RNA catalysis in various environments can have profound importance for many applications, ranging from the design of new biotechnologies to the unraveling of the evolutionary origin of life. An integral step in the nucleolytic RNA catalysis is self-cleavage of RNA strands by 2′-O-transphosphorylation. Key to elucidating a reaction mechanism is determining the molecular structure and bonding characteristics of transition state. A direct and powerful probe of transition state is measuring isotope effects on biochemical reactions, particularly if we can reproduce isotope effect values from quantum calculations. This paper significantly extends the scope of our previous joint experimental and theoretical work in examining isotope effects on enzymatic and non-enzymatic 2′-O-transphosphorylation reaction models that mimic reactions catalyzed by RNA enzymes (ribozymes), and protein enzymes such as ribonuclease A (RNase A). Native reactions are studied, as well as reactions with thio substitutions representing chemical modifications often used in experiments to probe mechanism. Here, we report and compare results from eight levels of electronic-structure calculations for constructing the potential energy surfaces in kinetic and equilibrium isotope effects (KIE and EIE) computations, including a “gold-standard” coupled-cluster level of theory [CCSD(T)]. In addition to the widely-used Bigeleisen equation for estimating KIE and EIE values, internuclear anharmonicity and quantum tunneling effects were also computed using our recently-developed ab initio path-integral method, i.e., automated integration-free path-integral (AIF-PI) method. The results of this work establish an important set of benchmarks that serve to guide calculations of KIE and EIE for RNA catalysis. PMID:24841935
Towards AB Initio Calculation of the Circular Dichroism of Peptides
NASA Astrophysics Data System (ADS)
Molteni, E.; Onida, G.; Tiana, G.
2012-08-01
In this work we plan to use ab initio spectroscopy calculations to compute circular dichroism (CD) spectra of peptides. CD provides information on protein secondary structure content; peptides, instead, remain difficult to address, due to their tendency to adopt multiple conformations in equilibrium. Therefore peptides are an interesting test-case for ab initio calculation of CD spectra. As a first application, we focus on the (83-92) fragment of HIV-1 protease, which is known to be involved in the folding and dimerization of this protein. As a preliminary step, we performed classical molecular dynamics (MD) simulations, in order to obtain a set of representative conformers of the peptide. Then, on some of the obtained conformations, we calculated absorption spectra at the independent particle, RPA and TDLDA levels, showing the presence of charge transfer excitations, and their influence on spectral features.
Ab-initio calculations on melting of thorium
NASA Astrophysics Data System (ADS)
Mukherjee, D.; Sahoo, B. D.; Joshi, K. D.; Kaushik, T. C.; Gupta, Satish C.
2016-05-01
Ab-initio molecular dynamics study has been performed on face centered cubic structured thorium to determine its melting temperature at room pressure. The ion-electron interaction potential energy calculated as a function of temperature for three volumes (a0)3 and (1.02a0)3 and (1.04a0)3 increases gradually with temperature and undergoes a sharp jump at ~2200 K, ~2100 K and ~1800 K, respectively. Here, a0 = 5.043 Å is the equilibrium lattice parameter at 0 K obtained from ab-initio calculations. These jumps in interaction energy are treated as due to the onset of melting and corresponding temperatures as melting point. The melting point of 2100 K is close to the experimental value of 2023K. Further, the same has been verified by plotting the atomic arrangement evolved at various temperatures and corresponding pair correlation functions.
Ab Initio Calculations of Water Line Strengths
NASA Technical Reports Server (NTRS)
Schwenke, David W.; Partridge, Harry
1998-01-01
We report on the determination of a high quality ab initiu potential energy surface (PES) and dipole moment function for water. This PES is empirically adjusted to improve the agreement between the computed line positions and those from the HITRAN 92 data base with J less than 6 for H2O. The changes in the PES are small, nonetheless including an estimate of core (oxygen 1s) electron correlation greatly improves the agreement with experiment. Using this adjusted PES, we can match 30,092 of the 30,117 transitions in the HITRAN 96 data base for H2O with theoretical lines. The 10,25,50,75, and 90 percentiles of the difference between the calculated and tabulated line positions are -0.11, -0.04, -0.01, 0.02, and 0.07 l/cm. Non-adiabatic effects are not explicitly included. About 3% of the tabulated line positions appear to be incorrect. Similar agreement using this adjusted PES is obtained for the oxygen 17 and oxygen 18 isotopes. For HDO, the agreement is not as good, with root-mean-square error of 0.25 l/cm for lines with J less than 6. This error is reduced to 0.02 l/cm by including a small asymmetric correction to the PES, which is parameterized by simultaneously fitting to HDO md D2O data. Scaling this correction by mass factors yields good results for T2O and HTO. The intensities summed over vibrational bands are usually in good agreement between the calculations and the tabulated results, but individual lines strengths can differ greatly. A high temperature list consisting of 307,721,352 lines is generated for H2O using our PES and dipole moment function.
Ab-initio modeling of an anion C- 60 pseudopotential for fullerene-based compounds
NASA Astrophysics Data System (ADS)
Vrubel, Ivan I.; Polozkov, Roman G.; Ivanov, Vadim K.
2016-08-01
An anion C- 60 pseudopotential is determined from an ab-initio-based approach. First, ab-initio calculations are performed to calculate the electronic charge density and the total electrostatic potential. Second, the effective dependence of the pseudopotential on the radial degree of freedom is extracted from the angular average of the total electrostatic potential. Finally, the resulting effective pseudopotential is fitted to a simple analytical form which can be applied in further dynamical simulations of fullerene-based compounds.
Ab initio calculations of reactions with light nuclei
NASA Astrophysics Data System (ADS)
Quaglioni, Sofia; Hupin, Guillaume; Calci, Angelo; Navrátil, Petr; Roth, Robert
2016-03-01
An ab initio (i.e., from first principles) theoretical framework capable of providing a unified description of the structure and low-energy reaction properties of light nuclei is desirable to further our understanding of the fundamental interactions among nucleons, and provide accurate predictions of crucial reaction rates for nuclear astrophysics, fusion-energy research, and other applications. In this contribution we review ab initio calculations for nucleon and deuterium scattering on light nuclei starting from chiral two- and three-body Hamiltonians, obtained within the framework of the ab initio no-core shell model with continuum. This is a unified approach to nuclear bound and scattering states, in which square-integrable energy eigenstates of the A-nucleon system are coupled to (A-a)+a target-plus-projectile wave functions in the spirit of the resonating group method to obtain an efficient description of the many-body nuclear dynamics both at short and medium distances and at long ranges.
Ab initio calculations of nuclear reactions important for astrophysics
NASA Astrophysics Data System (ADS)
Navratil, Petr; Dohet-Eraly, Jeremy; Calci, Angelo; Horiuchi, Wataru; Hupin, Guillaume; Quaglioni, Sofia
2016-09-01
In recent years, significant progress has been made in ab initio nuclear structure and reaction calculations based on input from QCD employing Hamiltonians constructed within chiral effective field theory. One of the newly developed approaches is the No-Core Shell Model with Continuum (NCSMC), capable of describing both bound and scattering states in light nuclei simultaneously. We will present NCSMC results for reactions important for astrophysics that are difficult to measure at relevant low energies, such as 3He(α,γ)7Be and 3H(α,γ)7Li and 11C(p,γ)12N radiative capture, as well as the 3H(d,n)4He fusion. We will also address prospects of calculating the 2H(α,γ)6Li capture reaction within the NCSMC formalism. Prepared in part by LLNL under Contract DE-AC52-07NA27344. Supported by the U.S. DOE, OS, NP, under Work Proposal No. SCW1158, and by the NSERC Grant No. SAPIN-2016-00033. TRIUMF receives funding from the NRC Canada.
Ab Initio Calculations Applied to Problems in Metal Ion Chemistry
NASA Technical Reports Server (NTRS)
Bauschlicher, Charles W., Jr.; Langhoff, Stephen R.; Partridge, Harry; Arnold, James O. (Technical Monitor)
1994-01-01
Electronic structure calculations can provide accurate spectroscopic data (such as molecular structures) vibrational frequencies, binding energies, etc.) that have been very useful in explaining trends in experimental data and in identifying incorrect experimental measurements. In addition, ab initio calculations. have given considerable insight into the many interactions that make the chemistry of transition metal systems so diverse. In this review we focus on cases where calculations and experiment have been used to solve interesting chemical problems involving metal ions. The examples include cases where theory was used to differentiate between disparate experimental values and cases where theory was used to explain unexpected experimental results.
Ab initio calculations of free-energy reaction barriers.
Bucko, T
2008-02-13
The theoretical description of chemical reactions was until recently limited to a 'static' approach in which important parameters such as the rate constant are deduced from the local topology of the potential energy surface close to minima and saddle points. Such an approach has, however, serious limitations. The growing computational power allows us now to use advanced simulation techniques to determine entropic effects accurately for medium-sized systems at ab initio level. Recently, we have implemented free-energy simulation techniques based on molecular dynamics, in particular on the blue-moon ensemble technique and on metadynamics, in the popular DFT code VASP. In the thermodynamic integration (blue-moon ensemble) technique, the free-energy profile is calculated as the path integral over the restoring forces along a parametrized reaction coordinate. In metadynamics, an image of the free-energy surface is constructed on the fly during the simulation by adding small repulsive Gaussian-shaped hills to the Lagrangian driving the dynamics. The two methods are tested on a simple chemical reaction-the nucleophilic substitution of methyl chloride by a chlorine anion.
Ab initio calculations of free-energy reaction barriers
NASA Astrophysics Data System (ADS)
Bucko, T.
2008-02-01
The theoretical description of chemical reactions was until recently limited to a 'static' approach in which important parameters such as the rate constant are deduced from the local topology of the potential energy surface close to minima and saddle points. Such an approach has, however, serious limitations. The growing computational power allows us now to use advanced simulation techniques to determine entropic effects accurately for medium-sized systems at ab initio level. Recently, we have implemented free-energy simulation techniques based on molecular dynamics, in particular on the blue-moon ensemble technique and on metadynamics, in the popular DFT code VASP. In the thermodynamic integration (blue-moon ensemble) technique, the free-energy profile is calculated as the path integral over the restoring forces along a parametrized reaction coordinate. In metadynamics, an image of the free-energy surface is constructed on the fly during the simulation by adding small repulsive Gaussian-shaped hills to the Lagrangian driving the dynamics. The two methods are tested on a simple chemical reaction—the nucleophilic substitution of methyl chloride by a chlorine anion.
NASA Astrophysics Data System (ADS)
Polyansky, Oleg L.; Ovsyannikov, Roman I.; Kyuberis, Aleksandra A.; Lodi, Lorenzo; Tennyson, Jonathan; Zobov, Nikolai F.
2013-10-01
A recently computed, high-accuracy ab initio Born-Oppenheimer (BO) potential energy surface (PES) for the water molecule is combined with relativistic, adiabatic, quantum electrodynamics, and, crucially, nonadiabatic corrections. Calculations of ro-vibrational levels are presented for several water isotopologues and shown to have unprecedented accuracy. A purely ab initio calculation reproduces some 200 known band origins associated with seven isotopologues of water with a standard deviation (σ) of about 0.35 cm-1. Introducing three semiempirical scaling parameters, two affecting the BO PES and one controlling nonadiabatic effects, reduces σ below 0.1 cm-1. Introducing one further rotational nonadiabatic parameter gives σ better than 0.1 cm-1 for all observed ro-vibrational energy levels up to J = 25. We conjecture that the energy levels of closed-shell molecules with roughly the same number of electrons as water, such as NH3, CH4, and H3O+, could be calculated to this accuracy using an analogous procedure. This means that near-ab initio calculations are capable of predicting transition frequencies with an accuracy only about a factor of 5 worse than high resolution experiments.
NASA Astrophysics Data System (ADS)
Nishioka, Hirotaka; Ando, Koji
2011-05-01
By making use of an ab initio fragment-based electronic structure method, fragment molecular orbital-linear combination of MOs of the fragments (FMO-LCMO), developed by Tsuneyuki et al. [Chem. Phys. Lett. 476, 104 (2009)], 10.1016/j.cplett.2009.05.069, we propose a novel approach to describe long-distance electron transfer (ET) in large system. The FMO-LCMO method produces one-electron Hamiltonian of whole system using the output of the FMO calculation with computational cost much lower than conventional all-electron calculations. Diagonalizing the FMO-LCMO Hamiltonian matrix, the molecular orbitals (MOs) of the whole system can be described by the LCMOs. In our approach, electronic coupling TDA of ET is calculated from the energy splitting of the frontier MOs of whole system or perturbation method in terms of the FMO-LCMO Hamiltonian matrix. Moreover, taking into account only the valence MOs of the fragments, we can considerably reduce computational cost to evaluate TDA. Our approach was tested on four different kinds of model ET systems with non-covalent stacks of methane, non-covalent stacks of benzene, trans-alkanes, and alanine polypeptides as their bridge molecules, respectively. As a result, it reproduced reasonable TDA for all cases compared to the reference all-electron calculations. Furthermore, the tunneling pathway at fragment-based resolution was obtained from the tunneling current method with the FMO-LCMO Hamiltonian matrix.
Ab initio and RRKM calculations of o-benzyne pyrolysis
NASA Astrophysics Data System (ADS)
Deng, Wei-Qiao; Han, Ke-Li; Zhan, Ji-Ping; He, Guo-Zhong
1998-05-01
Recently, a new mechanism has been provided in the phenyl pyrolysis, that is, the phenyl dissociation favours the benzyne channel by losing an H atom [H. Wang, M. Frenklach, J. Phys. Chem., 98 (1994) 11465]. In this Letter, the dissociation of o-benzyne has been investigated by means of ab initio theory. The geometries and structures of o-benzyne with its pyrolysis products C 4H 2, C 2H 2 and also the transition state were optimized at the UHF/6-31G* level. The single point energies were refined by B3LYP/6-31G* calculations. The unimolecular rate constants for o-benzyne pyrolysis in different pressures were calculated by the Rice-Ramsperger-Kassel-Marcus (RRKM) method.
Motif based Hessian matrixfor ab initio geometry optimization ofnanostructures
Zhao, Zhengji; Wang, Lin-Wang; Meza, Juan
2006-04-05
A simple method to estimate the atomic degree Hessian matrixof a nanosystem is presented. The estimated Hessian matrix, based on themotif decomposition of the nanosystem, can be used to accelerate abinitio atomic relaxations with speedups of 2 to 4 depending on the sizeof the system. In addition, the programing implementation for using thismethod in a standard ab initio package is trivial.
Ab Initio Calculations of Excited Carrier Dynamics in Gallium Nitride
NASA Astrophysics Data System (ADS)
Jhalani, Vatsal; Bernardi, Marco
Bulk wurtzite GaN is the primary material for blue light-emission technology. The radiative processes in GaN are regulated by the dynamics of excited (or so-called ``hot'') carriers, through microscopic processes not yet completely understood. We present ab initio calculations of electron-phonon (e-ph) scattering rates for hot carriers in GaN. Our work combines density functional theory to compute the electronic states, and density functional perturbation theory to obtain the phonon dispersions and e-ph coupling matrix elements. These quantities are interpolated on fine Brillouin zone grids with maximally localized Wannier functions, to converge the e-ph scattering rates within 5 eV of the band edges. We resolve the contribution of the different phonon modes to the total scattering rate, and study the impact on the relaxation times of the long-range Fröhlich interaction due to the longitudinal-optical phonon modes.
Quantum plasmonics: from jellium models to ab initio calculations
NASA Astrophysics Data System (ADS)
Varas, Alejandro; García-González, Pablo; Feist, Johannes; García-Vidal, F. J.; Rubio, Angel
2016-08-01
Light-matter interaction in plasmonic nanostructures is often treated within the realm of classical optics. However, recent experimental findings show the need to go beyond the classical models to explain and predict the plasmonic response at the nanoscale. A prototypical system is a nanoparticle dimer, extensively studied using both classical and quantum prescriptions. However, only very recently, fully ab initio time-dependent density functional theory (TDDFT) calculations of the optical response of these dimers have been carried out. Here, we review the recent work on the impact of the atomic structure on the optical properties of such systems. We show that TDDFT can be an invaluable tool to simulate the time evolution of plasmonic modes, providing fundamental understanding into the underlying microscopical mechanisms.
Interatomic Coulombic decay widths of helium trimer: Ab initio calculations
Kolorenč, Přemysl; Sisourat, Nicolas
2015-12-14
We report on an extensive study of interatomic Coulombic decay (ICD) widths in helium trimer computed using a fully ab initio method based on the Fano theory of resonances. Algebraic diagrammatic construction for one-particle Green’s function is utilized for the solution of the many-electron problem. An advanced and universal approach to partitioning of the configuration space into discrete states and continuum subspaces is described and employed. Total decay widths are presented for all ICD-active states of the trimer characterized by one-site ionization and additional excitation of an electron into the second shell. Selected partial decay widths are analyzed in detail, showing how three-body effects can qualitatively change the character of certain relaxation transitions. Previously unreported type of three-electron decay processes is identified in one class of the metastable states.
Ab initio calculation of the shock Hugoniot of bulk silicon
NASA Astrophysics Data System (ADS)
Strickson, Oliver; Artacho, Emilio
2016-03-01
We describe how ab initio molecular dynamics can be used to determine the Hugoniot locus (states accessible by a shock wave) for materials with a number of stable phases, and with an approximate treatment of plasticity and yield, without having to simulate these phenomena directly. We consider the case of bulk silicon, with forces from density-functional theory, up to 70 GPa. The fact that shock waves can split into multiple waves due to phase transitions or yielding is taken into account here by specifying the strength of any preceding waves explicitly based on their yield strain. Points corresponding to uniaxial elastic compression along three crystal axes and a number of postshock phases are given, including a plastically yielded state, approximated by an isotropic stress configuration following an elastic wave of predetermined strength. The results compare well to existing experimental data for shocked silicon.
Interatomic Coulombic decay widths of helium trimer: Ab initio calculations.
Kolorenč, Přemysl; Sisourat, Nicolas
2015-12-14
We report on an extensive study of interatomic Coulombic decay (ICD) widths in helium trimer computed using a fully ab initio method based on the Fano theory of resonances. Algebraic diagrammatic construction for one-particle Green's function is utilized for the solution of the many-electron problem. An advanced and universal approach to partitioning of the configuration space into discrete states and continuum subspaces is described and employed. Total decay widths are presented for all ICD-active states of the trimer characterized by one-site ionization and additional excitation of an electron into the second shell. Selected partial decay widths are analyzed in detail, showing how three-body effects can qualitatively change the character of certain relaxation transitions. Previously unreported type of three-electron decay processes is identified in one class of the metastable states.
An ab initio-based Er–He interatomic potential in hcp Er
Yang, Li; ye, Yeting; Fan, K. M.; Shen, Huahai; Peng, Shuming; Long, XG; Zhou, X. S.; Zu, Xiaotao; Gao, Fei
2014-09-01
We have developed an empirical erbium-helium (Er-He) potential by fitting to the results calculated from ab initio method. Based on the electronic hybridization between Er and He atoms, an s-band model, along with a repulsive pair potential, has been derived to describe the Er-He interaction. The atomic configurations and the formation energies of single He defects, small He interstitial clusters (Hen) and He-vacancy (HenV ) clusters obtained by ab initio calculations are used as the fitting database. The binding energies and relative stabilities of the HnVm clusters are studied by the present potential and compared with the ab initio calculations. The Er-He potential is also applied to study the migration of He in hcp-Er at different temperatures, and He clustering is found to occur at 600 K in hcp Er crystal, which may be due to the anisotropic migration behavior of He interstitials.
NASA Astrophysics Data System (ADS)
Hong, Qi-Jun; van de Walle, Axel
2015-07-01
Using electronic structure calculations, we conduct an extensive investigation into the Hf-Ta-C system, which includes the compounds that have the highest melting points known to date. We identify three major chemical factors that contribute to the high melting temperatures. Based on these factors, we propose a class of materials that may possess even higher melting temperatures and explore it via efficient ab initio molecular dynamics calculations in order to identify the composition maximizing the melting point. This study demonstrates the feasibility of automated and high-throughput materials screening and discovery via ab initio calculations for the optimization of "higher-level" properties, such as melting points, whose determination requires extensive sampling of atomic configuration space.
NASA Astrophysics Data System (ADS)
Demkov, Alexander A.; Navrotsky, Alexandra
2001-03-01
The International Technology Roadmap for Semiconductors (ITRS) predicts that the strategy of scaling complementary metal-oxide-semiconductor (CMOS) devices will come to an abrupt end around the year 2012. The main reason for this will be the unacceptably high leakage current through the silicon dioxide gate with a thickness below 20 ÅFinding a gate insulator alternative to SiO2 has proven to be far from trivial. Hafnium and zirconium dioxides and silicates have been recently considered as gate dielectrics with intermediate dielectric constants. Hafnia and ziconia are important ceramic materials as well, and their phase relations are rather well studied. There is also interest in hafnia as a constituent of ceramic waste forms for plutonium, based on its refractory nature and high neutron absorption cross section. We use a combination of the ab-initio calculations and calorimetry to investigate thermodynamic and electronic properties of hafnia and zirconia. We describe the cubic to tetragonal phase transition in the fluorite structure by computing the total energy surface for zone-edge distortions correct to fourth order in the soft-mode displacement with the strain coupling renormalization included. We compare the two materials using some simple chemical concepts.
Surface Segregation Energies of BCC Binaries from Ab Initio and Quantum Approximate Calculations
NASA Technical Reports Server (NTRS)
Good, Brian S.
2003-01-01
We compare dilute-limit segregation energies for selected BCC transition metal binaries computed using ab initio and quantum approximate energy method. Ab initio calculations are carried out using the CASTEP plane-wave pseudopotential computer code, while quantum approximate results are computed using the Bozzolo-Ferrante-Smith (BFS) method with the most recent parameterization. Quantum approximate segregation energies are computed with and without atomistic relaxation. The ab initio calculations are performed without relaxation for the most part, but predicted relaxations from quantum approximate calculations are used in selected cases to compute approximate relaxed ab initio segregation energies. Results are discussed within the context of segregation models driven by strain and bond-breaking effects. We compare our results with other quantum approximate and ab initio theoretical work, and available experimental results.
Ab initio calculations in a uniform magnetic field using periodic supercells
Cai, W; Galli, G
2003-10-21
We present a formulation of ab initio electronic structure calculations in a finite magnetic field, which retains the simplicity and efficiency of techniques widely used in first principles molecular dynamics simulations, based on plane-wave basis sets and Fourier transforms. In addition we discuss results obtained with this method for the energy spectrum of interacting electrons in quantum wells, and for the electronic properties of dense fluid deuterium in a uniform magnetic field.
Ab initio calculations on the magnetic properties of transition metal complexes
Bodenstein, Tilmann; Fink, Karin
2015-12-31
We present a protocol for the ab initio determination of the magnetic properties of mono- and polynuclear transition metal compounds. First, we obtain the low lying electronic states by multireference methods. Then, we include spin-orbit coupling and an external magnetic field for the determination of zero-field splitting and g-tensors. For the polynuclear complexes the magnetic exchange coupling constants are determined by a modified complete active space self consistent field method. Based on the results of the ab initio calculations, magnetic data such as magnetic susceptibility or magnetization are simulated and compared to experimental data. The results obtained for the polynuclear complexes are further analysed by calculations on model complexes where part of the magnetic centers are substituted by diamagnetic ions. The methods are applied to different Co and Ni containing transition metal complexes.
Development of an ab-initio calculation method for 2D layered materials-based optoelectronic devices
NASA Astrophysics Data System (ADS)
Kim, Han Seul; Kim, Yong-Hoon
We report on the development of a novel first-principles method for the calculation of non-equilibrium nanoscale device operation process. Based on region-dependent Δ self-consistent field method beyond the standard density functional theory (DFT), we will introduce a novel method to describe non-equilibrium situations such as external bias and simultaneous optical excitations. In particular, we will discuss the limitation of conventional method and advantage of our scheme in describing 2D layered materials-based devices operations. Then, we investigate atomistic mechanism of optoelectronic effects from 2D layered materials-based devices and suggest the optimal material and architecture for such devices.
NASA Astrophysics Data System (ADS)
Xie, Z. L.; Dy, K. S.; Wu, S. Y.
1996-03-01
An efficient ab initio scheme has been developed by using TB LMTO method to calculate the electronic structure and the full electron density of atomic clusters. In this scheme, the real-space Green's function (RSG) is used to obtain the full electronic density which includes the correction for non-spherical effects. Meanwhile, the real space Green's function can facilitate the calculation of the electronic structure for large clusters with restricted computer memory and CPU resources by using matrix reversion method developed by Wu and his co-workers. With the full electron density of atomic clusters, the total energies of clusters can be calculated. We have applied this ab initio scheme to evaluate the structural properties for Si clusters of 11-14 atoms. Diffrent geometrical arrangements have been examined as posible candidates of the most stable structure for each size cluster, and by calculating their cohesive energies, the most stable structure can be determined for a given size cluster. Our calculations have given similar results of previous investigations using the tight-binding molecular dynamics method.
FTIR, Raman spectra and ab initio calculations of 2-mercaptobenzothiazole.
Rai, Amareshwar K; Singh, Rachana; Singh, K N; Singh, V B
2006-02-01
FTIR and Raman spectra of a rubber vulcanization accelerator, 2-mercaptobenzothiazole (MBT), were recorded in the solid phase. The harmonic vibrational wavenumbers, for both the toutomeric forms of MBT, as well as for its dimeric complex, have been calculated, using ab initio RHF and density functional B3LYP methods invoking different basis sets upto RHF/6-31G** and B3LYP/6-31G** and the results were compared with the experimental values. Conformational studies have been also carried out regarding its toutomeric monomer forms and its dimer form. With all the basis sets the thione form of MBT (II) is predicted to be more stable than thiol form (I) and dimeric conformation (III) is predicted to be more stable with monomeric conformations (I) and (II). Vibrational assignments have been made, and it has been found that the calculated normal mode frequencies of dimeric conformation (III) are required for the analysis of IR and Raman bands of the MBT. The predicted shift in NH- stretching vibration towards the lower wave number side with the B3LYP/6-31G** calculations for the most stable dimer form (III), is in better agreement with experimental results. The intermolecular sulfur-nitrogen distance in N-H...S hydrogen bond was found to be 3.35 angstroms from these calculations, is also in agreement to the experimental value. PMID:16098806
Li, Y. Q.; Zhang, P. Y.; Han, K. L.
2015-03-28
A global many-body expansion potential energy surface is reported for the electronic ground state of CH{sub 2}{sup +} by fitting high level ab initio energies calculated at the multireference configuration interaction level with the aug-cc-pV6Z basis set. The topographical features of the new global potential energy surface are examined in detail and found to be in good agreement with those calculated directly from the raw ab initio energies, as well as previous calculations available in the literature. In turn, in order to validate the potential energy surface, a test theoretical study of the reaction CH{sup +}(X{sup 1}Σ{sup +})+H({sup 2}S)→C{sup +}({sup 2}P)+H{sub 2}(X{sup 1}Σ{sub g}{sup +}) has been carried out with the method of time dependent wavepacket on the title potential energy surface. The total integral cross sections and the rate coefficients have been calculated; the results determined that the new potential energy surface can both be recommended for dynamics studies of any type and as building blocks for constructing the potential energy surfaces of larger C{sup +}/H containing systems.
Ab initio calculation of infrared intensities for hydrogen peroxide
NASA Technical Reports Server (NTRS)
Rogers, J. D.; Hillman, J. J.
1982-01-01
Results of an ab initio SCF quantum mechanical study are used to derive estimates for the infrared intensities of the fundamental vibrations of hydrogen peroxide. Atomic polar tensors (APTs) were calculated on the basis of a 4-31G basis set, and used to derive absolute intensities for the vibrational transitions. Comparison of the APTs calculated for H2O2 with those previously obtained for H2O and CH3OH, and of the absolute intensities derived from the H2O2 APTs with those derived from APTs transferred from H2O and CH3OH, reveals the sets of values to differ by no more than a factor of two, supporting the validity of the theoretical calculation. Values of the infrared intensities obtained correspond to A1 = 14.5 km/mol, A2 = 0.91 km/mol, A3 = 0.058 km/mol, A4 = 123 km/mol, A5 = 46.2 km/mol, and A6 = 101 km/mol. Charge, charge flux and overlap contributions to the dipole moment derivatives are also computed.
Engineering Room-temperature Superconductors Via ab-initio Calculations
NASA Astrophysics Data System (ADS)
Gulian, Mamikon; Melkonyan, Gurgen; Gulian, Armen
The BCS, or bosonic model of superconductivity, as Little and Ginzburg have first argued, can bring in superconductivity at room temperatures in the case of high-enough frequency of bosonic mode. It was further elucidated by Kirzhnitset al., that the condition for existence of high-temperature superconductivity is closely related to negative values of the real part of the dielectric function at finite values of the reciprocal lattice vectors. In view of these findings, the task is to calculate the dielectric function for real materials. Then the poles of this function will indicate the existence of bosonic excitations which can serve as a "glue" for Cooper pairing, and if the frequency is high enough, and the dielectric matrix is simultaneously negative, this material is a good candidate for very high-Tc superconductivity. Thus, our approach is to elaborate a methodology of ab-initio calculation of the dielectric function of various materials, and then point out appropriate candidates. We used the powerful codes (TDDF with the DP package in conjunction with ABINIT) for computing dielectric responses at finite values of the wave vectors in the reciprocal lattice space. Though our report is concerned with the particular problem of superconductivity, the application range of the data processing methodology is much wider. The ability to compute the dielectric function of existing and still non-existing (though being predicted!) materials will have many more repercussions not only in fundamental sciences but also in technology and industry.
Liu, Miao; Rong, Ziqin; Malik, Rahul; Canepa, Pieremanuele; Jain, Anubhav; Ceder, Gerbrand; Persson, Kristin A.
2014-12-16
In this study, batteries that shuttle multivalent ions such as Mg^{2+} and Ca^{2+} ions are promising candidates for achieving higher energy density than available with current Li-ion technology. Finding electrode materials that reversibly store and release these multivalent cations is considered a major challenge for enabling such multivalent battery technology. In this paper, we use recent advances in high-throughput first-principles calculations to systematically evaluate the performance of compounds with the spinel structure as multivalent intercalation cathode materials, spanning a matrix of five different intercalating ions and seven transition metal redox active cations. We estimate the insertion voltage, capacity, thermodynamic stability of charged and discharged states, as well as the intercalating ion mobility and use these properties to evaluate promising directions. Our calculations indicate that the Mn_{2}O_{4} spinel phase based on Mg and Ca are feasible cathode materials. In general, we find that multivalent cathodes exhibit lower voltages compared to Li cathodes; the voltages of Ca spinels are ~0.2 V higher than those of Mg compounds (versus their corresponding metals), and the voltages of Mg compounds are ~1.4 V higher than Zn compounds; consequently, Ca and Mg spinels exhibit the highest energy densities amongst all the multivalent cation species. The activation barrier for the Al³⁺ ion migration in the Mn₂O₄ spinel is very high (~1400 meV for Al^{3+} in the dilute limit); thus, the use of an Al based Mn spinel intercalation cathode is unlikely. Amongst the choice of transition metals, Mn-based spinel structures rank highest when balancing all the considered properties.
Jung, Julie; Guennic, Boris Le; Fedin, Matvey V; Ovcharenko, Victor I; Calzado, Carmen J
2015-07-20
The gradual magnetostructural transition in breathing crystals based on copper(II) and pyrazolyl-substituted nitronyl nitroxides has been analyzed by means of DDCI quantum chemistry calculations. The magnetic coupling constants (J) within the spin triads of Cu(hfac)2L(Bu)·0.5C8H18 have been evaluated for the X-ray structures reported at different temperatures. The coupling is strongly antiferromagnetic at low temperature and becomes ferromagnetic when the temperature increases. The intercluster magnetic coupling (J') is antiferromagnetic and shows a marked dependence on temperature. The magnetostructural transition can be reproduced using the calculated J values for each structure in the simulation of the magnetic susceptibility. However, the μ(T) curve can be improved nicely by considering the coexistence of two phases in the transition region, whose ratio varies with temperature corresponding to both the weakly and strongly coupled spin states. These results complement a recent VT-FTIR study on the parent Cu(hfac)2L(Pr) compound with a gradual magnetostructural transition.
Ab initio calculations of As-vacancy interactions in silicon
Xie, J.; Chen, S.P.
1999-04-01
Atomistic simulation of a vacancy-assisted dopant diffusion in silicon needs details of the dopant-vacancy interaction, i.e., the potential as a functional of dopant-vacancy separations. In this paper, the authors present a detailed study on the energetics of As-vacancy reaction in silicon and the lattice distortions surrounding the As-vacancy defect by using an ab initio plane wave pseudopotential method and the density functional theory (DFT). A potential-energy diagram as a function of As-vacancy separation is provided, which can be used in the atomistic diffusion simulations. The authors also calculate the binding energy and the formation energy of different complexes such as AsV, As{sub 2}V and AsV{sub 2} (V represents vacancy). They find that the stable configuration of As{sub 2}V is As-V-As, while the stable configuration of AsV{sub 2} is As-V-V. The nature of the binding between As and vacancy is explained from the lattice distortions and the change of chemical bond configuration introduced by the As-vacancy complex.
Rational design of electrolyte components by ab initio calculations
NASA Astrophysics Data System (ADS)
Johansson, Patrik; Jacobsson, Per
This paper is a small review of the use of computer simulations and especially the use of standard quantum-mechanical ab initio electronic structure calculations to rationally design and investigate different choices of chemicals/systems for lithium battery electrolytes. Covered systems and strategies to enhance the performance of electrolytes will range from assisting the interpretation of vibrational spectroscopy experiments over development of potentials for molecular dynamics simulations, to the design of new lithium salts and the lithium ion coordination in liquid, polymer, and gel polymer electrolytes. Examples of studied properties include the vibrational spectra of anions and ion pairs to characterize the nature and extent of the interactions present, the lithium ion affinities of anions, important for the salt solvation and the ability to provide a high concentration of charge carriers, the HOMO energies of the anions to estimate the stability versus oxidation, the anion volumes that correlate to the anion mobility, the lithium ion coordination and dynamics to reveal the limiting steps of lithium ion transport, etc.
Wu, Yao; Dai, Xiaodong; Huang, Niu; Zhao, Lifeng
2013-06-01
In force field parameter development using ab initio potential energy surfaces (PES) as target data, an important but often neglected matter is the lack of a weighting scheme with optimal discrimination power to fit the target data. Here, we developed a novel partition function-based weighting scheme, which not only fits the target potential energies exponentially like the general Boltzmann weighting method, but also reduces the effect of fitting errors leading to overfitting. The van der Waals (vdW) parameters of benzene and propane were reparameterized by using the new weighting scheme to fit the high-level ab initio PESs probed by a water molecule in global configurational space. The molecular simulation results indicate that the newly derived parameters are capable of reproducing experimental properties in a broader range of temperatures, which supports the partition function-based weighting scheme. Our simulation results also suggest that structural properties are more sensitive to vdW parameters than partial atomic charge parameters in these systems although the electrostatic interactions are still important in energetic properties. As no prerequisite conditions are required, the partition function-based weighting method may be applied in developing any types of force field parameters.
Structures of 13-atom clusters of fcc transition metals by ab initio and semiempirical calculations
NASA Astrophysics Data System (ADS)
Longo, R. C.; Gallego, L. J.
2006-11-01
We report the results of ab initio calculations of the structures and magnetic moments of Ni13 , Pd13 , Pt13 , Cu13 , Ag13 , and Au13 that were performed using a density-functional method that employs linear combinations of pseudoatomic orbitals as basis sets (SIESTA). Our structural results for Pt13 , Cu13 , Ag13 , and Au13 show that a buckled biplanar structure (BBP) is more stable than the icosahedral configuration, in keeping with results obtained recently by Chang and Chou [Phys. Rev. Lett. 93, 133401 (2004)] using the Vienna ab initio simulation package with a plane-wave basis. However, for Ni13 and Pd13 we found that the icosahedral structure is more stable than BBP. For all these clusters, two semiempirical methods based on spherically symmetric potentials both found the icosahedral structure to be the more stable, while the modified embedded atom model method, which uses a direction-dependent potential, found BBP to be the more stable structure. When low-energy structures found in recent ab initio studies of Pt13 , Cu13 , and Au13 other than Chang and Chou were optimized with SIESTA, those reported for Pt13 and Cu13 were found to be less stable than BBP, but the two-dimensional planar configuration reported for Au13 proved to be more stable than BBP.
Almeida, D; Kinzel, D; Ferreira da Silva, F; Puschnigg, B; Gschliesser, D; Scheier, P; Denifl, S; García, G; González, L; Limão-Vieira, P
2013-07-21
Electron transfer and dissociative electron attachment to 3-methyluracil (3meU) and 1-methylthymine (1meT) yielding anion formation have been investigated in atom-molecule collision and electron attachment experiments, respectively. The former has been studied in the collision energy range 14-100 eV whereas the latter in the 0-15 eV incident electron energy range. In the present studies, emphasis is given to the reaction channel resulting in the loss of the methyl group from the N-sites with the extra charge located on the pyrimidine ring. This particular reaction channel has neither been approached in the context of dissociative electron attachment nor in atom-molecule collisions yet. Quantum chemical calculations have been performed in order to provide some insight into the dissociation mechanism involved along the N-CH3 bond reaction coordinate. The calculations provide support to the threshold value derived from the electron transfer measurements, allowing for a better understanding of the role of the potassium cation as a stabilising agent in the collision complex. The present comparative study gives insight into the dynamics of the decaying transient anion and more precisely into the competition between dissociation and auto-detachment.
Almeida, D; Kinzel, D; Ferreira da Silva, F; Puschnigg, B; Gschliesser, D; Scheier, P; Denifl, S; García, G; González, L; Limão-Vieira, P
2013-07-21
Electron transfer and dissociative electron attachment to 3-methyluracil (3meU) and 1-methylthymine (1meT) yielding anion formation have been investigated in atom-molecule collision and electron attachment experiments, respectively. The former has been studied in the collision energy range 14-100 eV whereas the latter in the 0-15 eV incident electron energy range. In the present studies, emphasis is given to the reaction channel resulting in the loss of the methyl group from the N-sites with the extra charge located on the pyrimidine ring. This particular reaction channel has neither been approached in the context of dissociative electron attachment nor in atom-molecule collisions yet. Quantum chemical calculations have been performed in order to provide some insight into the dissociation mechanism involved along the N-CH3 bond reaction coordinate. The calculations provide support to the threshold value derived from the electron transfer measurements, allowing for a better understanding of the role of the potassium cation as a stabilising agent in the collision complex. The present comparative study gives insight into the dynamics of the decaying transient anion and more precisely into the competition between dissociation and auto-detachment. PMID:23743926
Ab initio calculations in three-body cluster systems
Romero-Redondo, C.; Navratil, P.; Quaglioni, S.
2013-06-10
In this work we briefly outline the extension of the ab initio no-core shell model/Resonating group method (NCSM/RGM) to three-body cluster states. We present the results for {sup 6}He ground state within a {sup 4}He+n+n cluster basis under this approach.
Ab Initio Electronic Structure Calculations of Cytochrome P450 -- Ligand Interactions
NASA Astrophysics Data System (ADS)
Segall, M. D.; Payne, M. C.; Ellis, S. W.; Tucker, G. T.
1997-03-01
The Cytochrome P450 superfamily of enzymes are of great interest in pharmacology as they participate in an enormous range of physiological processes including drug deactivation and xenobiotic detoxification. We apply ab initio electronic structure calculations to model the interactions of the haem molecule at the P450 active site with substrate and inhibitor ligands. These calculations, based on density function theory, were performed with the CETEP code which uses a plane wave basis set and pseudopotentials to perform efficient LDA, GGA and spin dependent calculations. A change in the spin state of the haem iron atom is observed on binding of a substrate molecule, consistent with the accepted reaction mechanism.
NASA Astrophysics Data System (ADS)
Anoufa, M.; Kiat, J. M.; Kornev, I.; Bogicevic, C.
2013-10-01
In this paper, we want to emphasize the fact that many experimental properties of ceramics can be explained by the existence of a core-shell structure of the grains, particularly at small sizes. In this framework, we have studied BaTiO3 (BT) ceramics constituted of core-shell nanoparticles, nanowires, or nanoplanes by using ab initio derived effective Hamiltonian calculations whose application range is for large values of shell thickness and low values of shell permittivity. Many differences and new features compared to the situation of nanodots are induced by the core-shell structure. For instance, phase sequences are different; there is also a coexistence of vortices found by Naumov, Bellaiche, and Fu [I. I. Naumov, L. Bellaiche, and H. Fu, Nature (London)10.1038/nature03107 432, 737 (2004)] in the case of isolated dots with a homogeneous polarization, a transition from cubic paraelectric phase towards nonpolar rhombohedral phase, anomalies in dielectric permittivity associated with the onset of toroidal moments, etc. Afterwards, we compare these results with those obtained by the Landau theory of core-shell ceramics we have recently published. However, the ab initio calculations fail to capture the physics at small shell thickness and/or high shell permittivity, whereas the Landau theory fails to predict the peculiar properties of the phases in which vortices exist. Therefore, in a tentative way to build a global theory, we have constructed a Landau potential using both the polarization and the toroidal moment as competing order parameters, which allows us to propose a phase diagram, whatever the thickness and permittivity of the shell are.
Ouk, Chanda-Malis; Zvereva-Loëte, Natalia; Scribano, Yohann; Bussery-Honvault, Béatrice
2012-10-30
Multireference single and double configuration interaction (MRCI) calculations including Davidson (+Q) or Pople (+P) corrections have been conducted in this work for the reactants, products, and extrema of the doublet ground state potential energy surface involved in the N((2)D) + CH(4) reaction. Such highly correlated ab initio calculations are then compared with previous PMP4, CCSD(T), W1, and DFT/B3LYP studies. Large relative differences are observed in particular for the transition state in the entrance channel resolving the disagreement between previous ab initio calculations. We confirm the existence of a small but positive potential barrier (3.86 ± 0.84 kJ mol(-1) (MR-AQCC) and 3.89 kJ mol(-1) (MRCI+P)) in the entrance channel of the title reaction. The correlation is seen to change significantly the energetic position of the two minima and five saddle points of this system together with the dissociation channels but not their relative order. The influence of the electronic correlation into the energetic of the system is clearly demonstrated by the thermal rate constant evaluation and it temperature dependance by means of the transition state theory. Indeed, only MRCI values are able to reproduce the experimental rate constant of the title reaction and its behavior with temperature. Similarly, product branching ratios, evaluated by means of unimolecular RRKM theory, confirm the NH production of Umemoto et al., whereas previous works based on less accurate ab initio calculations failed. We confirm the previous findings that the N((2)D) + CH(4) reaction proceeds via an insertion-dissociation mechanism and that the dominant product channels are CH(2)NH + H and CH(3) + NH.
Electronic states of Zn2 - Ab initio calculations of a prototype for Hg2
NASA Technical Reports Server (NTRS)
Hay, P. J.; Dunning, T. H., Jr.; Raffenetti, R. C.
1976-01-01
The electronic states of Zn2 are investigated by ab initio polarization configuration-interaction calculations. Molecular states dissociating to Zn(1S) + Zn(1S, 3P, 1P) and Zn(3P) + Zn(3P) are treated. Important effects from states arising from Zn(+)(25) + Zn(-)(2P) are found in the potential-energy curves and electronic-transition moments. A model calculation for Hg2 based on the Zn2 curves and including spin-orbit coupling leads to a new interpretation of the emission bands in Hg vapor.
Ab Initio Nuclear Structure and Reaction Calculations for Rare Isotopes
Draayer, Jerry P.
2014-09-28
We have developed a novel ab initio symmetry-adapted no-core shell model (SA-NCSM), which has opened the intermediate-mass region for ab initio investigations, thereby providing an opportunity for first-principle symmetry-guided applications to nuclear structure and reactions for nuclear isotopes from the lightest p-shell systems to intermediate-mass nuclei. This includes short-lived proton-rich nuclei on the path of X-ray burst nucleosynthesis and rare neutron-rich isotopes to be produced by the Facility for Rare Isotope Beams (FRIB). We have provided ab initio descriptions of high accuracy for low-lying (including collectivity-driven) states of isotopes of Li, He, Be, C, O, Ne, Mg, Al, and Si, and studied related strong- and weak-interaction driven reactions that are important, in astrophysics, for further understanding stellar evolution, X-ray bursts and triggering of s, p, and rp processes, and in applied physics, for electron and neutrino-nucleus scattering experiments as well as for fusion ignition at the National Ignition Facility (NIF).
Ab-initio study of napthelene based conducting polymer
Ruhela, Ankur; Kanchan, Reena; Srivastava, Anurag; Sinha, O. P.
2014-04-24
In this paper, we have identified structural and electronic properties of conducting polymers by using DFT based ATK-VNL ab-initio tool. Naphthalene derivative structures were stabilized by varying the bond length between two atoms of the molecule C-N and C-C. We have also studied the molecular energy spectrum of naphthalene derivatives and found the HOMOLUMO for the same. A comparison of structural and electronic properties of naphthalene derivatives by attaching the functional group of amine, have been performed and found that they show good semi conducting properties.
NASA Astrophysics Data System (ADS)
Liu, Hanchao; Wang, Yimin; Bowman, Joel M.
2015-05-01
The calculation and characterization of the IR spectrum of liquid water have remained a challenge for theory. In this paper, we address this challenge using a combination of ab initio approaches, namely, a quantum treatment of IR spectrum using the ab initio WHBB water potential energy surface and a refined ab initio dipole moment surface. The quantum treatment is based on the embedded local monomer method, in which the three intramolecular modes of each embedded H2O monomer are fully coupled and also coupled singly to each of six intermolecular modes. The new dipole moment surface consists of a previous spectroscopically accurate 1-body dipole moment surface and a newly fitted ab initio intrinsic 2-body dipole moment. A detailed analysis of the new dipole moment surface in terms of the coordinate dependence of the effective atomic charges is done along with tests of it for the water dimer and prism hexamer double-harmonic spectra against direct ab initio calculations. The liquid configurations are taken from previous molecular dynamics calculations of Skinner and co-workers, using the TIP4P plus E3B rigid monomer water potential. The IR spectrum of water at 300 K in the range of 0-4000 cm-1 is calculated and compared with experiment, using the ab initio WHBB potential and new ab initio dipole moment, the q-TIP4P/F potential, which has a fixed-charged description of the dipole moment, and the TTM3-F potential and dipole moment surfaces. The newly calculated ab initio spectrum is in very good agreement with experiment throughout the above spectral range, both in band positions and intensities. This contrasts to results with the other potentials and dipole moments, especially the fixed-charge q-TIP4P/F model, which gives unrealistic intensities. The calculated ab initio spectrum is analyzed by examining the contribution of various transitions to each band.
Liu, Hanchao; Wang, Yimin; Bowman, Joel M.
2015-05-21
The calculation and characterization of the IR spectrum of liquid water have remained a challenge for theory. In this paper, we address this challenge using a combination of ab initio approaches, namely, a quantum treatment of IR spectrum using the ab initio WHBB water potential energy surface and a refined ab initio dipole moment surface. The quantum treatment is based on the embedded local monomer method, in which the three intramolecular modes of each embedded H{sub 2}O monomer are fully coupled and also coupled singly to each of six intermolecular modes. The new dipole moment surface consists of a previous spectroscopically accurate 1-body dipole moment surface and a newly fitted ab initio intrinsic 2-body dipole moment. A detailed analysis of the new dipole moment surface in terms of the coordinate dependence of the effective atomic charges is done along with tests of it for the water dimer and prism hexamer double-harmonic spectra against direct ab initio calculations. The liquid configurations are taken from previous molecular dynamics calculations of Skinner and co-workers, using the TIP4P plus E3B rigid monomer water potential. The IR spectrum of water at 300 K in the range of 0–4000 cm{sup −1} is calculated and compared with experiment, using the ab initio WHBB potential and new ab initio dipole moment, the q-TIP4P/F potential, which has a fixed-charged description of the dipole moment, and the TTM3-F potential and dipole moment surfaces. The newly calculated ab initio spectrum is in very good agreement with experiment throughout the above spectral range, both in band positions and intensities. This contrasts to results with the other potentials and dipole moments, especially the fixed-charge q-TIP4P/F model, which gives unrealistic intensities. The calculated ab initio spectrum is analyzed by examining the contribution of various transitions to each band.
Ab Initio Calculations of the Interaction between CO _{2} and the Acetate Ion
Steckel, Janice A.
2012-11-29
A series of ab initio calculations designed to investigate the interaction of CO{sub 2} with acetate are presented. The lowest energy structure, AC–CO{sub 2}-η{sup 2}, is predicted by CCSD(T)/aVTZ to be bound by -10.6 kcal/mol. Six of the bound complexes have binding energies on the order of -8 kcal/mol, but analysis shows that the η{sup 1}-CT complex is fundamentally different from the others. The η{sup 1}-CT complex is characterized by geometric distortion, large polarization and induction effects and charge transfer whereas the other five complexes have little geometric distortion and negligible charge transfer. The amount of charge that is transferred from the anion to the CO{sub 2} in the η{sup 1}-CT complex is estimated to be about half an electron by NPA, DMA, CHELPG, and Mulliken analyses, whereas the EDA-ALMO-CTA (B3LYP) approach predicts a charge transfer of 75 me{sup –}. However, the transfer of this small amount of charge leads to an energy lowering of -56 kcal/mol, without which the complex would not be bound. The RI-MP2 geometries closely approximate those resulting from the CCSD optimizations, and the optimized second-order opposite spin (O2) method performs well for all the complexes except for the η{sup 1}-CT complex. DFT methods do not reproduce all the ab initio geometries, binding energies and/or energy ordering of these complexes although the range-separated hybrid meta-GGA (M11) and nonlocal (VV10 and vdwDF10) functionals are shown to yield results significantly better than other functionals considered for this system. The fact that there is such variation among DFT methods has implications for DFT-based ab initio molecular dynamics simulations and for the parametrization of classical force fields based on DFT calculations.
NASA Astrophysics Data System (ADS)
Jung, Jeil; Leconte, Nicolas; Lebegue, Sebastien; Gould, Timothy
Stacking-dependent interlayer interactions are important for understanding the structural and electronic properties in incommensurable two dimensional material assemblies where long-range moiré patterns arise due to small lattice constant mismatch or twist angles. We study the stacking-dependent interlayer coupling energies between graphene (G) and hexagonal boron nitride (BN) single layers for different possible combinations such as G/G, G/BN and BN/BN using high-level EXX+RPA ab initio calculations. The total energies differ substantially when compared with conventional LDA, but for stacking-dependent total energy differences we find that the dominance of short-range covalent-type binding over the longer-ranged van der Waals tails near equilibrium geometries renders the LDA as a reasonable starting point for ab initio calculation based analyses for the systems we have studied. Our calculations are useful input for study of strains originated by interlayer interactions in incommensurable 2D van der Waals crystals.
Muller, R P; Warshel, A
1996-01-01
This paper describes a hybrid ab initio quantum mechanical/molecular mechanics (QM/MM) method for calculating activation free energies of chemical reactions in solution, using molecular mechanics force fields for the solvent and an ab initio technique that incorporates the potential from the solvent in its Hamiltonian for the solute. The empirical valence bond (EVB) method is used as a reference potential for the ab initio free energy calculation, and drives the reaction along the proper coordinate, thus overcoming problems encountered by direct attempts to use molecular orbital methods in calculations of activation free energies. The utility of our method is illustrated by calculating the activation free energy for proton transfer between fluoride ions in the [FHF]-system, in both polar and nonpolar solution.
Ab Initio Force Fields for Imidazolium-Based Ionic Liquids.
McDaniel, Jesse G; Choi, Eunsong; Son, Chang Yun; Schmidt, J R; Yethiraj, Arun
2016-07-21
We develop ab initio force fields for alkylimidazolium-based ionic liquids (ILs) that predict the density, heats of vaporization, diffusion, and conductivity that are in semiquantitative agreement with experimental data. These predictions are useful in light of the scarcity of and sometimes inconsistency in experimental heats of vaporization and diffusion coefficients. We illuminate physical trends in the liquid cohesive energy with cation chain length and anion. These trends are different than those based on the experimental heats of vaporization. Molecular dynamics prediction of the room temperature dynamics of such ILs is more difficult than is generally realized in the literature due to large statistical uncertainties and sensitivity to subtle force field details. We believe that our developed force fields will be useful for correctly determining the physics responsible for the structure/property relationships in neat ILs.
Density-matrix based determination of low-energy model Hamiltonians from ab initio wavefunctions
Changlani, Hitesh J.; Zheng, Huihuo; Wagner, Lucas K.
2015-09-14
We propose a way of obtaining effective low energy Hubbard-like model Hamiltonians from ab initio quantum Monte Carlo calculations for molecular and extended systems. The Hamiltonian parameters are fit to best match the ab initio two-body density matrices and energies of the ground and excited states, and thus we refer to the method as ab initio density matrix based downfolding. For benzene (a finite system), we find good agreement with experimentally available energy gaps without using any experimental inputs. For graphene, a two dimensional solid (extended system) with periodic boundary conditions, we find the effective on-site Hubbard U{sup ∗}/t to be 1.3 ± 0.2, comparable to a recent estimate based on the constrained random phase approximation. For molecules, such parameterizations enable calculation of excited states that are usually not accessible within ground state approaches. For solids, the effective Hamiltonian enables large-scale calculations using techniques designed for lattice models.
Density-matrix based determination of low-energy model Hamiltonians from ab initio wavefunctions.
Changlani, Hitesh J; Zheng, Huihuo; Wagner, Lucas K
2015-09-14
We propose a way of obtaining effective low energy Hubbard-like model Hamiltonians from ab initio quantum Monte Carlo calculations for molecular and extended systems. The Hamiltonian parameters are fit to best match the ab initio two-body density matrices and energies of the ground and excited states, and thus we refer to the method as ab initio density matrix based downfolding. For benzene (a finite system), we find good agreement with experimentally available energy gaps without using any experimental inputs. For graphene, a two dimensional solid (extended system) with periodic boundary conditions, we find the effective on-site Hubbard U(∗)/t to be 1.3 ± 0.2, comparable to a recent estimate based on the constrained random phase approximation. For molecules, such parameterizations enable calculation of excited states that are usually not accessible within ground state approaches. For solids, the effective Hamiltonian enables large-scale calculations using techniques designed for lattice models. PMID:26374007
Symmetry-Adapted Ab Initio Shell Model for Nuclear Structure Calculations
NASA Astrophysics Data System (ADS)
Draayer, J. P.; Dytrych, T.; Launey, K. D.; Langr, D.
2012-05-01
An innovative concept, the symmetry-adapted ab initio shell model, that capitalizes on partial as well as exact symmetries that underpin the structure of nuclei, is discussed. This framework is expected to inform the leading features of nuclear structure and reaction data for light and medium mass nuclei, which are currently inaccessible by theory and experiment and for which predictions of modern phenomenological models often diverge. We use powerful computational and group-theoretical algorithms to perform ab initio CI (configuration-interaction) calculations in a model space spanned by SU(3) symmetry-adapted many-body configurations with the JISP16 nucleon-nucleon interaction. We demonstrate that the results for the ground states of light nuclei up through A = 16 exhibit a strong dominance of low-spin and high-deformation configurations together with an evident symplectic structure. This, in turn, points to the importance of using a symmetry-adapted framework, one based on an LS coupling scheme with the associated spatial configurations organized according to deformation.
Ab initio quantum Monte Carlo calculations of ground-state properties of manganese's oxides
NASA Astrophysics Data System (ADS)
Sharma, Vinit; Krogel, Jaron T.; Kent, P. R. C.; Reboredo, Fernando A.
One of the critical scientific challenges of contemporary research is to obtain an accurate theoretical description of the electronic properties of strongly correlated systems such as transition metal oxides and rare-earth compounds, since state-of-art ab-initio methods based on approximate density functionals are not always sufficiently accurate. Quantum Monte Carlo (QMC) methods, which use statistical sampling to evaluate many-body wave functions, have the potential to answer this challenge. Owing to the few fundamental approximations made and the direct treatment of electron correlation, QMC methods are among the most accurate electronic structure methods available to date. We assess the accuracy of the diffusion Monte Carlo method in the case of rocksalt manganese oxide (MnO). We study the electronic properties of this strongly-correlated oxide, which has been identified as a suitable candidate for many applications ranging from catalysts to electronic devices. ``This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.'' Ab initio quantum Monte Carlo calculations of ground-state properties of manganese's oxides.
Heats of Segregation of BCC Binaries from Ab Initio and Quantum Approximate Calculations
NASA Technical Reports Server (NTRS)
Good, Brian S.
2003-01-01
We compare dilute-limit segregation energies for selected BCC transition metal binaries computed using ab initio and quantum approximate energy methods. Ab initio calculations are carried out using the CASTEP plane-wave pseudopotential computer code, while quantum approximate results are computed using the Bozzolo-Ferrante-Smith (BFS) method with the most recent parameters. Quantum approximate segregation energies are computed with and without atomistic relaxation. Results are discussed within the context of segregation models driven by strain and bond-breaking effects. We compare our results with full-potential quantum calculations and with available experimental results.
A-dependence of the Spectra of the F Isotopes from ab initio Calculations
NASA Astrophysics Data System (ADS)
Barrett, Bruce R.; Dikmen, Erdal; Maris, Pieter; Vary, James P.; Shirokov, Andrey M.
2016-03-01
Using a succession of Okubo-Lee-Suzuki transformations within the No Core Shell Model (NCSM) formalism, we derive an ab initio, non-perturbative procedure for calculating the input for standard shell-model (SSM) calculations within one major shell. We have used this approach for calculating the spectra of the F isotopes from A=18 to A=25, so as to study the A-dependence of the results. In particular, we are interested in seeing if the theoretical input is weak enough, so that a single set of two-body effective interactions can be used for all of the F isotopes investigated. We will present results from SSM calculations based on input obtained with the JISP16 nucleon-nucleon interaction in an initial 4 ℏΩ NCSM basis space. This work supported in part by TUBITAK-BIDEB, the US DOE, the US NSF, NERSC, and the Russian Ministry of Education and Science.
Ab initio molecular dynamics calculations of ion hydration free energies
Leung, Kevin; Rempe, Susan B.; Lilienfeld, O. Anatole von
2009-05-28
We apply ab initio molecular dynamics (AIMD) methods in conjunction with the thermodynamic integration or '{lambda}-path' technique to compute the intrinsic hydration free energies of Li{sup +}, Cl{sup -}, and Ag{sup +} ions. Using the Perdew-Burke-Ernzerhof functional, adapting methods developed for classical force field applications, and with consistent assumptions about surface potential ({phi}) contributions, we obtain absolute AIMD hydration free energies ({Delta}G{sub hyd}) within a few kcal/mol, or better than 4%, of Tissandier et al.'s [J. Phys. Chem. A 102, 7787 (1998)] experimental values augmented with the SPC/E water model {phi} predictions. The sums of Li{sup +}/Cl{sup -} and Ag{sup +}/Cl{sup -} AIMD {Delta}G{sub hyd}, which are not affected by surface potentials, are within 2.6% and 1.2 % of experimental values, respectively. We also report the free energy changes associated with the transition metal ion redox reaction Ag{sup +}+Ni{sup +}{yields}Ag+Ni{sup 2+} in water. The predictions for this reaction suggest that existing estimates of {Delta}G{sub hyd} for unstable radiolysis intermediates such as Ni{sup +} may need to be extensively revised.
NASA Astrophysics Data System (ADS)
Panikar, Savitha S.; Guirgis, Gamil A.; Sheehan, Tracie G.; Durig, Douglas T.; Durig, James R.
2012-05-01
The infrared spectra of gaseous and solid N-bromo-hexafluoro-2-propanimine, (CF3)2Cdbnd NBr, have been obtained from 2000 to 50 cm-1. The vibrational assignment for the normal modes is proposed based on infrared band contours, group frequencies and normal coordinate calculations utilizing Cs symmetry. The structural parameters have been obtained from ab initio MP2(full)/6-311 + G(d,p) calculations employing the Gaussian-03 program. Additionally, the frequencies and potential energy distributions for the normal modes have been calculated with the MP2(full)/6-31G(d). All of these results are compared to the corresponding data for some similar molecules.
NASA Astrophysics Data System (ADS)
Katayama-Yoshida, Hiroshi; Nakanishi, Akitaka; Uede, Hiroki; Takawashi, Yuki; Fukushima, Tetsuya; Sato, Kazunori
2014-03-01
Based upon ab initio electronic structure calculation, I will discuss the general rule of negative effective U system by (1) exchange-correlation-induced negative effective U caused by the stability of the exchange-correlation energy in Hund's rule with high-spin ground states of d5 configuration, and (2) charge-excitation-induced negative effective U caused by the stability of chemical bond in the closed-shell of s2, p6, and d10 configurations. I will show the calculated results of negative effective U systems such as hole-doped CuAlO2 and CuFeS2. Based on the total energy calculations of antiferromagnetic and ferromagnetic states, I will discuss the magnetic phase diagram and superconductivity upon hole doping. I also discuss the computational materials design method of high-Tc superconductors by ab initio calculation to go beyond LDA and multi-scale simulations.
Bulk Modulus of Sc2O3: Ab initio Calculations and Experimental Results
S Barzilai; I Halvey; O Yeheskel
2011-12-31
The bulk modulus of scandia is evaluated by ab initio calculation, based on density functional theory, and compared with bulk modulus measurement on nearly fully dense scandia and with the value attained from the equation of state based on diamond anvil cell measurements. The current results are in the upper range of the bulk moduli results in the literature. The scatter in the literature results might be explained by the differences in the specific volumes of the measured and calculated cases. For the specific volume of 59.65{+-}0.07 {angstrom}{sup 3} the average measured isothermal bulk modulus of scandia from the present study and recent literature results is 188{+-}10 GPa.
NASA Astrophysics Data System (ADS)
Maes, G.; Smets, J.; Adamowicz, L.; McCarthy, W.; Van Bael, M. K.; Houben, L.; Schoone, K.
1997-06-01
Correlations between selected ab initio predicted and experimentally observed properties of 1:1 H-bonded complexes of pyridines, pyrimidines, and imidazoles with water are investigated. Relationships are found between the experimental properties of proton affinity and water frequency shift, and the ab initio calculated bond distances, interaction energies and water frequency shifts. It is also found that well-defined relations can be established between calculated and observed properties for the pyridine complexes, but these cannot be reliably extended to the other N-base systems. The similarities demonstrate that the presently available ab initio methods are useful in predicting the experimental behaviour of H-bonded systems, but only for closely related molecules.
High-pressure elastic properties of cubic Ir2P from ab initio calculations
NASA Astrophysics Data System (ADS)
Sun, Xiao-Wei; Bioud, Nadhira; Fu, Zhi-Jian; Wei, Xiao-Ping; Song, Ting; Li, Zheng-Wei
2016-10-01
A study of the high-pressure elastic properties of new synthetic Ir2P in the anti-fluorite structure is conducted using ab initio calculations based on density functional theory. The elastic constants C11, C12 and C44 for the cubic Ir2P are obtained by the stress-strain method and the elastic stability calculations under pressure indicate that it is stable at least 100 GPa. Additionally, the electronic density of states, the aggregate elastic moduli, that is bulk modulus, shear modulus, and Young's modulus along with the Debye temperature, Poisson's ratio, and elastic anisotropy factor are all successfully obtained. Moreover, the pressure dependence of the longitudinal and shear wave velocities in three different directions [100], [110], and [111] for Ir2P are also predicted for the first time.
The Use of Ab Initio Wavefunctions in Line-Shape Calculations for Water Vapor
NASA Astrophysics Data System (ADS)
Gamache, Robert R.; Lamouroux, Julien; Schwenke, David W.
2014-06-01
In semi-classical line-shape calculations, the internal motions of the colliding pair are treated via quantum mechanics and the collision trajectory is determined by classical dynamics. The quantum mechanical component, i.e. the determination of reduced matrix elements (RME) for the colliding pair, requires the wavefunctions of the radiating and the perturbing molecules be known. Here the reduced matrix elements for collisions in the ground vibrational state of water vapor are calculated by two methods and compared. First, wavefunctions determined by diagonalizing an effective (Watson) Hamiltonian are used to calculate the RMEs and, second, the ab initio wavefunctions of Partridge and Schwenke are used. While the ground vibrational state will yield the best approximation of the wavefunctions from the effective Hamiltonian approach, this study clearly identifies problems for states not included in the fit of the Hamiltonian and for extrapolated states. RMEs determined using ab initio wavefunctions use ˜100000 times more computational time; however, all ro-vibrational interactions are included. Hence, the ab initio approach will yield better RMEs as the number of vibrational quanta exchanged in the optical transition increases, resulting in improvements in calculated half-widths and line shifts. It is important to note that even for pure rotational transitions the use of ab initio wavefunctions will yield improved results.
NASA Astrophysics Data System (ADS)
Chibotaru, L. F.; Ungur, L.
2012-08-01
A methodology for the rigorous nonperturbative derivation of magnetic pseudospin Hamiltonians of mononuclear complexes and fragments based on ab initio calculations of their electronic structure is described. It is supposed that the spin-orbit coupling and other relativistic effects are already taken fully into account at the stage of quantum chemistry calculations of complexes. The methodology is based on the establishment of the correspondence between the ab initio wave functions of the chosen manifold of multielectronic states and the pseudospin eigenfunctions, which allows to define the pseudospin Hamiltonians in the unique way. Working expressions are derived for the pseudospin Zeeman and zero-field splitting Hamiltonian corresponding to arbitrary pseudospins. The proposed calculation methodology, already implemented in the SINGLE_ANISO module of the MOLCAS-7.6 quantum chemistry package, is applied for a first-principles evaluation of pseudospin Hamiltonians of several complexes exhibiting weak, moderate, and very strong spin-orbit coupling effects.
NASA Astrophysics Data System (ADS)
Borges, P. D.; Scolfaro, L.
2014-12-01
The thermoelectric properties of indium nitride in the most stable wurtzite phase (w-InN) as a function of electron and hole concentrations and temperature were studied by solving the semiclassical Boltzmann transport equations in conjunction with ab initio electronic structure calculations, within Density Functional Theory. Based on maximally localized Wannier function basis set and the ab initio band energies, results for the Seebeck coefficient are presented and compared with available experimental data for n-type as well as p-type systems. Also, theoretical results for electric conductivity and power factor are presented. Most cases showed good agreement between the calculated properties and experimental data for w-InN unintentionally and p-type doped with magnesium. Our predictions for temperature and concentration dependences of electrical conductivity and power factor revealed a promising use of InN for intermediate and high temperature thermoelectric applications. The rigid band approach and constant scattering time approximation were utilized in the calculations.
Borges, P. D. E-mail: lscolfaro@txstate.edu; Scolfaro, L. E-mail: lscolfaro@txstate.edu
2014-12-14
The thermoelectric properties of indium nitride in the most stable wurtzite phase (w-InN) as a function of electron and hole concentrations and temperature were studied by solving the semiclassical Boltzmann transport equations in conjunction with ab initio electronic structure calculations, within Density Functional Theory. Based on maximally localized Wannier function basis set and the ab initio band energies, results for the Seebeck coefficient are presented and compared with available experimental data for n-type as well as p-type systems. Also, theoretical results for electric conductivity and power factor are presented. Most cases showed good agreement between the calculated properties and experimental data for w-InN unintentionally and p-type doped with magnesium. Our predictions for temperature and concentration dependences of electrical conductivity and power factor revealed a promising use of InN for intermediate and high temperature thermoelectric applications. The rigid band approach and constant scattering time approximation were utilized in the calculations.
Ab initio Calculations of Solvation Processes in Volcanic Gases
NASA Astrophysics Data System (ADS)
Lemke, K.; Seward, T.
2006-12-01
The structures and thermochemical properties of hydrated ions and neutral molecules play an important role in our understanding of solvent clustering and hydrogen bonding in the gas phase. Considerable effort therefore has been devoted to both the experimental and theoretical determination of stepwise hydration energies of geochemically important ions and neutral molecules with solvents, for instance H2O or H2S, over a broad range of temperatures typical of those encountered in volcanic gases. Because volcanic gases contain mutiple solute and solvent components which are subject to proton transfer, competive solvation and solvent switching, characterizing individual clusters has been a fundamental challenge to a molecular-level understanding of high temperature gas-phase solvation. However, recent advances in computational chemistry methods, especially Pople´s Gaussian (G-n) and complete basis set limit (CBS-x) model chemistries, now allow characterization of the dominant cluster structures and thermochemical properties of solute-solvent and solvent-solvent interactions in high temperature volcanic gases. Building on reported measurements of volcanic gases at Vesuvio, Italy, and Showa-Shinzan, Japan, as well as our recent investigations of ion-hydration we have re-examined the high temperature clustering equilibria of the small hydronium (H3O+) and ammonium (NH4+) ions as well as neutral ammonia and sulphur species with H2O and/or H2S using ab initio quantum chemical methods. From our study, we find that most of the gas phase ions tend to associate with a small number of H2O and H2S molecules to yield a hydrated ion cluster even at low humidities. Furthermore, inspection of van´t Hoff data demonstrate that (1) hydration energies of ions are shifted to less exergonic values as the solvent shell grows and the composition shifts from water-rich to hydrogen sulphide rich, (2) ion-cluster size increases with decreasing temperature at constant humidity, (3) attachment
Assessing the elastic properties and ductility of Fe-Cr-Al alloys from ab initio calculations
NASA Astrophysics Data System (ADS)
Nurmi, E.; Wang, G.; Kokko, K.; Vitos, L.
2016-01-01
Fe-Al is one of the best corrosion resistant alloys at high temperatures. The flip side of Al addition to Fe is the deterioration of the mechanical properties. This problem can be solved by adding a suitable amount of third alloying component. In the present work, we use ab initio calculations based on density functional theory to study the elastic properties of Fe?Cr?Al? alloys for Al and Cr contents up to 20 at.%. We assess the ductility as a function of chemistry by making use of the semi-empirical correlations between the elastic parameters and mechanical properties. In particular, we derive the bulk modulus to shear modulus ratio and the Cauchy pressure and monitor their trends in terms of chemical composition. The present findings are contrasted with the previously established oxidation resistance of Fe-Cr-Al alloys.
Barker, John R; Nguyen, Thanh Lam; Stanton, John F
2012-06-21
Calculations were carried out for 25 isotopologues of the title reaction for various combinations of (35)Cl, (37)Cl, (12)C, (13)C, (14)C, H, and D. The computed rate constants are based on harmonic vibrational frequencies calculated at the CCSD(T)/aug-cc-pVTZ level of theory and X(ij) vibrational anharmonicity coefficients calculated at the CCSD(T) /aug-cc-pVDZ level of theory. For some reactions, anharmonicity coefficients were also computed at the CCSD(T)/aug-cc-pVTZ level of theory. The classical reaction barrier was taken from Eskola et al. [J. Phys. Chem. A 2008, 112, 7391-7401], who extrapolated CCSD(T) calculations to the complete basis set limit. Rate constants were calculated for temperatures from ∼100 to ∼2000 K. The computed ab initio rate constant for the normal isotopologue is in good agreement with experiments over the entire temperature range (∼10% lower than the recommended experimental value at 298 K). The ab initio H/D kinetic isotope effects (KIEs) for CH(3)D, CH(2)D(2), CHD(3), and CD(4) are in very good agreement with literature experimental data. The ab initio (12)C/(13)C KIE is in error by ∼2% at 298 K for calculations using X(ij) coefficients computed with the aug-cc-pVDZ basis set, but the error is reduced to ∼1% when X(ij) coefficients computed with the larger aug-cc-pVTZ basis set are used. Systematic improvements appear to be possible. The present SCTST results are found to be more accurate than those from other theoretical calculations. Overall, this is a very promising method for computing ab initio kinetic isotope effects.
An Ab Initio Based Potential Energy Surface for Water
NASA Technical Reports Server (NTRS)
Partridge, Harry; Schwenke, David W.; Langhoff, Stephen R. (Technical Monitor)
1996-01-01
We report a new determination of the water potential energy surface. A high quality ab initio potential energy surface (PES) and dipole moment function of water have been computed. This PES is empirically adjusted to improve the agreement between the computed line positions and those from the HITRAN 92 data base. The adjustment is small, nonetheless including an estimate of core (oxygen 1s) electron correlation greatly improves the agreement with experiment. Of the 27,245 assigned transitions in the HITRAN 92 data base for H2(O-16), the overall root mean square (rms) deviation between the computed and observed line positions is 0.125/cm. However the deviations do not correspond to a normal distribution: 69% of the lines have errors less than 0.05/cm. Overall, the agreement between the line intensities computed in the present work and those contained in the data base is quite good, however there are a significant number of line strengths which differ greatly.
Neves, Amanda P; Vargas, Maria D; Téllez Soto, Claudio A; Ramos, Joanna M; Visentin, Lorenzo do C; Pinheiro, Carlos B; Mangrich, Antônio S; de Rezende, Edivaltrys I P
2012-08-01
Zinc(II) and copper(II) complexes of a tridentate Mannich base L1 derived from 2-hydroxy-1,4-naphthoquinone, pyridinecarboxyaldehyde and 2-aminomethylpyridine, [ZnL1Cl(2)]·H(2)O 1 and [CuL1Cl(2)]·2H(2)O 2, have been synthesized and fully characterized. The structure of complex 1 has been elucidated by a single crystal X-ray diffraction study: the zinc atom is pentacoordinate and the coordination geometry is a distorted square base pyramid, with a geometric structural parameter τ equal to 0.149. Vibrational spectroscopy and ab initio DFT calculations of both compounds have confirmed that the two complexes exhibit similar structures. Full assignment of the vibrational spectra was also supported by careful analysis of the distorted geometries generated by the normal modes. PMID:22513170
Ab initio based force field and molecular dynamics simulations of crystalline TATB.
Gee, Richard H; Roszak, Szczepan; Balasubramanian, Krishnan; Fried, Laurence E
2004-04-15
An all-atom force field for 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is presented. The classical intermolecular interaction potential for TATB is based on single-point energies determined from high-level ab initio calculations of TATB dimers. The newly developed potential function is used to examine bulk crystalline TATB via molecular dynamics simulations. The isobaric thermal expansion and isothermal compression under hydrostatic pressures obtained from the molecular dynamics simulations are in good agreement with experiment. The calculated volume-temperature expansion is almost one dimensional along the c crystallographic axis, whereas under compression, all three unit cell axes participate, albeit unequally. PMID:15267608
On limits of ab initio calculations of pairing gap in nuclei
Saperstein, E. E.; Baldo, M.; Lombardo, U.; Pankratov, S. S.; Zverev, M. V.
2011-11-15
A brief review of recent microscopic calculations of nuclear pairing gap is given. A semi-microscopic model is suggested in which the ab initio effective pairing interaction is supplemented with a small phenomenological addendum. It involves a parameter which is universal for all medium and heavy nuclei. Calculations for several isotopic and isotonic chains of semi-magic nuclei confirm the relevance of the model.
Ab initio calculation of relative ion concentrations of protonated water clusters at equilibrium
NASA Astrophysics Data System (ADS)
Lee, E. P. F.; Dyke, J. M.; Wilders, A. E.; Watts, P.
Relative concentrations of protonated water clusters, H(H2O)+n, are determined for the equilibria H(H2O)+n-1 + H2O ⇌ H(H2O)+n (for n = 1, …, 5), by ab initio molecular-orbital calculations (at the MP2/6-31G* level), using standard thermodynamic and statistical-mechanical methods. The calculated relative cluster-ion concentrations, at water concentrations of between 1 and 90 ppm at 308 K, are compared with the corresponding relative ion intensities measured with an ion-mobility mass spectrometer. The comparison shows that the observed cluster-ion intensity distributions agree well with those found from ab initio calculations for an equilibrium mixture of protonated water clusters.
Ab initio calculations on twisted graphene/hBN: Electronic structure and STM image simulation
NASA Astrophysics Data System (ADS)
Correa, J. D.; Cisternas, E.
2016-09-01
By performing ab initio calculations we obtained theoretical scanning tunneling microscopy (STM) images and studied the electronic properties of graphene on a hexagonal boron-nitrite (hBN) layer. Three different stack configurations and four twisted angles were considered. All calculations were performed using density functional theory, including van der Waals interactions as implemented in the SIESTA ab initio package. Our results show that the electronic structure of graphene is preserved, although some small changes are induced by the interaction with the hBN layer, particularly in the total density of states at 1.5 eV under the Fermi level. When layers present a twisted angle, the density of states shows several van Hove singularities under the Fermi level, which are associated to moiré patterns observed in theoretical STM images.
Like-charge guanidinium pairing from molecular dynamics and ab initio calculations.
Vazdar, Mario; Vymětal, Jiři; Heyda, Jan; Vondrášek, Jiři; Jungwirth, Pavel
2011-10-20
Pairing of guanidinium moieties in water is explored by molecular dynamics simulations of short arginine-rich peptides and ab initio calculations of a pair of guanidinium ions in water clusters of increasing size. Molecular dynamics simulations show that, in an aqueous environment, the diarginine guanidinium like-charged ion pairing is sterically hindered, whereas in the Arg-Ala-Arg tripeptide, this pairing is significant. This result is supported by the survey of protein structure databases, where it is found that stacked arginine pairs in dipeptide fragments exist solely as being imposed by the protein structure. In contrast, when two arginines are separated by a single amino acid, their guanidinium groups can freely approach each other and they frequently form stacked pairs. Molecular dynamics simulations results are also supported by ab initio calculations, which show stabilization of stacked guanidinium pairs in sufficiently large water clusters.
NASA Astrophysics Data System (ADS)
Kon, Masao; Kurokawa, Hideki; Takeuchi, Hiroshi; Konaka, Shigehiro
1992-04-01
Conformational properties of methyl nicotinate and methyl isonicotinate have been studied by liquid crystal 1H-NMR spectroscopy combined with ab initio calculations. The solvent used is a mixture of 80 mol.% of EBBA and 20 mol.% of MBBA.Ab initio calculations have been performed with 4-21G and MINI-4 basis sets to estimate molecular structures and the potential functions for internal rotation. Some structural parameters and the energy difference between rotational isomers have been refined by using observed dipolar coupling constants. The correlation between internal rotation and reorientational molecular motion has been taken into account according to the theory of Emsley, Luckhurst and Stockley. The parameters of the mean external potential are found to take similar values for methyl nicotinate and methyl isonicotinate. The energy difference of the two stable conformers of methyl nicotinate is in agreement with the analysis neglecting the correlation between the two motions.
Ab Initio Many-Body Calculations Of Nucleon-Nucleus Scattering
Quaglioni, S; Navratil, P
2008-12-17
We develop a new ab initio many-body approach capable of describing simultaneously both bound and scattering states in light nuclei, by combining the resonating-group method with the use of realistic interactions, and a microscopic and consistent description of the nucleon clusters. This approach preserves translational symmetry and Pauli principle. We outline technical details and present phase shift results for neutron scattering on {sup 3}H, {sup 4}He and {sup 10}Be and proton scattering on {sup 3,4}He, using realistic nucleon-nucleon (NN) potentials. Our A = 4 scattering results are compared to earlier ab initio calculations. We find that the CD-Bonn NN potential in particular provides an excellent description of nucleon-{sup 4}He S-wave phase shifts. We demonstrate that a proper treatment of the coupling to the n-{sup 10}Be continuum is successful in explaining the parity-inverted ground state in {sup 11}Be.
Ab initio calculation of valley splitting in monolayer δ-doped phosphorus in silicon
2013-01-01
The differences in energy between electronic bands due to valley splitting are of paramount importance in interpreting transport spectroscopy experiments on state-of-the-art quantum devices defined by scanning tunnelling microscope lithography. Using vasp, we develop a plane-wave density functional theory description of systems which is size limited due to computational tractability. Nonetheless, we provide valuable data for the benchmarking of empirical modelling techniques more capable of extending this discussion to confined disordered systems or actual devices. We then develop a less resource-intensive alternative via localised basis functions in siesta, retaining the physics of the plane-wave description, and extend this model beyond the capability of plane-wave methods to determine the ab initio valley splitting of well-isolated δ-layers. In obtaining an agreement between plane-wave and localised methods, we show that valley splitting has been overestimated in previous ab initio calculations by more than 50%. PMID:23445785
Ab Initio No-Core Shell Model Calculations Using Realistic Two- and Three-Body Interactions
Navratil, P; Ormand, W E; Forssen, C; Caurier, E
2004-11-30
There has been significant progress in the ab initio approaches to the structure of light nuclei. One such method is the ab initio no-core shell model (NCSM). Starting from realistic two- and three-nucleon interactions this method can predict low-lying levels in p-shell nuclei. In this contribution, we present a brief overview of the NCSM with examples of recent applications. We highlight our study of the parity inversion in {sup 11}Be, for which calculations were performed in basis spaces up to 9{Dirac_h}{Omega} (dimensions reaching 7 x 10{sup 8}). We also present our latest results for the p-shell nuclei using the Tucson-Melbourne TM three-nucleon interaction with several proposed parameter sets.
Ab initio calculation of valley splitting in monolayer δ-doped phosphorus in silicon.
Drumm, Daniel W; Budi, Akin; Per, Manolo C; Russo, Salvy P; L Hollenberg, Lloyd C
2013-02-27
: The differences in energy between electronic bands due to valley splitting are of paramount importance in interpreting transport spectroscopy experiments on state-of-the-art quantum devices defined by scanning tunnelling microscope lithography. Using vasp, we develop a plane-wave density functional theory description of systems which is size limited due to computational tractability. Nonetheless, we provide valuable data for the benchmarking of empirical modelling techniques more capable of extending this discussion to confined disordered systems or actual devices. We then develop a less resource-intensive alternative via localised basis functions in siesta, retaining the physics of the plane-wave description, and extend this model beyond the capability of plane-wave methods to determine the ab initio valley splitting of well-isolated δ-layers. In obtaining an agreement between plane-wave and localised methods, we show that valley splitting has been overestimated in previous ab initio calculations by more than 50%.
Ab initio calculations of the photoionization of diatomic molecules
NASA Astrophysics Data System (ADS)
Lefebvre-Brion, Helene; Raşeev, Georges
2003-01-01
A review is presented of the calculation of photoionization spectra, particularly in the spectral range where electron autoionization of diatomic molecules takes place. In addition to some interesting results obtained over years that compare favourably with experiment, the emphasis here is put on the relation between the methods developed for the calculation of observables associated with the continuum energy spectrum of the electrons and the Alchemy system of programs. This system of programs serves as a basis for initial and intermediate calculations. The examples presented show that diatomic molecules not only in gas phase but also oriented in space or physisorbed at surfaces may be studied readily.
Electron transport in extended carbon-nanotube/metal contacts: Ab initio based Green function method
NASA Astrophysics Data System (ADS)
Fediai, Artem; Ryndyk, Dmitry A.; Cuniberti, Gianaurelio
2015-04-01
We have developed a new method that is able to predict the electrical properties of the source and drain contacts in realistic carbon nanotube field effect transistors (CNTFETs). It is based on large-scale ab initio calculations combined with a Green function approach. For the first time, both internal and external parts of a realistic CNT-metal contact are taken into account at the ab initio level. We have developed the procedure allowing direct calculation of the self-energy for an extended contact. Within the method, it is possible to calculate the transmission coefficient through a contact of both finite and infinite length; the local density of states can be determined in both free and embedded CNT segments. We found perfect agreement with the experimental data for Pd and Al contacts. We have explained why CNTFETs with Pd electrodes are p -type FETs with ohmic contacts, which can carry current close to the ballistic limit (provided contact length is large enough), whereas in CNT-Al contacts transmission is suppressed to a significant extent, especially for holes.
Dissipative Particle Dynamics interaction parameters from ab initio calculations
NASA Astrophysics Data System (ADS)
Sepehr, Fatemeh; Paddison, Stephen J.
2016-02-01
Dissipative Particle Dynamics (DPD) is a commonly employed coarse-grained method to model complex systems. Presented here is a pragmatic approach to connect atomic-scale information to the meso-scale interactions defined between the DPD particles or beads. Specifically, electronic structure calculations were utilized for the calculation of the DPD pair-wise interaction parameters. An implicit treatment of the electrostatic interactions for charged beads is introduced. The method is successfully applied to derive the parameters for a hydrated perfluorosulfonic acid ionomer with absorbed vanadium cations.
Efficient Ab-initio Calculation of the Anomalous Hall Conductivity of Fe by Wannier Interpolation
NASA Astrophysics Data System (ADS)
Wang, Xinjie; Vanderbilt, David; Yates, Jonathan; Souza, Ivo
2006-03-01
Recently, a first-principles calculation of the anomalous Hall conductivity (AHC) of Fe as a Brillouin-zone integral of the Berry curvature was carried out and found to be in reasonable agreement with experimental results. However, these authors observed extraordinarily strong and rapid variations of the Berry curvature with wavevector k in the vicinity of avoided crossings and near-degeneracies in reciprocal space. A conventional first-principles calculation thus requires an extremely dense k-point mesh and is quite time-consuming. Here, we present an efficient first-principles approach for computing the AHC based on Wannier interpolation. First, a conventional electronic-structure calculation is performed for Fe, with spin-orbit included, on a relatively coarse k-point mesh. Second, maximally-localized Wannier functions are constructed by a post-processing step, thus transforming the full ab-initio problem into an effective tight- binding form. Finally, the needed quantities such as Berry potentials and curvatures are interpolated onto a fine k-point mesh and used to compute the AHC. Our approach gives good agreement with conventional, less efficient first-priciples calculations. Y. Yao et al., Phys. Rev. Lett. 92, 037204 (2004). I. Souza,N. Marzari, and D. Vanderbilt, Phys. Rev. B 65, 035109 (2001).
Ab Initio Calculations for the Surface Energy of Silver Nanoclusters
NASA Astrophysics Data System (ADS)
Medasani, Bharat; Vasiliev, Igor; Park, Young Ho
2007-03-01
We apply first principles computational methods to study the surface energy and the surface stress of silver nanoparticles. The structures, energies and lattice contractions of spherical Ag nanoclusters are calculated in the framework of density functional theory combined with the generalized gradient approximation. Our calculations predict the surface energies of Ag nanoclusters to be in the range of 1-2 J/m^2. These values are close to the bulk surface energy of silver, but are significantly lower than the recently reported value of 7.2 J/m^2 derived from the Kelvin equation for free Ag nanoparticles. From the lattice contraction and the nearest neighbor interatomic distance, we estimate the surface stress of the silver nanoclusters to be in the the range of 1-1.45 N/m. This result suggests that a liquid droplet model can be employed to evaluate the surface energy and the surface stress of Ag nanoparticles. K. K. Nanda et al., Phys. Rev. Lett. 91, 106102 (2003).
Ab Initio Calculations Of Nuclear Reactions And Exotic Nuclei
Quaglioni, S.
2014-05-05
Our ultimate goal is to develop a fundamental theory and efficient computational tools to describe dynamic processes between nuclei and to use such tools toward supporting several DOE milestones by: 1) performing predictive calculations of difficult-to-measure landmark reactions for nuclear astrophysics, such as those driving the neutrino signature of our sun; 2) improving our understanding of the structure of nuclei near the neutron drip line, which will be the focus of the DOE’s Facility for Rare Isotope Beams (FRIB) being constructed at Michigan State University; but also 3) helping to reveal the true nature of the nuclear force. Furthermore, these theoretical developments will support plasma diagnostic efforts at facilities dedicated to the development of terrestrial fusion energy.
Electronic Structure of Silicon Nanowires Matrix from Ab Initio Calculations.
Monastyrskii, Liubomyr S; Boyko, Yaroslav V; Sokolovskii, Bogdan S; Potashnyk, Vasylyna Ya
2016-12-01
An investigation of the model of porous silicon in the form of periodic set of silicon nanowires has been carried out. The electronic energy structure was studied using a first-principle band method-the method of pseudopotentials (ultrasoft potentials in the basis of plane waves) and linearized mode of the method of combined pseudopotentials. Due to the use of hybrid exchange-correlation potentials (B3LYP), the quantitative agreement of the calculated value of band gap in the bulk material with experimental data is achieved. The obtained results show that passivation of dangling bonds with hydrogen atoms leads to substantial transformation of electronic energy structure. At complete passivation of the dangling silicon bonds by hydrogen atoms, the band gap value takes the magnitude which substantially exceeds that for bulk silicon. The incomplete passivation gives rise to opposite effect when the band gap value decreases down the semimetallic range.
Ab-initio Calculation of the XANES of Lithium Phosphates and LiFePO4
NASA Astrophysics Data System (ADS)
Yiu, Y. M.; Yang, Songlan; Wang, Dongniu; Sun, Xueliang; Sham, T. K.
2013-04-01
Lithium iron phosphate has been regarded as a promising cathode material for the next generation lithium ion batteries due to its high specific capacity, superior thermal and cyclic stability [1]. In this study, the XANES (X-ray Absorption Near Edge Structure) spectra of lithium iron phosphate and lithium phosphates of various compositions at the Li K, P L3,2, Fe M3,2 and O K-edges have been simulated self-consistently using ab-initio calculations based on multiple scattering theory (the FEFF9 code) and DFT (Density Functional Theory, the Wien2k code). The lithium phosphates under investigation include LiFePO4, γ-Li3PO4, Li4P2O7 and LiPO3. The calculated spectra are compared to the experimental XANES recorded in total electron yield (TEY) and fluorescence yield (FLY). This work was carried out to assess the XANES of possible phases presented in LiFePO4 based Li ion battery applications [2].
Ab initio calculations of three-nucleon scattering
Witala, H. Golak, J.; Skibiński, R.; Topolnicki, K.
2015-10-15
Results on three-nucleon (3N) elastic scattering below the pion production threshold are discussed with an emphasis on the need for a three-nucleon force (3NF). The large discrepancies found between a theory based on numerical solutions of 3N Faddeev equations with (semi)phenomenological NN potentials only and data point to the need for 3NF’s. This notion is supported by the fact that another possible reason for the discrepancies in elastic nucleon-deuteron (Nd) scattering, relativistic effects, turned out to be small. Results for new generation of chiral NN forces (up to N{sup 4}LO) together with theoretical truncation errors are shown.
Structure and lattice dynamics of PrFe3(BO3)4: Ab initio calculation
NASA Astrophysics Data System (ADS)
Chernyshev, V. A.; Nikiforov, A. E.; Petrov, V. P.
2016-06-01
The crystal structure and phonon spectrum of PrFe3(BO3)4 are ab initio calculated in the context of the density functional theory. The ion coordinates in the unit cell of a crystal and the lattice parameters are evaluated from the calculations. The types and frequencies of the fundamental vibrations, as well as the line intensities of the IR spectrum, are determined. The elastic constants of the crystal are calculated. A "seed" frequency of the vibration strongly interacting with the electron excitation on the praseodymium ion is obtained for low-frequency A 2 mode. The calculated results are in agreement with the known experimental data.
Chen Shilu; Fang Weihai
2009-08-07
In the present paper we report a theoretical study on mechanistic photodissociation of acetaldehyde (CH{sub 3}CHO). Stationary structures for H{sub 2} and CO eliminations in the ground state (S{sub 0}) have been optimized with density functional theory method, which is followed by the intrinsic reaction coordinate and ab initio molecular dynamics calculations to confirm the elimination mechanism. Equilibrium geometries, transition states, and intersection structures for the C-C and C-H dissociations in excited states were determined by the complete-active-space self-consistent field (CASSCF) method. Based on the CASSCF optimized structures, the potential energy profiles for the dissociations were refined by performing the single-point calculations using the multireference configuration interaction method. Upon the low-energy irradiation of CH{sub 3}CHO (265 nm<{lambda}<318 nm), the T{sub 1} C-C bond fission following intersystem crossing from the S{sub 1} state is the predominant channel and the minor channel, the ground-state elimination to CH{sub 4}+CO after internal conversion (IC) from S{sub 1} to S{sub 0}, could not be excluded. With the photon energy increasing, another pathway of IC, achieved via an S{sub 1}/S{sub 0} intersection point resulting from the S{sub 1} C-C bond fission, becomes accessible and increases the yield of CH{sub 4}+CO.
Band offset of the ZnO/Cu2O heterojunction from ab initio calculations
NASA Astrophysics Data System (ADS)
Zemzemi, M.; Alaya, S.
2013-12-01
The ZnO/Cu2O system has known a recent revival of interest in solar cells for its potential use as a heterojunction able to highly perform under visible light. In this work, we are interested on the characterization of the interface through nanoscale modelization based on ab initio (Density Functional Theory (DFT), Local Density Approximation (LDA), Generalized Gradient Approximation (GGA-PBE), and Pseudopotential (PP)). This work aims to build a supercell containing a heterojunction ZnO/Cu2O and study the structural properties and the discontinuity of the valence band (band offset) from a semiconductor to another. We built a zinc oxide in the wurtzite structure along the [0 0 0 1] on which we placed the copper oxide in the hexagonal structure (CdI2-type). We choose the method of Van de Walle and Martin to calculate the energy offset. This approach fits well with the DFT. Our calculations of the band offset gave us a value that corresponds to other experimental and theoretical values.
Ab initio calculations of many-body interactions for compressed solid argon.
Tian, Chunling; Liu, Fusheng; Cai, Lingcang; Yuan, Hongkuan; Chen, Hong; Zhong, Mingmin
2015-11-01
An investigation on many-body effects of solid argon at high pressure was conducted based on a many-body expansion of interaction energy. The three- and four-body terms in the expansion were calculated using the coupled-cluster method with single, double, and noniterative triple theory and incremental method, in which the configurations of argon trimers and tetramers were chosen as the same as those in the actual lattice. The four-body interactions in compressed solid argon were estimated for the first time, and the three-body interaction ab initio calculations were extended to a small distance. It shows that the four-body contribution is repulsive at high densities and effectively cancels the three-body lattice energy. The dimer potential plus three-body interaction can well reproduce the measurements of equation of state at pressure approximately lower than ∼60 GPa, when including the four-body effects extends the agreement up to the maximum experimental pressure of 114 GPa. PMID:26547175
NASA Astrophysics Data System (ADS)
Dézerald, Lucile; Ventelon, Lisa; Willaime, François; Clouet, Emmanuel; Rodney, David
2014-06-01
Ab initio methods, based on the Density Functional Theory (DFT), have been extensively used to study point defects and defect clusters in materials. Present HPC resources and DFT codes now allow similar investigations to be performed on dislocations. The study of these extended defects requires not only larger simulation cells but also a higher accuracy because the energy differences, which are involved, are rather small, typically 50-to-100 meV for supercells containing 50-to-500 atoms. The topology of the Peierls potential of screw dislocations with 1/2 <111>Burgers vector, i.e. the 2D energy landscape seen by these dislocations, is being completely revisited by DFT calculations. From results obtained in all body-centered cubic (bcc) transition metals, except Cr (V, Nb, Ta, Mo, W and Fe), using the PWSCF code, which is part of the Quantum-Espresso package, we concluded that the 2D Peierls potentials have two common features: the single-hump shape of the barrier between two minima of the potential, and the presence of a maximum - and not a minimum as predicted by most empirical potentials - around the split core. In iron, the topology of the Peierls potential is reversed compared to the classical sinusoidal picture: the location of the saddle point and the maximum are indeed inverted with unexpected flat regions. The first results obtained within the framework of the PRACE project, DIMAIM (DIslocations in Metals using Ab Initio Methods), started at the beginning of 2013, will also be presented. In particular, in order to address the twinning-antitwinning asymmetry often observed in bcc metals and regarded as the major contribution to the breakdown of Schmid's law, we have determined the crystal orientation dependence of the Peierls stress, i.e. the critical stress required for dislocation motion. These computationally most expensive simulations were performed on the PRACE Tier-0 system at Barcelona Supercomputing Center (Marenostrum III). The scalability results
Ab initio R-matrix calculations of e+-molecule scattering
NASA Technical Reports Server (NTRS)
Danby, Grahame; Tennyson, Jonathan
1990-01-01
The adaptation of the molecular R-matrix method, originally developed for electron-molecule collision studies, to positron scattering is discussed. Ab initio R-matrix calculations are presented for collisions of low energy positrons with a number of diatomic systems including H2, HF and N2. Differential elastic cross sections for positron-H2 show a minimum at about 45 deg for collision energies between 0.3 and 0.5 Ryd. The calculations predict a bound state of positronHF. Calculations on inelastic processes in N2 and O2 are also discussed.
NASA Astrophysics Data System (ADS)
Kianfar, Ali Hossein; Ramazani, Shapour; Fath, Roghaye Hashemi; Roushani, Mahmoud
2013-03-01
Attempts to spectroscopic and structural study of copper complexes, some Cu(II) Schiff base complexes were synthesized and characterized by means of electronic, IR, 1HNMR spectra and elemental analysis. The thermal analyses of the complexes were investigated and the first order kinetic parameters were derived for them. The cyclic voltammetric studies in acetonitrile were proposed a monomeric structure for complexes. The structures of compounds were determined by ab initio calculations. In the solid state, the ligands exist as keto-amine/enol-imine tautomeric forms with an intramolecular hydrogen bond (Nsbnd H⋯O) between amine and carbonyl group. The kinetic studies of the tautomerism and equilibrium constant of the reactions were calculated using transition state theory. The optimized molecular geometry and atomic charges were calculated using MP2 method with 6-31G(d) basis set for H, C, N and O atoms and LANL2DZ for the Cu atom. The results suggested that, in the complexes, Cu(II) ion is in pseudo square-planar NO3 coordination geometry. Also the bond lengths and angles were studied and compared.
Espinosa-Garcia, J; Rangel, C; Corchado, J C
2016-06-22
We report an analytical full-dimensional potential energy surface for the GeH4 + OH → GeH3 + H2O reaction based on ab initio calculations. It is a practically barrierless reaction with very high exothermicity and the presence of intermediate complexes in the entrance and exit channels, reproducing the experimental evidence. Using this surface, thermal rate constants for the GeH4 + OH/OD isotopic reactions were calculated using two approaches: variational transition state theory (VTST) and quasi-classical trajectory (QCT) calculations, in the temperature range 200-1000 K, and results were compared with the only experimental data at 298 K. Both methods showed similar values over the whole temperature range, with differences less than 30%; and the experimental data was reproduced at 298 K, with negative temperature dependence below 300 K, which is associated with the presence of an intermediate complex in the entrance channel. However, while the QCT approach reproduced the experimental kinetic isotope effect, the VTST approach underestimated it. We suggest that this difference is associated with the harmonic approximation used in the treatment of vibrational frequencies.
Espinosa-Garcia, J; Rangel, C; Corchado, J C
2016-06-22
We report an analytical full-dimensional potential energy surface for the GeH4 + OH → GeH3 + H2O reaction based on ab initio calculations. It is a practically barrierless reaction with very high exothermicity and the presence of intermediate complexes in the entrance and exit channels, reproducing the experimental evidence. Using this surface, thermal rate constants for the GeH4 + OH/OD isotopic reactions were calculated using two approaches: variational transition state theory (VTST) and quasi-classical trajectory (QCT) calculations, in the temperature range 200-1000 K, and results were compared with the only experimental data at 298 K. Both methods showed similar values over the whole temperature range, with differences less than 30%; and the experimental data was reproduced at 298 K, with negative temperature dependence below 300 K, which is associated with the presence of an intermediate complex in the entrance channel. However, while the QCT approach reproduced the experimental kinetic isotope effect, the VTST approach underestimated it. We suggest that this difference is associated with the harmonic approximation used in the treatment of vibrational frequencies. PMID:27292879
Solvation properties of microhydrated sulfate anion clusters: insights from ab initio calculations.
Wan, Quan; Spanu, Leonardo; Galli, Giulia
2012-08-01
Sulfate-water clusters play an important role in environmental and industrial processes, yet open questions remain on their physical and chemical properties. We investigated the smallest hydrated sulfate anion clusters believed to have a full solvation shell, with 12 or 13 water molecules. We used ab initio molecular dynamics and electronic structure calculations based on density functional theory, with semilocal and hybrid functionals. At both levels of theory we found that configurations with the anion at the surface of the cluster are energetically favored compared to fully solvated ones, which are instead metastable. We show that infrared spectra of the anion with different solvation shells have similar vibrational signatures, indicating that a mixture of surface and internally solvated geometries are likely to be present in the experimental samples at low temperature. In addition, the computed electronic density of states of surface and internally solvated clusters are hardly distinguishable at finite temperature, with the highest occupied molecular orbital belonging to the anion in all cases. The equilibrium structure determined for SO(4)(2-)·(H(2)O)(13) differs from that previously reported; we find that the addition of one water molecule to a 12-water cluster modifies its hydration shell and that water-water bonds are preferred over water-anion bonds.
Ab initio calculations of the ground and excited states of I 2- and ICl -
NASA Astrophysics Data System (ADS)
Maslen, P. E.; Faeder, J.; Parson, R.
1996-12-01
We performed all-electron ab initio calculations of the first six states of I 2- and ICl - using a multi-reference configuration interaction method. Spin-orbit coupling is included via an empirical one-electron operator and has a large effect on the dissociation energy. The ground state dissociation energies were in error by 20-30%, probably due to deficiencies in the one electron basis sets. The electronic wavefunctions at the equilibrium geometry were used to calculate the electronic absorption spectrum from the ground state, and good agreement was found with the experimental data.
Kurova, N. V. Burdov, V. A.
2013-12-15
The results of ab initio calculations of the electronic structure of Si nanocrystals doped with shallow donors (Li, P) are reported. It is shown that phosphorus introduces much more significant distortions into the electronic structure of the nanocrystal than lithium, which is due to the stronger central cell potential of the phosphorus ion. It is found that the Li-induced splitting of the ground state in the conduction band of the nanocrystal into the singlet, doublet, and triplet retains its inverse structure typical for bulk silicon.
Matrix IR spectrum and ab initio SCF calculations of molecular SiS sub 2
Schnoeckel, H.; Koeppe, R. )
1989-06-21
In solid argon molecular SiS{sub 2} is generated by a reaction of SiS with S atoms. The antisymmetric stretching vibration {nu}{sub as}(SiS) is observed at 918 cm{sup {minus}1}. Bonding and structure (force constants from experimentally observed frequencies and results from ab initio SCF calculations) of SiS{sub 2} are compared with that of the similar molecules: CO, CS, CO{sub 2}, COS, CS{sub 2}, SiO, SiS, SiO{sub 2}, and SiOS.
Shaughnessy, M C; Jones, R E
2016-02-01
We develop and demonstrate a method to efficiently use density functional calculations to drive classical dynamics of complex atomic and molecular systems. The method has the potential to scale to systems and time scales unreachable with current ab initio molecular dynamics schemes. It relies on an adapting dataset of independently computed Hellmann-Feynman forces for atomic configurations endowed with a distance metric. The metric on configurations enables fast database lookup and robust interpolation of the stored forces. We discuss mechanisms for the database to adapt to the needs of the evolving dynamics, while maintaining accuracy, and other extensions of the basic algorithm.
NASA Astrophysics Data System (ADS)
Feshin, V. P.; Feshina, E. V.
2000-07-01
The results of ab initio calculations at the RHF/6-31G ∗ level of 1-methyl-4-chloro- and -5-chloroimidazoles as well as of 1-methyl-4,5-dichloroimidazoles with total optimization of their geometry were presented. They were used for the interpretation of peculiarities of an influence of the "pyridine" and "pyrrole" N atoms on the electron distribution of the Cl atoms in these molecules and of their 35Cl NQR frequencies. These peculiarities are caused by the different space electron distribution of these N atoms that causes the different polarization of the geminal Cl atom p-electron shell.
Shaughnessy, M C; Jones, R E
2016-02-01
We develop and demonstrate a method to efficiently use density functional calculations to drive classical dynamics of complex atomic and molecular systems. The method has the potential to scale to systems and time scales unreachable with current ab initio molecular dynamics schemes. It relies on an adapting dataset of independently computed Hellmann-Feynman forces for atomic configurations endowed with a distance metric. The metric on configurations enables fast database lookup and robust interpolation of the stored forces. We discuss mechanisms for the database to adapt to the needs of the evolving dynamics, while maintaining accuracy, and other extensions of the basic algorithm. PMID:26669825
Ab initio no core calculations of light nuclei and preludes to Hamiltonian quantum field theory
Vary, J.P.; Maris, P.; Shirokov, A.M.; Honkanen, H.; li, J.; Brodsky, S.J.; Harindranath, A.; Teramond, G.F.de; /Costa Rica U.
2009-08-03
Recent advances in ab initio quantum many-body methods and growth in computer power now enable highly precise calculations of nuclear structure. The precision has attained a level sufficient to make clear statements on the nature of 3-body forces in nuclear physics. Total binding energies, spin-dependent structure effects, and electroweak properties of light nuclei play major roles in pinpointing properties of the underlying strong interaction. Eventually,we anticipate a theory bridge with immense predictive power from QCD through nuclear forces to nuclear structure and nuclear reactions. Light front Hamiltonian quantum field theory offers an attractive pathway and we outline key elements.
Ab Initio Calculations of Singlet and Triplet Excited States of Chlorine Nitrate and Nitric Acid
NASA Technical Reports Server (NTRS)
Grana, Ana M.; Lee, Timothy J.; Head-Gordon, Martin; Langhoff, Stephen R. (Technical Monitor)
1994-01-01
Ab initio calculations of vertical excitations to singlet and triplet excited states of chlorine nitrate and nitric acid are reported. The nature of the electronic transitions are examined by decomposing the difference density into the sum of detachment and attachment densities. Counterparts for the three lowest singlet excited states of nitric acid survive relatively unperturbed in chlorine nitrate, while other low-lying singlet states of chlorine nitrate appear to be directly dissociative in the ClO chromophore. These results suggest an assignment of the two main peaks in the experimental chlorine nitrate absorption spectrum. In addition, triplet vertical excitations and the lowest optimized triplet geometries of both molecules are studied.
Ab initio calculations of non-radiative carrier trapping due to deep impurity levels
NASA Astrophysics Data System (ADS)
Wang, Lin-Wang; Shi, Lin
2013-03-01
Non-radiative carrier decay due to deep impurity levels in semiconductors is an important process which affects the efficiencies of devices from solar cells to light emitting diode. This process is due to multiple phonon emission. Despite of the fact the analytical formalisms have been derived long time ago, so far there is no direct ab initio calculations due to the high cost of calculating all the electron-phonon coupling constants. Here we introduce an algorithm which calculates all the electron-phonon coupling constants at once, hence allows the ab initio calculations of such processes. Another approximation is introduced to calculate the phonon modes of a given impurity system. We use a ZnGa-VN paired defect in GaN as an example to study this process. We found that while most of the promoting phonon modes (used to promote the transition with the electron-phonon coupling) come from the optical modes, the accepting phonon modes (used to satisfy the energy conservation) come mostly from the acoustic phonons. This work is supported by SC/BES/MSED of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231, and by the National High Technology Research and Development Program of China (863 Program) (No. 2011AA03A103)
Evolved chiral NN +3N Hamiltonians for ab initio nuclear structure calculations
NASA Astrophysics Data System (ADS)
Roth, Robert; Calci, Angelo; Langhammer, Joachim; Binder, Sven
2014-08-01
We discuss the building blocks for a consistent inclusion of chiral three-nucleon (3N) interactions into ab initio nuclear structure calculations beyond the lower p shell. We highlight important technical developments, such as the similarity renormalization group (SRG) evolution in the 3N sector, a JT-coupled storage scheme for 3N matrix elements with efficient on-the-fly decoupling, and the importance-truncated no-core shell model with 3N interactions. Together, these developments make converged ab initio calculations with explicit 3N interactions possible also beyond the lower p shell. We analyze in detail the impact of various truncations of the SRG-evolved Hamiltonian, in particular the truncation of the harmonic-oscillator model space used for solving the SRG flow equations and the omission of the induced beyond-3N contributions of the evolved Hamiltonian. Both truncations lead to sizable effects in the upper p shell and beyond and we present options to remedy these truncation effects. The analysis of the different truncations is a first step towards a systematic uncertainty quantification of all stages of the calculation.
Higher-order elastic constants and megabar pressure effects of bcc tungsten: Ab initio calculations
NASA Astrophysics Data System (ADS)
Vekilov, Yu. Kh.; Krasilnikov, O. M.; Lugovskoy, A. V.; Lozovik, Yu. E.
2016-09-01
The general method for the calculation of n th (n ≥2 ) order elastic constants of the loaded crystal is given in the framework of the nonlinear elasticity theory. For the crystals of cubic symmetry under hydrostatic compression, the two schemes of calculation of the elastic constants of second, third, and fourth order from energy-finite strain relations and stress-finite strain relations are implemented. Both techniques are applied for the calculation of elastic constants of orders from second to fourth to the bcc phase of tungsten at a 0-600 GPa pressure range. The energy and stress at the various pressures and deformations are obtained ab initio in the framework of projector augmented wave+generalized gradient approximation (PAW+GGA) method, as implemented in Vienna Ab initio Simulation Package (VASP) code. Using the obtained results, we found the pressure dependence of Grüneisen parameters for long-wave acoustic modes in this interval. The Lamé constants of second and third order were estimated for polycrystalline tungsten. The proposed method is applicable for crystals with arbitrary symmetry.
Ab initio calculation of oxygen self-diffusion coefficient in uranium dioxide UO2
NASA Astrophysics Data System (ADS)
Dorado, Boris; Garcia, Philippe; Torrent, Marc
Uranium dioxide UO2 is the most widely used nuclear fuel worldwide and its atomic transport properties are relevant to practically all engineering aspects of the material. Although transport properties have already been studied in UO2 by means of first-principles calculations, the ab initio determination of self-diffusion coefficients has up to now remained unreachable because the relevant computational tools were neither available or adapted. The present work reports our results related to the ab initio calculation of the oxygen self-diffusion coefficient in UO2. We first determine the Gibbs free energies of formation of oxygen charged defects by calculating both the electronic and vibrational (hence entropic) contributions. Then, we use the transition state theory in order to compute the effective jump frequency of the defects, which in turn provides us with the value of the pre-exponential factor. The results are compared to self-diffusion data obtained experimentally with a careful monitoring of the relevant thermodynamic conditions (oxygen partial pressure, temperature, impurity content).
Ab initio calculations of the optical properties of crystalline and liquid InSb
Sano, Haruyuki; Mizutani, Goro
2015-11-15
Ab initio calculations of the electronic and optical properties of InSb were performed for both the crystalline and liquid states. Two sets of atomic structure models for liquid InSb at 900 K were obtained by ab initio molecular dynamics simulations. To reduce the effect of structural peculiarities in the liquid models, an averaging of the two sets of the calculated electronic and optical properties corresponding to the two liquid models was performed. The calculated results indicate that, owing to the phase transition from crystal to liquid, the density of states around the Fermi level increases. As a result, the energy band gap opening near the Fermi level disappears. Consequently, the optical properties change from semiconductor to metallic behavior. Namely, owing to the melting of InSb, the interband transition peaks disappear and a Drude-like dispersion is observed in the optical dielectric functions. The optical absorption at a photon energy of 3.06 eV, which is used in Blu-ray Disc systems, increases owing to the melting of InSb. This increase in optical absorption is proposed to result from the increased optical transitions below 2 eV.
Ab initio calculation of thermodynamic, transport, and optical properties of CH{sub 2} plastics
Knyazev, D. V.; Levashov, P. R.
2015-05-15
This work covers an ab initio calculation of thermodynamic, transport, and optical properties of plastics of the effective composition CH{sub 2} at density 0.954 g/cm{sup 3} in the temperature range from 5 kK up to 100 kK. The calculation is based on the quantum molecular dynamics, density functional theory, and the Kubo-Greenwood formula. The temperature dependence of the static electrical conductivity σ{sub 1{sub D{sub C}}}(T) has a step-like shape: σ{sub 1{sub D{sub C}}}(T) grows rapidly for 5 kK ≤ T ≤ 10 kK and is almost constant for 20 kK ≤ T ≤ 60 kK. The additional analysis based on the investigation of the electron density of states (DOS) is performed. The rapid growth of σ{sub 1{sub D{sub C}}}(T) at 5 kK ≤ T ≤ 10 kK is connected with the increase of DOS at the electron energy equal to the chemical potential ϵ = μ. The frequency dependence of the dynamic electrical conductivity σ{sub 1}(ω) at 5 kK has the distinct non-Drude shape with the peak at ω ≈ 10 eV. This behavior of σ{sub 1}(ω) was explained by the dip at the electron DOS.
Hydrogen Bonds in Crystalline Imidazoles Studied by 15N NMR and ab initio MO Calculations
NASA Astrophysics Data System (ADS)
Ueda, Takahiro; Nagatomo, Shigenori; Masui, Hirotsugu; Nakamura, Nobuo; Hayashi, Shigenobu
1999-07-01
Intermolecular hydrogen bonds of the type N-H...N in crystals of imidazole and its 4-substituted and 4,5-disubstituted derivatives were studied by 15N CP/MAS NMR and an ab initio molecular orbital (MO) calculation. In the 15N CP/MAS NMR spectrum of each of the imidazole derivatives, two peaks due to the two different functional groups, >NH and =N-, were observed. The value of the 15N isotropic chemical shift for each nitrogen atom depends on both the length of the intermolecular hydrogen bond and the kind of the substituent or substituents. It was found that the difference between the experimen-tal chemical shifts of >NH and =N-varies predominantly with the hydrogen bond length but does not show any systematic dependence on the kind of substituent. The ab initio MO calculations suggest that the hydrogen bond formation influences the 15N isotropic chemical shift predominantly, and that the difference between the 15N isotropic chemical shift of >NH and =N-varies linearly with the hydrogen bond length.
Petersen, Philippe A D; Silva, Andreia S; Gonçalves, Marcos B; Lapolli, André L; Ferreira, Ana Maria C; Carbonari, Artur W; Petrilli, Helena M
2014-06-01
In this work, perturbed angular correlation (PAC) spectroscopy is used to study differences in the nuclear quadrupole interactions of Cd probes in DNA molecules of mice infected with the Y-strain of Trypanosoma cruzi. The possibility of investigating the local genetic alterations in DNA, which occur along generations of mice infected with T. cruzi, using hyperfine interactions obtained from PAC measurements and density functional theory (DFT) calculations in DNA bases is discussed. A comparison of DFT calculations with PAC measurements could determine the type of Cd coordination in the studied molecules. To the best of our knowledge, this is the first attempt to use DFT calculations and PAC measurements to investigate the local environment of Cd ions bound to DNA bases in mice infected with Chagas disease. The obtained results also allowed the detection of local changes occurring in the DNA molecules of different generations of mice infected with T. cruzi, opening the possibility of using this technique as a complementary tool in the characterization of complicated biological systems.
NASA Technical Reports Server (NTRS)
Smith, Grant D.; Jaffe, R. L.; Yoon, D. Y.; Arnold, James O. (Technical Monitor)
1994-01-01
Molecular dynamics simulations of POE melts have been performed utilizing a potential force field parameterized to reproduce conformer energies and rotational energy barriers in dimethoxyethane as determined from ab initio electronic structure calculations. Chain conformations and dimensions of POE from the simulations were found to be in good agreement with predictions of a rotational isomeric state (RIS) model based upon the ab initio conformational. energies. The melt chains were found to be somewhat extended relative to chains at theta conditions. This effect will be discussed in light of neutron scattering experiments which indicate that POE chains are extended in the melt relative to theta solutions. The conformational characteristics of POE chains will also be compared with those of other poly (alkylethers), namely poly(oxymethylene), poly(oxytrimethylene) and poly(oxytetramethylene). Local conformational dynamics were found to be more rapid than in polymethylene. Calculated C-H vector correlation times were found to be in reasonable agreement with experimental values from C-13 NMR spin-lattice relaxation times. The influence of ionic salts on local conformations and dynamics will also be discussed.
NASA Astrophysics Data System (ADS)
Kim, Bog G.
2011-05-01
We have studied the detailed mechanism of epitaxial strain induced ferroelectricity in rocksalt binary compound by ab initio calculation and soft mode group theory analysis. By applying compressive strain, cubic binary rocksalt (F m3m) transforms into tetragonal (I 4/mmm) structure. With increasing compressive strain, tetragonal structure becomes unstable against spontaneous transformation to lower symmetry tetragonal structure (I 4/mm), evident both from ab initio calculation and from soft mode group theory analysis. For the tensile strain, phase transition sequence can be cubic binary rocksalt to tetragonal (I 4/mmm) and to orthorhombic structure (I m2m). From ab initio calculation and space group analysis, we propose that the epitaxial strain induced ferroelectricity of rocksalt binary compound is the generic property.
The Crystal Structure of Impurity Centers Tm^{2+} and Eu^{2+} in SrCl2 : Ab Initio Calculations
NASA Astrophysics Data System (ADS)
Chernyshev, V. A.; Serdcev, A. V.; Petrov, V. P.; Nikiforov, A. E.
2016-01-01
Ab initio calculations of the impurity centers Tm^{2+} thulium and europium Eu^{2+} in SrCl2 and MeF2 (Me = Ca, Sr, Ba) were carried out at low (zero) temperature. The crystal structure of impurity centers was investigated. Charge density maps show that the bonds formed by the rare-earth ions have an ionic character. The crystal structures, lattice dynamics, and band structures of MeF2 and SrCl2 were calculated at low temperature. Ab initio calculations were performed in periodic CRYSTAL code within the framework of the MO LCAO approach by using hybrid DFT functionals.
Ab Initio Calculations of the Electronic Structures and Biological Functions of Protein Molecules
NASA Astrophysics Data System (ADS)
Zheng, Haoping
2003-04-01
The self-consistent cluster-embedding (SCCE) calculation method reduces the computational effort from M3 to about M1 (M is the number of atoms in the system) with unchanged calculation precision. So the ab initio, all-electron calculation of the electronic structure and biological function of protein molecule becomes a reality, which will promote new proteomics considerably. The calculated results of two real protein molecules, the trypsin inhibitor from the seeds of squash Cucurbita maxima (CMTI-I, 436 atoms) and the Ascaris trypsin inhibitor (912 atoms, two three-dimensional structures), are presented. The reactive sites of the inhibitors are determined and explained. The precision of structure determination of inhibitors are tested theoretically.
Ab Initio Calculations of the Electronic Structures and Biological Functions of Protein Molecules
NASA Astrophysics Data System (ADS)
Zheng, Haoping
The self-consistent cluster-embedding (SCCE) calculation method reduces the computational effort from M3 to about M1 (M is the number of atoms in the system) with precise calculations. Thus the ab initio, all-electron calculation of the electronic structure and biological function of protein molecule has become a reality, which will promote new proteomics considerably. The calculated results of two real protein molecules, the trypsin inhibitor from the seeds of squash Cucurbita maxima (CMTI-I, 436 atoms) and the ascaris trypsin inhibitor (912 atoms, two three-dimensional structures), will be presented in this paper. The reactive sites of the inhibitors are determined and explained. The accuracy of structure determination of the inhibitors are tested theoretically.
Ab initio/DFT calculations of butyl ammonium salt of O,O'-dibornyl dithiophosphate.
Kart, H H; Ozdemir Kart, S; Karakuş, M; Kurt, M
2014-08-14
O,O'-dibornyl dithiophosphete has been synthesized by the reaction of P2S5 and borneol in toluene. Fourier Transform Infrared spectra (FT-IR) of the title compound are measured. The molecular geometry, vibrational frequencies, infrared intensities and NMR spectrum of the title compound in the ground state have been calculated by using the density functional theory (DFT) and ab initio Hartree-Fock (HF) methods with the basis set of 6-31G(d). The computed bond lengths and bond angles show the good agreement with the experimental data. Moreover, the vibrational frequencies are calculated and the scaled values have been compared with experimental FT-IR spectra. Assignments of the vibrational modes are made on the basis of total energy distribution (TED) calculated with scaled quantum mechanical (SQM) method. The observed and calculated FT-IR and NMR spectra are in good agreement with each other. PMID:24747929
Ab initio/DFT calculations of butyl ammonium salt of O,O‧-dibornyl dithiophosphate
NASA Astrophysics Data System (ADS)
Kart, H. H.; Özdemir Kart, S.; Karakuş, M.; Kurt, M.
2014-08-01
O,O‧-dibornyl dithiophosphete has been synthesized by the reaction of P2S5 and borneol in toluene. Fourier Transform Infrared spectra (FT-IR) of the title compound are measured. The molecular geometry, vibrational frequencies, infrared intensities and NMR spectrum of the title compound in the ground state have been calculated by using the density functional theory (DFT) and ab initio Hartree-Fock (HF) methods with the basis set of 6-31G(d). The computed bond lengths and bond angles show the good agreement with the experimental data. Moreover, the vibrational frequencies are calculated and the scaled values have been compared with experimental FT-IR spectra. Assignments of the vibrational modes are made on the basis of total energy distribution (TED) calculated with scaled quantum mechanical (SQM) method. The observed and calculated FT-IR and NMR spectra are in good agreement with each other.
Emergent properties of nuclei from ab initio coupled-cluster calculations
NASA Astrophysics Data System (ADS)
Hagen, G.; Hjorth-Jensen, M.; Jansen, G. R.; Papenbrock, T.
2016-06-01
Emergent properties such as nuclear saturation and deformation, and the effects on shell structure due to the proximity of the scattering continuum and particle decay channels are fascinating phenomena in atomic nuclei. In recent years, ab initio approaches to nuclei have taken the first steps towards tackling the computational challenge of describing these phenomena from Hamiltonians with microscopic degrees of freedom. This endeavor is now possible due to ideas from effective field theories, novel optimization strategies for nuclear interactions, ab initio methods exhibiting a soft scaling with mass number, and ever-increasing computational power. This paper reviews some of the recent accomplishments. We also present new results. The recently optimized chiral interaction NNLO{}{{sat}} is shown to provide an accurate description of both charge radii and binding energies in selected light- and medium-mass nuclei up to 56Ni. We derive an efficient scheme for including continuum effects in coupled-cluster computations of nuclei based on chiral nucleon-nucleon and three-nucleon forces, and present new results for unbound states in the neutron-rich isotopes of oxygen and calcium. The coupling to the continuum impacts the energies of the {J}π =1/{2}-,3/{2}-,7/{2}-,3/{2}+ states in {}{17,23,25}O, and—contrary to naive shell-model expectations—the level ordering of the {J}π =3/{2}+,5/{2}+,9/{2}+ states in {}{53,55,61}Ca. ).
NASA Astrophysics Data System (ADS)
Roy, Tufan; Pandey, Dhanshree; Chakrabarti, Aparna
2016-05-01
Using first-principles calculations based on density functional theory, we have studied the mechanical, electronic, and magnetic properties of Heusler alloys, namely, Ni2B C and Co2B C (B = Sc, Ti, V, Cr, and Mn as well as Y, Zr, Nb, Mo, and Tc; C = Ga and Sn). On the basis of electronic structure (density of states) and mechanical properties (tetragonal shear constant), as well as magnetic interactions (Heisenberg exchange coupling parameters), we probe the properties of these materials in detail. We calculate the formation energy of these alloys in the (face-centered) cubic austenite structure to probe the stability of all these materials. From the energetic point of view, we have studied the possibility of the electronically stable alloys having a tetragonal phase lower in energy compared to the respective cubic phase. A large number of the magnetic alloys is found to have the cubic phase as their ground state. On the other hand, for another class of alloys, the tetragonal phase has been found to have lower energy compared to the cubic phase. Further, we find that the values of tetragonal shear constant show a consistent trend: a high positive value for materials not prone to tetragonal transition and low or negative for others. In the literature, materials which have been seen to undergo the martensite transition are found to be metallic in nature. We probe here if there is any Heusler alloy which has a tendency to undergo a tetragonal transition and at the same time possesses a high spin polarization at the Fermi level. From our study, it is found that out of the four materials which exhibit a martensite phase as their ground state, three of these, namely, Ni2MnGa , Ni2MoGa , and Co2NbSn have a metallic nature; on the contrary, Co2MoGa exhibits a high spin polarization.
Ab-initio calculations and phase diagram assessments of An-Al systems (An = U, Np, Pu)
NASA Astrophysics Data System (ADS)
Sedmidubský, D.; Konings, R. J. M.; Souček, P.
2010-02-01
The enthalpies of formation of binary intermetallic compounds AnAl n(n=2,3,4, An=U,Np,Pu) were assessed from first principle calculations of total energies performed using full potential APW + lo technique within density functional theory ( WIEN2k). The substantial contribution to entropies, S298°, arising from lattice vibrations was calculated by direct method within harmonic crystal approximation ( Phonon software + VASP for obtaining Hellmann-Feynman forces). The electronic heat capacity and the corresponding contribution to entropy were estimated from the density of states at Fermi level obtained from electronic structure calculations. The phase diagrams of the relevant systems An-Al were calculated based on the thermodynamic data assessed from ab-initio calculations, known equilibrium and calorimetry data by employing the FactSage program.
Structure and lattice dynamics of rare-earth ferroborate crystals: Ab initio calculation
NASA Astrophysics Data System (ADS)
Chernyshev, V. A.; Nikiforov, A. E.; Petrov, V. P.; Serdtsev, A. V.; Kashchenko, M. A.; Klimin, S. A.
2016-08-01
The ab initio calculation of the crystal structure and the phonon spectrum of crystals RFe3(BO3)4 ( R = Pr, Nd, Sm) has been performed in the framework of the density functional theory. The ion coordinates in the unit cell, the lattice parameters, the frequencies and the types of fundamental vibrations, and also the intensities of lines in the Raman spectrum and infrared reflection spectra have been found. The elastic constants of the crystals have been calculated. For low-frequency A 2 mode in PrFe3(BO3)4, a "seed" vibration frequency that strongly interacts with the electronic excitation on a praseodymium ion was found. The calculation results satisfactory agree with the experimental data.
Dos, Alexandra; Schimming, Volkmar; Tosoni, Sergio; Limbach, Hans-Heinrich
2008-12-11
The interactions of the 15N-labeled amino groups of dry solid poly-L-lysine (PLL) with various halogen and oxygen acids HX and the relation to the secondary structure have been studied using solid-state 15N and 13C CPMAS NMR spectroscopy (CP = cross polarization and MAS = magic angle spinning). For comparison, 15N NMR spectra of an aqueous solution of PLL were measured as a function of pH. In order to understand the effects of protonation and hydration on the 15N chemical shifts of the amino groups, DFT and chemical shielding calculations were performed on isolated methylamine-acid complexes and on periodic halide clusters of the type (CH3NH3(+)X(-))n. The combined experimental and computational results reveal low-field shifts of the amino nitrogens upon interaction with the oxygen acids HX = HF, H2SO4, CH3COOH, (CH3)2POOH, H3PO4, HNO3, and internal carbamic acid formed by reaction of the amino groups with gaseous CO2. Evidence is obtained that only hydrogen-bonded species of the type (Lys-NH2***H-X)n are formed in the absence of water. 15N chemical shifts are maximum when H is located in the hydrogen bond center and then decrease again upon full protonation, as found for aqueous solution at low pH. By contrast, halogen acids interact in a different way. They form internal salts of the type (Lys-NH3(+)X(-))n via the interaction of many acid-base pairs. This salt formation is possible only in the beta-sheet conformation. By contrast, the formation of hydrogen-bonded complexes can occur both in beta-sheet domains as well as in alpha-helical domains. The 15N chemical shifts of the protonated ammonium groups increase when the size of the interacting halogen anions is increased from chloride to iodide and when the number of the interacting anions is increased. Thus, the observed high-field 15N shift of ammonium groups upon hydration is the consequence of replacing interacting halogen atoms by oxygen atoms.
Ab initio calculation of ICD widths in photoexcited HeNe
Jabbari, G.; Klaiman, S.; Chiang, Y.-C.; Gokhberg, K.; Trinter, F.; Jahnke, T.
2014-06-14
Excitation of HeNe by synchrotron light just below the frequency of the 1s → 3p transition of isolated He has been recently shown to be followed by resonant interatomic Coulombic decay (ICD). The vibrationally resolved widths of the ICD states were extracted with high precision from the photoion spectra. In this paper, we report the results of ab initio calculations of these widths. We show that interaction between electronic states at about the equilibrium distance of HeNe makes dark states of He accessible for the photoexcitation and subsequent electronic decay. Moreover, the values of the calculated widths are shown to be strongly sensitive to the presence of the non-adiabatic coupling between the electronic states participating in the decay. Therefore, only by considering the complete manifold of interacting decaying electronic states a good agreement between the measured and computed ICD widths can be achieved.
Ab initio cluster calculations of hydrogenated GaAs(001) surfaces
NASA Astrophysics Data System (ADS)
Fu, Q.; Li, L.; Hicks, R. F.
2000-04-01
Hydrogen adsorption on the (2×4) and (4×2) reconstructions of gallium arsenide (001) has been studied by internal reflectance infrared spectroscopy and ab initio cluster calculations with density-functional theory. The calculations are made on Ga5As4H11,13, Ga4As5H11,13, and Ga7As8H19 clusters, which model the arsenic- and gallium-dimer termination of the semiconductor surface. Excellent agreement has been achieved between the vibrational frequencies predicted by the theory and those observed in experiments. On the (2×4), hydrogen adsorbs on arsenic dimers to form isolated and coupled arsenic-monohydrogen bonds, and arsenic-dihydrogen bonds. Conversely, on the (4×2), hydrogen adsorbs on gallium dimers to form terminal and bridged gallium hydrides. The latter species occur in isolated or coupled structures involving two or three Ga atoms.
Ab Initio Calculation of Structure and Thermodynamic Properties of Zintl Aluminide SrAl2
NASA Astrophysics Data System (ADS)
Fu, Zhi-Jian; Jia, Li-Jun; Xia, Ji-Hong; Tang, Ke; Li, Zhao-Hong; Sun, Xiao-Wei; Chen, Qi-Feng
2015-12-01
The structural and thermodynamic properties of the orthorhombic and cubic structure SrAl2 at pressure and temperature are investigated by using the ab initio plane-wave pseudopotential density functional theory methodwithin the generalised gradient approximation (GGA). The calculated lattice parameters are in agreement with the available experimental data and other theoretical results. The phase transition predicted takes place at 0.5 GPa from the orthorhombic to the cubic structure at zero temperature. The thermodynamic properties of the zinc-blende structure SrAl2 are calculated by the quasi-harmonic Debye model. The pressure-volume relationship and the variations inthe thermal expansion α are obtained systematically in the pressure and temperature ranges of 0-5 GPa and 0-500 K, respectively.
Ab initio molecular orbital calculations of the vibrational frequencies of XY4/sup -n/ anions
NASA Astrophysics Data System (ADS)
Curtiss, L. A.; Nichols, R.
The vibrational frequencies of a seris of XY4/sup -n/ anions (BeF4(-2), BF4(-), AlF4(-), MgCl4(-2), and AlCl4(-)) have been calculated by ab initio molecular orbital theory using the 3-21G and 6-31G* basis sets. The predicted harmonic frequencies are for the most part in good agreement with the observed frequencies of these anions in molten alkali halide mixtures. At the 3-21G basis set level the average difference between the observed and predicted frequencies is 12% while at the 6-31G* basis set level the average difference is 6%. Calculations of this type may be helpful in predicting the vibrational frequencies of other anions in molten salts.
Ab-initio calculations of electronic, transport, and structural properties of boron phosphide
Ejembi, J. I.; Nwigboji, I. H.; Franklin, L.; Malozovsky, Y.; Zhao, G. L.; Bagayoko, D.
2014-09-14
We present results from ab-initio, self-consistent density functional theory calculations of electronic and related properties of zinc blende boron phosphide (zb-BP). We employed a local density approximation potential and implemented the linear combination of atomic orbitals formalism. This technique follows the Bagayoko, Zhao, and Williams method, as enhanced by the work of Ekuma and Franklin. The results include electronic energy bands, densities of states, and effective masses. The calculated band gap of 2.02 eV, for the room temperature lattice constant of a=4.5383 Å, is in excellent agreement with the experimental value of 2.02±0.05 eV. Our result for the bulk modulus, 155.7 GPa, agrees with experiment (152–155 GPa). Our predictions for the equilibrium lattice constant and the corresponding band gap, for very low temperatures, are 4.5269 Å and 2.01 eV, respectively.
Ab initio calculation and spectral properties of nano- and bulk materials
Kulagin, N. A.
2013-01-15
This paper presents the development of ab initio calculation of the electronic structure of either clusters, nano-crystals, doped and unperfected bulk crystals. In addition, analysis of selected experimental data for {gamma}- or plasma irradiated pure and doped wide-band gap oxides such as sapphire, {alpha}-Al{sub 2}O{sub 3}, garnet, Y{sub 3}Al{sub 5}O{sub 12}, Gd{sub 3}Sc{sub 2}Al{sub 3}O{sub 12} and perovskites YAlO{sub 3}, SrTiO{sub 3} is presented. Change in the crystals surface morphology and spectroscopic properties of sapphire, perovskites, garnets as well as ion oxidation state in pure and doped {gamma}- and plasma irradiated crystals are discussed in detail using the optical and X ray spectroscopy experimental results.
Optical spectroscopy of the bulk and interfacial hydrated electron from ab initio calculations.
Uhlig, Frank; Herbert, John M; Coons, Marc P; Jungwirth, Pavel
2014-09-01
The optical spectrum of the hydrated (aqueous) electron, e(aq)(–), is the primary observable by means of which this species is detected, monitored, and studied. In theoretical calculations, this spectrum has most often been simulated using one-electron models. Here, we present ab initio simulations of that spectrum in both bulk water and, for the first time, at the water/vapor interface, using density functional theory and its time-dependent variant. Our results indicate that this approach provides a reliable description, and quantitative agreement with the experimental spectrum for the bulk species is obtained using a “tuned” long-range corrected functional. The spectrum of the interfacial electron is found to be very similar to the bulk spectrum.
Ab initio calculations on etching of graphite and diamond surfaces by atomic hydrogen
Kanai, C.; Watanabe, K.; Takakuwa, Y.
2001-06-15
Etching of graphite and hydrogenated diamond C(100) 2{times}1 surfaces by irradiating atomic hydrogen, which is one of the key reactions to promote epitaxial diamond growth by chemical vapor deposition, has been investigated by ab initio pseudopotential calculations. We demonstrate the reaction pathways and determine the activation energies for breaking C-C bonds on the surfaces by irradiating hydrogen atoms. The activation energy for C-C bond breaking on graphite is found to be only one-half of that on the hydrogenated diamond surface. This indicates that graphite, which is a typical nondiamond phase unnecessarily generated on the diamond surface during epitaxial growth, can be selectively eliminated by atomic hydrogen, resulting in methane desorption. Our result supports the growth rate enhancement in diamond epitaxy observed in a recent experiment by gas-source molecular beam epitaxy under hydrogen beam irradiation.
Ab Initio Infrared Spectra and Electronic Response Calculations for the Insulating Phases of VO2
NASA Astrophysics Data System (ADS)
Hendriks, Christopher; Huffman, Tyler; Walter, Eric; Qazilbash, Mumtaz; Krakauer, Henry
Previous studies have shown that, under doping or tensile strain and upon heating, the well-known vanadium dioxide (VO2) transition from an insulating monoclinic (M1) to a metallic rutile (R) phase progresses through a triclinic symmetry (T) phase and a magnetic monoclinic phase (M2), both of which are insulating. Structurally, this progression from M1 to R through T and M2 can be characterized by the progressive breaking of the V dimers. Investigation of the effect of these structural changes on the insulating phases of VO2 may help resolve questions surrounding the long-debated issue of the respective roles of electronic correlation and Peierls mechanisms in driving the MIT. We investigated electronic and vibrational properties of the insulating phases of VO2 in the framework of DFT+U. We will present ab initio calculations of infrared spectra and optical electronic responses for the insulating phases and compare these to available experimental measurements. Supported by ONR.
Far-infrared spectrum and ab initio calculations for vinylene carbonate
NASA Astrophysics Data System (ADS)
Autrey, D.; del Rosario, A.; Laane, J.
2000-09-01
The far-infrared spectrum of vinylene carbonate shows five closely spaced bands near 233 cm -1, characteristic of a nearly harmonic ring-puckering potential energy function. Using a coordinate dependent kinetic energy expansion, the potential energy function was determined to be V ( cm-1)=1.652×10 5x 2-1.416×10 5x 4 where x is the puckering coordinate in Å. This shows the molecule to be much more rigid than the similar 3-cyclopenten-1-one due to π bonding interactions involving the oxygen atoms adjacent to the carbonyl group. Ab initio calculations with a B3LYP/6-311++G ∗∗ basis set yield structural data in good agreement with the previous microwave work and also predict vibrational frequencies that correspond closely to the experimental values.
Stress reduction of Cu-doped diamond-like carbon films from ab initio calculations
Li, Xiaowei; Ke, Peiling; Wang, Aiying
2015-01-15
Structure and properties of Cu-doped diamond-like carbon films (DLC) were investigated using ab initio calculations. The effect of Cu concentrations (1.56∼7.81 at.%) on atomic bond structure was mainly analyzed to clarify the residual stress reduction mechanism. Results showed that with introducing Cu into DLC films, the residual compressive stress decreased firstly and then increased for each case with the obvious deterioration of mechanical properties, which was in agreement with the experimental results. Structural analysis revealed that the weak Cu-C bond and the relaxation of both the distorted bond angles and bond lengths accounted for the significant reduction of residual compressive stress, while at the higher Cu concentration the increase of residual stress attributed to the existence of distorted Cu-C structures and the increased fraction of distorted C-C bond lengths.
NASA Astrophysics Data System (ADS)
Guo, Guang-Yu; Ishibashi, Shoji; Tamura, Tomoyuki; Terakura, Kiyoyuki
2007-03-01
Since the discovery of carbon nanotubes (CNTs) in 1991 by Iijima, carbon and other nanotubes have attracted considerable interest worldwide because of their unusual properties and also great potentials for technological applications. Though CNTs continue to attract great interest, other nanotubes such as BN nanotubes (BN-NTs) may offer different opportunities that CNTs cannot provide. In this contribution, we present the results of our recent systematic ab initio calculations of the static dielectric constant, electric polarizability, Born dynamical charge, electrostriction coefficient and piezoelectric constant of BN-NTs using the latest crystalline finite electric field theory [1]. [1] I. Souza, J. Iniguez, and D. Vanderbilt, Phys. Rev. Lett. 89, 117602 (2002); P. Umari and A. Pasquarello, Phys. Rev. Lett. 89, 157602 (2002).
Ab initio calculations of the reaction pathways for methane decomposition over the Cu (111) surface
NASA Astrophysics Data System (ADS)
Gajewski, Grzegorz; Pao, Chun-Wei
2011-08-01
Growth of large-area, few-layer graphene has been reported recently through the catalytic decomposition of methane (CH4) over a Cu surface at high temperature. In this study, we used ab initio calculations to investigate the minimum energy pathways of successive dehydrogenation reactions of CH4 over the Cu (111) surface. The geometries and energies of all the reaction intermediates and transition states were identified using the climbing image nudged elastic band method. The activation barriers for CH4 decomposition over this Cu surface are much lower than those in the gas phase; furthermore, analysis of electron density differences revealed significant degrees of charge transfer between the adsorbates and the Cu atoms along the reaction path; these features reveal the role of Cu as the catalytic material for graphene growth. All the dehydrogenation reactions are endothermic, except for carbon dimer (C2) formation, which is, therefore, the most critical step for subsequent graphene growth, in particular, on Cu (111) surface.
Ab Initio Calculated and Experimentally Measured Raman Spectra of Spodumene (LiAlSi2O6)
NASA Astrophysics Data System (ADS)
Stangarone, C.; Prencipe, M.; Mantovani, L.; Bersani, D.; Tribaudino, M.; Lottici, P. P.
2014-06-01
Polarization Raman measurements on spodumene enabled the identification of all 30 active modes (14 Ag and 16 Bg). Ab initio CRYSTAL14 calculations (WC1LYP hamiltonian) give very good agreement for frequencies (ǀΔǀ< 4.8 cm^-1) and intensities.
NASA Astrophysics Data System (ADS)
Yang, Shenyuan; Yoon, Mina; Hicke, Christian; Zhang, Zhenyu; Wang, Enge
2008-09-01
Endohedral metallofullerenes constitute an appealing class of nanoscale building blocks for fabrication of a wide range of materials. One open question of fundamental importance is the precise nature of charge redistribution within the carbon cages (Cn) upon metal encapsulation. Using ab initio density functional theory, we systematically study the electronic structure of metallofullerenes, focusing on the spatial charge redistribution. For large metallofullerenes (n>32) , the valence electrons of the metal atoms are all transferred to the fullerene states. Surprisingly, the transferred charge is found to be highly localized inside the cage near the metal cations rather than uniformly distributed on the surfaces of the carbon cage as traditionally believed. This counterintuitive charge localization picture is attributed to the strong metal-cage interactions within the systems. These findings may prove to be instrumental in the design of fullerene-based functional nanomaterials.
Trends in magnetism of free Rh clusters via relativistic ab-initio calculations.
Šipr, O; Ebert, H; Minár, J
2015-02-11
A fully relativistic ab-initio study on free Rh clusters of 13-135 atoms is performed to identify general trends concerning their magnetism and to check whether concepts which proved to be useful in interpreting magnetism of 3d metals are applicable to magnetism of 4d systems. We found that there is no systematic relation between local magnetic moments and coordination numbers. On the other hand, the Stoner model appears well-suited both as a criterion for the onset of magnetism and as a guide for the dependence of local magnetic moments on the site-resolved density of states at the Fermi level. Large orbital magnetic moments antiparallel to spin magnetic moments were found for some sites. The intra-atomic magnetic dipole Tz term can be quite large at certain sites but as a whole it is unlikely to affect the interpretation of x-ray magnetic circular dichroism experiments based on the sum rules.
NASA Astrophysics Data System (ADS)
Andriyevsky, B.; Doll, K.; Cakmak, G.; Jansen, M.; Niemer, A.; Betzler, K.
2011-09-01
An ab initio density functional theory-based study of the electronic band structure, the elastic, electric, elastoelectric, and linear and nonlinear optical properties of the new ion conductor LiB6O9F, has been performed. The computed band structure reveals a wide direct band gap. The coefficients of the second order nonlinear susceptibility χ(2) were found to be comparable to those of KH2PO4. Corresponding experimental investigations of second harmonic generation comply with the respective ab initio calculations.
NASA Astrophysics Data System (ADS)
Pagès, O.; Souhabi, J.; Torres, V. J. B.; Postnikov, A. V.; Rustagi, K. C.
2012-07-01
We report on the detailed assignment of various features observed in the Raman spectra of SiGe alloys along the linear chain approximation (LCA), as achieved based on remarkable intensity interplays with composition between such neighboring features known from the literature but which so far have not been fully exploited. Such an assignment is independently supported by ab initio calculation of the frequencies of bond-stretching modes taking place in different local environments, which we define at one dimension (1D) for consistency with the LCA. Fair contour modeling of the SiGe Raman spectra is eventually obtained via a so-called 1D-cluster version of the phenomenological (LCA-based) percolation scheme, as originally developed for zincblende alloys, after ab initio calibration of the intrinsic Si-Si, Si-Ge, and Ge-Ge Raman efficiencies. The 1D-cluster scheme introduces a seven-oscillator [1 × (Ge-Ge), 4 × (Si-Ge), 2 × (Si-Si)] Raman behavior for SiGe, which considerably deviates from the currently accepted six-oscillator [1 × (Ge-Ge), 1 × (Si-Ge), 4 × (Si-Si)] behavior. Different numbers of Raman modes per bond are interpreted as different sensitivities to the local environment of Ge-Ge (insensitive), Si-Si (sensitive to first neighbors), and Si-Ge (sensitive to second neighbors) bond stretching. The as-obtained SiGe 1D-cluster/percolation scheme is also compared with the current version for zincblende alloys, using GaAsP as a natural reference. A marked deviation is concerned with an inversion of the like phonon branches in each multiplet. This is attributed either to the considerable Si and Ge phonon dispersions (Si-Si doublet) or to a basic difference in the lattice relaxations of diamond and zincblende alloys (Si-Ge multiplet). The SiGe vs GaAsP comparison is supported by ab initio calculation of the local lattice relaxation/dynamics related to prototype impurity motifs that are directly transposable to the two crystal structures.
Emergent properties of nuclei from ab initio coupled-cluster calculations
Hagen, G.; Hjorth-Jensen, M.; Jansen, G. R.; Papenbrock, T.
2016-05-17
Emergent properties such as nuclear saturation and deformation, and the effects on shell structure due to the proximity of the scattering continuum and particle decay channels are fascinating phenomena in atomic nuclei. In recent years, ab initio approaches to nuclei have taken the first steps towards tackling the computational challenge of describing these phenomena from Hamiltonians with microscopic degrees of freedom. Our endeavor is now possible due to ideas from effective field theories, novel optimization strategies for nuclear interactions, ab initio methods exhibiting a soft scaling with mass number, and ever-increasing computational power. We review some of the recent accomplishments. We also present new results. The recently optimized chiral interaction NNLOmore » $${}_{{\\rm{sat}}}$$ is shown to provide an accurate description of both charge radii and binding energies in selected light- and medium-mass nuclei up to 56Ni. We derive an efficient scheme for including continuum effects in coupled-cluster computations of nuclei based on chiral nucleon–nucleon and three-nucleon forces, and present new results for unbound states in the neutron-rich isotopes of oxygen and calcium. Finally, the coupling to the continuum impacts the energies of the $${J}^{\\pi }=1/{2}^{-},3/{2}^{-},7/{2}^{-},3/{2}^{+}$$ states in $${}^{\\mathrm{17,23,25}}$$O, and—contrary to naive shell-model expectations—the level ordering of the $${J}^{\\pi }=3/{2}^{+},5/{2}^{+},9/{2}^{+}$$ states in $${}^{\\mathrm{53,55,61}}$$Ca.« less
Conductivity of carbon-based molecular junctions from ab-initio methods
NASA Astrophysics Data System (ADS)
Li, Xiao-Fei; Luo, Yi
2014-12-01
Carbon nanomaterials (CNMs) are prompting candidates for next generational electronics. In this review we provide a mini overview of recent results on the conductivity of carbon-based molecular junctions obtained from ab-initio methods. CNMs used as nanoelectrodes and molecular materials in molecular junctions are discussed. The functionalities that include the nanomechanically controlled molecular conductance switches, negative differential resistance devices, and electronic rectifiers realized by using CNMs have been demonstrated.
NASA Astrophysics Data System (ADS)
Xu, Li-Hong; Liu, Qiang; Suenram, R. D.; Lovas, F. J.; Hight Walker, A. R.; Jensen, J. O.; Samuels, A. C.
2004-12-01
The rotational spectra of three low-energy conformers of thiodiglycol (TDG) (HOCH 2CH 2SCH 2CH 2OH) have been measured in a molecular beam using a pulsed-nozzle Fourier-transform microwave spectrometer. To determine the likely conformational structures with ab initio approach, conformational structures of 2-(ethylthio)ethanol (HOEES) (CH 3CH 2SCH 2CH 2OH) were used as starting points together with the consideration of possible intramolecular hydrogen bonding in TDG. Three lower-energy conformers have been found for TDG at the MP2=Full/6311G** level and ab initio results agree nicely with experimentally determined rotational constants. In addition, Stark measurements were performed for two of the three conformers for dipole moment determinations, adding to our confidence of the conformational structure matches between experimental observations and ab initio calculations. Of the three lower-energy conformers, one displays a compact folded-like structure with strong hydrogen bonding between the two hydroxyl groups and the central sulfide atom. Two other conformers have relatively open chain-like structures with hydrogen bonding between each of the hydroxyl groups to the central sulfur atom, of which one has pure b-type dipole moment according to the ab initio results.
Moura, Gustavo L C; Simas, Alfredo M
2012-04-01
In this article, we advance the foundations of a strategy to develop a molecular mechanics method based not on classical mechanics and force fields but entirely on quantum mechanics and localized electron-pair orbitals, which we call quantum molecular mechanics (QMM). Accordingly, we introduce a new manner of calculating Hartree-Fock ab initio wavefunctions of closed shell systems based on variationally preoptimized nonorthogonal electron pair orbitals constructed by linear combinations of basis functions centered on the atoms. QMM is noniterative and requires only one extremely fast inversion of a single sparse matrix to arrive to the one-particle density matrix, to the electron density, and consequently, to the ab initio electrostatic potential around the molecular system, or cluster of molecules. Although QMM neglects the smaller polarization effects due to intermolecular interactions, it fully takes into consideration polarization effects due to the much stronger intramolecular geometry distortions. For the case of methane, we show that QMM was able to reproduce satisfactorily the energetics and polarization effects of all distortions of the molecule along the nine normal modes of vibration, well beyond the harmonic region. We present the first practical applications of the QMM method by examining, in detail, the cases of clusters of helium atoms, hydrogen molecules, methane molecules, as well as one molecule of HeH(+) surrounded by several methane molecules. We finally advance and discuss the potentialities of an exact formula to compute the QMM total energy, in which only two center integrals are involved, provided that the fully optimized electron-pair orbitals are known.
Gall, D.; Sta''dele, M.; Ja''rrendahl, K.; Petrov, I.; Desjardins, P.; Haasch, R. T.; Lee, T.-Y.; Greene, J. E.
2001-03-15
Experimental and ab initio computational methods are employed to conclusively show that ScN is a semiconductor rather than a semimetal; i.e., there is a gap between the N 2p and the Sc 3d bands. Previous experimental investigators reported, in agreement with band structure calculations showing a band overlap of 0.2 eV, that ScN is a semimetal while others concluded that it is a semiconductor with a band gap larger than 2 eV. We have grown high quality, single crystalline ScN layers on MgO(001) and on TiN(001) buffer layers on MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition. ScN optical properties were determined by transmission, reflection, and spectroscopic ellipsometry while in-situ x-ray and ultraviolet valence band photoelectron spectroscopy were used to determine the density of states (DOS) below the Fermi level. The measured DOS exhibits peaks at 3.8 and 5.2 eV stemming from the N 2p bands and at 15.3 eV due to the N 2s bands. The imaginary part of the measured dielectric function {epsilon}{sub 2} consists of two primary features due to direct X- and {Gamma}-point transitions at photon energies of 2.7 and 3.8 eV, respectively. For comparison, the ScN band structure was calculated using an ab initio Kohn--Sham approach which treats the exchange interactions exactly within density-functional theory. Calculated DOS and the complex dielectric function are in good agreement with our ScN valence-band photoelectron spectra and measured optical properties, respectively. We conclude, combining experimental and computational results, that ScN is a semiconductor with an indirect {Gamma}--X bandgap of 1.3{+-}0.3eV and a direct X-point gap of 2.4{+-}0.3eV.
NASA Astrophysics Data System (ADS)
Aghabozorg, Hossein; Manteghi, Faranak; Ghadermazi, Mohammad; Mirzaei, Masoud; Salimi, Ali R.; Shokrollahi, Ardeshir; Derki, Somayyeh; Eshtiagh-Hosseini, Hossein
2009-02-01
A novel compound with formula unit (pipzH 2)[Cd(pydc) 2]·6H 2O, was synthesized and characterized by IR, 1H NMR and 13C NMR spectroscopy, elemental analysis, single crystal X-ray diffractometry. Moreover, the solution behavior was studied, and ab initio HF, DFT calculations were carried out. The compound belongs to a great family of supramolecular metal complexes derived from a proton transfer ion pair i.e. (pipzH 2)(pydc), where pipz is piperazine and pydcH 2 is pyridine-2,6-dicarboxylic acid. The compound shows a distorted octahedral geometry around the six-coordinated Cd II atom. A variety of intermolecular O sbnd H···O, N sbnd H···O and C sbnd H···O hydrogen bonds are responsible to extend the supramolecular network of the compound. The geometry parameters of [Cd(pydc) 2] 2- complex and free (pydc) 2- ligand have been optimized with the B3LYP method of density functional theory (DFT) and ab initio Hartree-Fock (HF) methods for comparison. The effect of basis sets has been investigated using four combination basis sets. The electronic properties of the Cd II compound and free (pydc) 2- ligand have been investigated based on the natural bond orbital (NBO) analysis at the B3LYP/A level of theory which verifies that the synergistic effect have been occurred in the compound. In solution study, the protonation constants of pipz and pydc, the equilibrium constants for pydc/pipz proton transfer system and the stoichiometry and stability of the system with Cd 2+ ion in aqueous solution were investigated by potentiometric pH titrations. The stoichiometry of one of the most abundant complexed species in solution was found to be the same as that of the crystalline cadmium complex.
NASA Astrophysics Data System (ADS)
Iwano, Sakae; Kawashima, Yoshiyuki; Hirota, Eizi
2016-06-01
We have systematically investigated the van der Waals complexes consisting of the one from each of the two groups: (Rg, CO, N_2 or CO_2) and (dimethyl ether, dimethyl sulfide, ethylene oxide or ethylene sulfide), by using Fourier transform microwave spectroscopy supplemented by ab initio MO calculations, in order to understand the dynamical behavior of van der Waals complexes and to obtain information on the potential function to internal motions in complexes. Two examples of the N_2 complex were investigated: N_2-DME (dimethyl ether), for which we reported a preliminary result and N_2-EO (ethylene oxide). In the present study we focused attention to the N_2-ES (ethylene sulfide) complex. We have detected two sets of the {b}-type transitions for the 15N_2-ES in ortho and para states, and have analyzed them by using the asymmetric-rotor program of {A}-reduction. In contrast with the N_2-EO, for which each of the ortho and para states were found split into a strong/weak pair, only some transitions of the 15N_2-ES were accompanied by two or three components. The observed spectra of the 14N_2-ES were complicated because of hyperfine splittings due to the nuclear quadrupole coupling of the two nitrogen atoms. We concluded that the N_2 moiety was located in the plane perpendicular to the C-S-C plane and bisecting the CSC angle of the ES. Two isomers were expected to exist for 15NN-ES, one with 15N in the inner and the other in the outer position, and in fact two sets of the spectra were detected. We have carried out ab initio molecular orbital calculations at the level of MP2 with basis sets 6-311++G(d, p), aug-cc-pVDZ, and aug-cc-pVTZ, to complement the information on the intracomplex motions obtained from the observed rotational spectra. Y. Kawashima, A. Sato, Y. Orita, and E. Hirota, J. Phys. Chem. A, 2012 116, 1224 Y. Kawashima, Y. Tatamitani, Y. Morita, and E. Hirota, 61st International Symposium on Molecular Spectroscopy, TE10 (2006) Y. Kawashima and E. Hirota, J
Ab initio calculations of the electronic structure and bonding characteristics of LaB6
NASA Astrophysics Data System (ADS)
Hossain, Faruque M.; Riley, Daniel P.; Murch, Graeme E.
2005-12-01
Lanthanum hexaboride ( LaB6 , NIST SRM-660a) is widely used as a standard reference material for calibrating the line position and line shape parameters of powder diffraction instruments. The accuracy of this calibration technique is highly dependent on how completely the reference material is characterized. Critical to x-ray diffraction, this understanding must include the valence of the La atomic position, which in turn will influence the x-ray form factor (f) and hence the diffracted intensities. The electronic structure and bonding properties of LaB6 have been investigated using ab initio plane-wave pseudopotential total energy calculations. The electronic properties and atomic bonding characteristics were analyzed by estimating the energy band structure and the density of states around the Fermi energy level. The calculated energy band structure is consistent with previously reported experimental findings; de Haas-van Alphen and two-dimensional angular correlation of electron-positron annihilation radiation. In addition, the bond strengths and types of atomic bonds in the LaB6 compound were estimated by analyzing the Mulliken charge density population. The calculated result revealed the coexistence of covalent, ionic, and metallic bonding in the LaB6 system and partially explains its high efficiency as a thermionic emitter.
NASA Astrophysics Data System (ADS)
Freeman, Fillmore; Tsegai, Zufan M.; Kasner, Marc L.; Hehre, Warren J.
2000-05-01
Ab initio 6-31G(d) and MP2/6-31G(d)//6-31G(d) methods were used to calculate the energies of the rotamers of the chair conformers of alkylcyclohexanes and trimethylsilylcyclohexane. The MP2/6-31G(d)//6-31G(d) calculated conformational energies ( ? or A values, in kcal/mol) of the alkylcyclohexanes (Me = 1.96; Et = 1.80; Pr = 1.73 iso-Pr = 1.60; t-Bu = 5.45; neo-pent = 1.32) and trimethylsilylcyclohexane (SiMe3 = 2.69) are similar to the experimental values. Plots of the calculated conformational energies for the alkylcyclohexanes and trimethylsilylcyclohexane versus their experimental values are linear (slope = 1.253 and r = .993 for 6-31G(d) and slope = 1.114 and r = .982 for MP2/6-31G(d)//6-31G(d)). The conformational energies are determined primarily by steric effects which include gauche (synclinal) interactions and repulsive nonbonded interactions in both the axial and equatorial conformers.
Formation of star nanowires of sulfur-doped zinc oxide: Ab initio calculations
NASA Astrophysics Data System (ADS)
Lin, Qiu-Bao; Wu, Shun-Qing; Zhu, Zi-Zhong
2016-09-01
ZnO nanowires are hexagonally shaped under normal growth conditions, but are transformed from a hexagon to hexagram ones when sulfur dopants are added into the growth solution. The formation mechanism of the hexagram-shaped ZnO nanowires is further studied by the ab initio calculations in this paper. The present calculations support the fact that the hexagonally shaped ZnO nanowires are transformed to hexagram shaped ones when the O atoms on the side surfaces of the nanowires are replaced by S atoms in certain quantities. It indicates that the ratio of sulfur content plays an important role in the hexagram formation. The results of the electronic charge densities indicate that the charge transfer makes the S-Zn bond longer than that of O-Zn. The new charge distribution on the side planes due to the S atoms replacement leads to the formation of the hexagram-shaped nanowires. The calculation on the electronic properties shows that a sulfur-doped hexagram ZnO nanowire is an indirect band gap semiconductor with a narrow gap. When dopant is increased, the gap will decrease.
Ab-initio Calculation of Optoelectronic and Structural Properties of Cubic Lithium Oxide (Li2O)
NASA Astrophysics Data System (ADS)
Ziegler, Joshua; Polin, Daniel; Malozovsky, Yuriy; Bagayoko, Diola
Using the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF), we performed ab-initio, density functional theory (DFT) calculations of optoelectronic, transport, and bulk properties of Li2S. In so doing, we avoid ``band gap'' and problems plaguing many DET calculations [AIP Advances 4, 127104 (2014)]. We employed a local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO). With the BZW-EF method, our results possess the full, physical content of DFT and agree with available, corresponding experimental ones. In particular, we found a room temperature indirect band gap of 6.659 eV that compares favorably with experimental values ranging from 5 to 7.99 eV. We also calculated total and partial density of states (DOS and PDOS), effective masses of charge carriers, the equilibrium lattice constant, and the bulk modulus. Acknowledgments: This work was funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy - National, Nuclear Security Administration (NNSA) (Award Nos. DE-NA0001861 and DE- NA0002630), LaSPACE, and LONI-SUBR.
Integration of ab-initio nuclear calculation with derivative free optimization technique
Sharda, Anurag
2008-01-01
Optimization techniques are finding their inroads into the field of nuclear physics calculations where the objective functions are very complex and computationally intensive. A vast space of parameters needs searching to obtain a good match between theoretical (computed) and experimental observables, such as energy levels and spectra. Manual calculation defies the scope of such complex calculation and are prone to error at the same time. This body of work attempts to formulate a design and implement it which would integrate the ab initio nuclear physics code MFDn and the VTDIRECT95 code. VTDIRECT95 is a Fortran95 suite of parallel code implementing the derivative-free optimization algorithm DIRECT. Proposed design is implemented for a serial and parallel version of the optimization technique. Experiment with the initial implementation of the design showing good matches for several single-nucleus cases are conducted. Determination and assignment of appropriate number of processors for parallel integration code is implemented to increase the efficiency and resource utilization in the case of multiple nuclei parameter search.
Determination of a silane intermolecular force field potential model from an ab initio calculation
Li, Arvin Huang-Te; Chao, Sheng D.; Chang, Chien-Cheng
2010-12-15
Intermolecular interaction potentials of the silane dimer in 12 orientations have been calculated by using the Hartree-Fock (HF) self-consistent theory and the second-order Moeller-Plesset (MP2) perturbation theory. We employed basis sets from Pople's medium-size basis sets [up to 6-311++G(3df, 3pd)] and Dunning's correlation consistent basis sets (up to the triply augmented correlation-consistent polarized valence quadruple-zeta basis set). We found that the minimum energy orientations were the G and H conformers. We have suggested that the Si-H attractions, the central silicon atom size, and electronegativity play essential roles in weakly binding of a silane dimer. The calculated MP2 potential data were employed to parametrize a five-site force field for molecular simulations. The Si-Si, Si-H, and H-H interaction parameters in a pairwise-additive, site-site potential model for silane molecules were regressed from the ab initio energies.
Ab initio calculation of the electronic absorption spectrum of liquid water
NASA Astrophysics Data System (ADS)
Martiniano, Hugo F. M. C.; Galamba, Nuno; Cabral, Benedito J. Costa
2014-04-01
The electronic absorption spectrum of liquid water was investigated by coupling a one-body energy decomposition scheme to configurations generated by classical and Born-Oppenheimer Molecular Dynamics (BOMD). A Frenkel exciton Hamiltonian formalism was adopted and the excitation energies in the liquid phase were calculated with the equation of motion coupled cluster with single and double excitations method. Molecular dynamics configurations were generated by different approaches. Classical MD were carried out with the TIP4P-Ew and AMOEBA force fields. The BLYP and BLYP-D3 exchange-correlation functionals were used in BOMD. Theoretical and experimental results for the electronic absorption spectrum of liquid water are in good agreement. Emphasis is placed on the relationship between the structure of liquid water predicted by the different models and the electronic absorption spectrum. The theoretical gas to liquid phase blue-shift of the peak positions of the electronic absorption spectrum is in good agreement with experiment. The overall shift is determined by a competition between the O-H stretching of the water monomer in liquid water that leads to a red-shift and polarization effects that induce a blue-shift. The results illustrate the importance of coupling many-body energy decomposition schemes to molecular dynamics configurations to carry out ab initio calculations of the electronic properties in liquid phase.
Baima, Jacopo; Zelferino, Alessandro; Olivero, Paolo; Erba, Alessandro; Dovesi, Roberto
2016-01-21
Quantum-mechanical ab initio calculations are performed to elucidate the vibrational spectroscopic features of a common irradiation-induced defect in diamond, i.e. the neutral vacancy. Raman spectra are computed analytically through a Coupled-Perturbed-Hartree-Fock/Kohn-Sham approach as a function of both different defect spin states and defect concentration. The experimental Raman features of defective diamond located in the 400-1300 cm(-1) spectral range, i.e. below the first-order line of pristine diamond at 1332 cm(-1), are well reproduced, thus corroborating the picture according to which, at low damage densities, this spectral region is mostly affected by non-graphitic sp(3) defects. No peaks above 1332 cm(-1) are found, thus ruling out previous tentative assignments of different spectral features (at 1450 and 1490 cm(-1)) to the neutral vacancy. The perturbation introduced by the vacancy to the thermal nuclear motion of carbon atoms in the defective lattice is discussed in terms of atomic anisotropic displacement parameters (ADPs), computed from converged lattice dynamics calculations. PMID:26686374
Ab initio calculation of structural stability, electronic and optical properties of Ag{sub 2}Se
Rameshkumar, S.; Jayalakshmi, V.; Jaiganesh, G.; Palanivel, B.
2015-06-24
The structural stability, electronic and optical properties of Ag{sub 2}Se compound is studied using ab initio packages. Ag{sub 2}Se is found to crystallize in orthorhombic structure with two different space groups i.e. P2{sub 1}2{sub 1}2{sub 1} (No. 19) and P222{sub 1} (No. 17). For this compound in these two space groups, the total energy has been computed as a function of volume. Our calculated results suggest that the P2{sub 1}2{sub 1}2{sub 1}–phase is more stable than that of the P222{sub 1}–phase. The band structure calculation show that Ag{sub 2}Se is semimetallic with an overlap of about 0.014 eV in P2{sub 1}2{sub 1}2{sub 1}–phase whereas is metallic in nature in P222{sub 1}–phase. Moreover, the optical properties including the dielectric function, energy loss spectrum are obtained and analysed.
Ab initio Calculations of Charge Symmetry Breaking in the A =4 Hypernuclei
NASA Astrophysics Data System (ADS)
Gazda, Daniel; Gal, Avraham
2016-03-01
We report on ab initio no-core shell model calculations of the mirror Λ hypernuclei H4Λ and He4Λ , using the Bonn-Jülich leading-order chiral effective field theory hyperon-nucleon potentials plus a charge symmetry breaking Λ -Σ0 mixing vertex. In addition to reproducing rather well the 0g.s . + and 1exc+ binding energies, these four-body calculations demonstrate for the first time that the observed charge symmetry breaking splitting of mirror levels, reaching hundreds of keV for 0g.s . +, can be reproduced using realistic theoretical interaction models, although with a non-negligible momentum cutoff dependence. Our results are discussed in relation to recent measurements of the H4Λ(0g.s . +) binding energy at the Mainz Microtron [A. Esser et al. (A1 Collaboration), Phys. Rev. Lett. 114, 232501 (2015)] and the He4Λ(1exc+) excitation energy [T.O. Yamamoto et al. (J-PARC E13 Collaboration), Phys. Rev. Lett. 115, 222501 (2015)].
NASA Astrophysics Data System (ADS)
Barabash, Sergey V.; Pramanik, Dipankar
2015-03-01
Development of low-leakage dielectrics for semiconductor industry, together with many other areas of academic and industrial research, increasingly rely upon ab initio tunneling and transport calculations. Complex band structure (CBS) is a powerful formalism to establish the nature of tunneling modes, providing both a deeper understanding and a guided optimization of materials, with practical applications ranging from screening candidate dielectrics for lowest ``ultimate leakage'' to identifying charge-neutrality levels and Fermi level pinning. We demonstrate that CBS is prone to a particular type of spurious ``phantom'' solution, previously deemed true but irrelevant because of a very fast decay. We demonstrate that (i) in complex materials, phantom modes may exhibit very slow decay (appearing as leading tunneling terms implying qualitative and huge quantitative errors), (ii) the phantom modes are spurious, (iii) unlike the pseudopotential ``ghost'' states, phantoms are an apparently unavoidable artifact of large numerical basis sets, (iv) a presumed increase in computational accuracy increases the number of phantoms, effectively corrupting the CBS results despite the higher accuracy achieved in resolving the true CBS modes and the real band structure, and (v) the phantom modes cannot be easily separated from the true CBS modes. We discuss implications for direct transport calculations. The strategy for dealing with the phantom states is discussed in the context of optimizing high-quality high- κ dielectric materials for decreased tunneling leakage.
NASA Astrophysics Data System (ADS)
Mullaney, John C.; Zaleski, Daniel P.; Tew, David Peter; Walker, Nick; Legon, Anthony
2016-06-01
An isolated, gas-phase dimer of imidazole is generated through laser vaporisation of a solid rod containing a 1:1 mixture of imidazole and copper in the presence of an argon buffer gas undergoing supersonic expansion. The complex is characterised through broadband rotational spectroscopy and is shown to have a twisted, hydrogen-bonded geometry. Calculations at the CCSD(T)(F12*)/cc-pVDZ-F12 level of theory confirm this to be the lowest-energy conformer of the imidazole dimer. The distance between the respective centres of mass of the imidazole monomer subunits is determined to be 5.2751(1) Å, and the twist angle γ describing rotation of one monomer with respect to the other about a line connecting the centres of mass of the monomers is determined to be 87.9(4)o. Four out of six intermolecular parameters in the model geometry are precisely determined from the experimental rotational constants and are consistent with results calculated ab initio.
Varnali, Tereza; Edwards, Howell G M
2010-07-13
The recognition that scytonemin, the radiation protectant pigment produced by extremophilic cyanobacterial colonies in stressed terrestrial environments, is a key biomarker for extinct or extant life preserved in geological scenarios is critically important for the detection of life signatures by remote analytical instrumentation on planetary surfaces and subsurfaces. The ExoMars mission to seek life signatures on Mars is just one experiment that will rely upon the detection of molecules such as scytonemin in the Martian regolith. Following a detailed structural analysis of the parent scytonemin, we report here for the first time a similar analysis of several of its methoxy derivatives that have recently been extracted from stressed cyanobacteria. Ab initio calculations have been carried out to determine the most stable molecular configurations, and the implications of the structural changes imposed by the methoxy group additions on the spectral characteristics of the parent molecule are discussed. The calculated electronic absorption bands of the derivative molecules reveal that their capability of removing UVA wavelengths is removed while preserving the ability to absorb the shorter wavelength UVB and UVC radiation, in contrast to scytonemin itself. This is indicative of a special role for these molecules in the protective strategy of the cyanobacterial extremophiles. PMID:20529954
Ab initio calculation of the electronic absorption spectrum of liquid water
Martiniano, Hugo F. M. C.; Galamba, Nuno; Cabral, Benedito J. Costa
2014-04-28
The electronic absorption spectrum of liquid water was investigated by coupling a one-body energy decomposition scheme to configurations generated by classical and Born-Oppenheimer Molecular Dynamics (BOMD). A Frenkel exciton Hamiltonian formalism was adopted and the excitation energies in the liquid phase were calculated with the equation of motion coupled cluster with single and double excitations method. Molecular dynamics configurations were generated by different approaches. Classical MD were carried out with the TIP4P-Ew and AMOEBA force fields. The BLYP and BLYP-D3 exchange-correlation functionals were used in BOMD. Theoretical and experimental results for the electronic absorption spectrum of liquid water are in good agreement. Emphasis is placed on the relationship between the structure of liquid water predicted by the different models and the electronic absorption spectrum. The theoretical gas to liquid phase blue-shift of the peak positions of the electronic absorption spectrum is in good agreement with experiment. The overall shift is determined by a competition between the O–H stretching of the water monomer in liquid water that leads to a red-shift and polarization effects that induce a blue-shift. The results illustrate the importance of coupling many-body energy decomposition schemes to molecular dynamics configurations to carry out ab initio calculations of the electronic properties in liquid phase.
Transport coefficients of helium-argon mixture based on ab initio potential.
Sharipov, Felix; Benites, Victor J
2015-10-21
The viscosity, thermal conductivity, diffusion coefficient, and thermal diffusion factor of helium-argon mixtures are calculated for a wide range of temperature and for various mole fractions up to the 12th order of the Sonine polynomial expansion with an ab initio intermolecular potential. The calculated values for these transport coefficients are compared with other data available in the open literature. The comparison shows that the obtained transport coefficients of helium-argon mixture have the best accuracy for the moment. PMID:26493894
Joubert, J.-M.; Colinet, C.; Rodrigues, G.; Suzuki, P.A.; Nunes, C.A.; Coelho, G.C.; Tedenac, J.-C.
2012-06-15
The solid solution based on Nb{sub 5}Si{sub 3} (Cr{sub 5}B{sub 3} structure type, D8{sub l}, tI32, I4/mcm, No140, a=6.5767 A, c=11.8967 A) in the Nb-Si-B system was studied from the structural and thermodynamic point of view both experimentally and by ab initio calculations. Rietveld refinement of powder X-ray synchrotron data allowed to determine the boron to silicon substitution mechanism and the structural parameters. Ab initio calculations of different ordered compounds and selected disordered alloys allowed to obtain in addition to the enthalpy of formation of the solution, substitution mechanism and structural parameters which are in excellent agreement with the experimental data. The stability of the phase is discussed. - Graphial abstract: Valence-charge electron localization function in the z=0 plane of the D8{sub l} structure for the ordered compound Nb{sub 5}SiB{sub 2}. Highlights: Black-Right-Pointing-Pointer Coupling between ab initio data and experimental results from synchrotron powder diffraction. Black-Right-Pointing-Pointer Excellent agreement between the two techniques for the site occupancies and internal coordinates. Black-Right-Pointing-Pointer Explanation of the phase stability up to Nb{sub 5}SiB{sub 2}.
Ab initio calculations of the reaction pathways for methane decomposition over the Cu (111) surface.
Gajewski, Grzegorz; Pao, Chun-Wei
2011-08-14
Growth of large-area, few-layer graphene has been reported recently through the catalytic decomposition of methane (CH(4)) over a Cu surface at high temperature. In this study, we used ab initio calculations to investigate the minimum energy pathways of successive dehydrogenation reactions of CH(4) over the Cu (111) surface. The geometries and energies of all the reaction intermediates and transition states were identified using the climbing image nudged elastic band method. The activation barriers for CH(4) decomposition over this Cu surface are much lower than those in the gas phase; furthermore, analysis of electron density differences revealed significant degrees of charge transfer between the adsorbates and the Cu atoms along the reaction path; these features reveal the role of Cu as the catalytic material for graphene growth. All the dehydrogenation reactions are endothermic, except for carbon dimer (C(2)) formation, which is, therefore, the most critical step for subsequent graphene growth, in particular, on Cu (111) surface. PMID:21842949
Hydrogen-vacancy interaction in bcc iron: ab initio calculations and thermodynamics
NASA Astrophysics Data System (ADS)
Mirzaev, D. A.; Mirzoev, A. A.; Okishev, K. Yu.; Verkhovykh, A. V.
2014-07-01
The paper presents results of ab initio modelling of formation energies of vacancy-hydrogen complexes VHn and an extended variant of thermodynamic theory describing equilibrium concentrations of such complexes. A single H atom is shifted from vacancy to a neighbouring O-site by 1.19 Å. Two H atoms in a vacancy form a dumbbell with H-H distance of 2.38 Å being much greater than in H2 molecule. Configurations of three, four and five H atoms in a vacancy are more complex, and H-H distances gradually increase showing repulsion between hydrogen atoms. Binding energy of a VHn-1 complex with the next hydrogen atom to form VHn is 0.60, 0.61, 0.39, 0.37 and 0.31 for n = 1-5, which is close to other researchers' data. These results were used to construct an improved variant of thermodynamic description of vacancy-hydrogen interaction in a bcc solid solution taking into account both binding energies and hydrogen atom configurations in different VHn complexes. Calculations show that at low temperatures most vacancies are bound to several hydrogen atoms, and the equilibrium concentration of vacancies themselves significantly increases, in accordance with existing experiments.
Patil, Amol Baliram; Bhanage, Bhalchandra Mahadeo
2016-06-21
The nature of bonding interactions between the cation and the anion of an ionic liquid is at the heart of understanding ionic liquid properties. A particularly interesting case is a special class of ionic liquids known as protic ionic liquids. The extent of proton transfer in protic ionic liquids has been observed to vary according to the interacting species. Back proton transfer renders protic ionic liquids volatile and to be considered as inferior ionic liquids. We try to address this issue by employing modern ab initio valence bond theory calculations. The results indicate that the bonding in the cation and the anion of a prototypical ionic liquid, ethylammonium nitrate, is fundamentally different. It is neither characteristic of covalent/polar covalent bonding nor ionic bonding but rather charge shift bonding as a resonance hybrid of two competing ionic molecular electronic structure configurations. An investigation of other analogous protic ionic liquids reveals that this charge shift bonding seems to be a typical characteristic of protic ionic liquids while the ionic solid analogue compound ammonium nitrate has less charge shift bonding character as compared to protic ionic liquids. Further the extent of charge shift bonding character has been found to be congruent with the trends in many physicochemical properties such as melting point, conductivity, viscosity, and ionicity of the studied ionic liquids indicating that percentage charge shift character may serve as a key descriptor for large scale computational screening of ionic liquids with desired properties.
Patil, Amol Baliram; Bhanage, Bhalchandra Mahadeo
2016-06-21
The nature of bonding interactions between the cation and the anion of an ionic liquid is at the heart of understanding ionic liquid properties. A particularly interesting case is a special class of ionic liquids known as protic ionic liquids. The extent of proton transfer in protic ionic liquids has been observed to vary according to the interacting species. Back proton transfer renders protic ionic liquids volatile and to be considered as inferior ionic liquids. We try to address this issue by employing modern ab initio valence bond theory calculations. The results indicate that the bonding in the cation and the anion of a prototypical ionic liquid, ethylammonium nitrate, is fundamentally different. It is neither characteristic of covalent/polar covalent bonding nor ionic bonding but rather charge shift bonding as a resonance hybrid of two competing ionic molecular electronic structure configurations. An investigation of other analogous protic ionic liquids reveals that this charge shift bonding seems to be a typical characteristic of protic ionic liquids while the ionic solid analogue compound ammonium nitrate has less charge shift bonding character as compared to protic ionic liquids. Further the extent of charge shift bonding character has been found to be congruent with the trends in many physicochemical properties such as melting point, conductivity, viscosity, and ionicity of the studied ionic liquids indicating that percentage charge shift character may serve as a key descriptor for large scale computational screening of ionic liquids with desired properties. PMID:27229870
NASA Astrophysics Data System (ADS)
Salem, Mostafa E.; Ahmed, Ashour A.; Shaaban, Mohamed R.; Shibl, Mohamed F.; Farag, Ahmad M.
2015-09-01
Pyrazolo[1,5-a]pyrimidine, triazolo[1,5-a]pyrimidine, and pyrimido[1,2-a]benzimidazole, pyrido[1,2-a]benzimidazole ring systems incorporating phenylsulfonyl moiety were synthesized via the reaction of 3-(N,N-dimethylamino)-1-(thiophen-2-yl)-2-(phenylsulfonyl)prop-2-en-1-one derivatives with the appropriate aminoazoles as 1,3-binucleophiles and 1H-benzimidazol-2-ylacetonitrile using conventional methods as well as microwave irradiation. The regioselectivity of the cyclocondensation reactions was confirmed both experimentally by alternative synthesis of reaction products and theoretically using ab initio quantum chemical calculations namely the Density Functional Theory (DFT). The theoretical work was carried out using the Becke, three parameter, Lee-Yang-Parr hybrid functional (B3LYP) combined with the 6-311++G(d,p) basis set. It was found that the final cyclocondensation reaction product depends mainly on the initial addition to the activated double bond by the nitrogen atom of the 1,3-binucleophiles that has the higher electron density.
Zhao, Hailiang; Pierloot, Kristine; Langner, Ernie H G; Swarts, Jannie C; Conradie, Jeanet; Ghosh, Abhik
2012-04-01
Manganese(V)-oxo corrole and corrolazine have been studied with ab initio multiconfiguration reference methods (CASPT2 and RASPT2) and large atomic natural orbital (ANO) basis sets. The calculations confirm the expected singlet d(δ)(2) ground states for both complexes and rule out excited states within 0.5 eV of the ground states. The lowest excited states are a pair of Mn(V) triplet states with d(δ)(1)d(π)(1) configurations 0.5-0.75 eV above the ground state. Manganese(IV)-oxo macrocycle radical states are much higher in energy, ≥1.0 eV relative to the ground state. The macrocyclic ligands in the ground states of the complexes are thus unambiguously 'innocent'. The approximate similarity of the spin state energetics of the corrole and corrolazine complexes suggests that the latter macrocycle on its own does not afford any special stabilization for the Mn(V)O center. The remarkable stability of an Mn(V)O octaarylcorrolazine thus appears to be ascribable to the steric protection afforded by the β-aryl groups. PMID:22432719
Ab initio many-body calculations of nucleon-4He scattering with three-nucleon forces
Hupin, Guillaume; Langhammer, Joachim; Navratil, Petr; Quaglioni, Sofia; Calci, Angelo; Roth, Robert
2013-11-27
We extend the ab initio no-core shell model/resonating-group method to include three-nucleon (3N) interactions for the description of nucleon-nucleus collisions. We outline the formalism, give algebraic expressions for the 3N-force integration kernels, and discuss computational aspects of two alternative implementations. The extended theoretical framework is then applied to nucleon-4He elastic scattering using similarity-renormalization-group (SRG)-evolved nucleon-nucleon plus 3N potentials derived from chiral effective field theory. We analyze the convergence properties of the calculated phase shifts and explore their dependence upon the SRG evolution parameter. We include up to six excited states of the 4He target and find significant effects from themore » inclusion of the chiral 3N force, e.g., it enhances the spin-orbit splitting between the 3/2– and 1/2– resonances and leads to an improved agreement with the phase shifts obtained from an accurate R-matrix analysis of the five-nucleon experimental data. As a result, we find remarkably good agreement with measured differential cross sections at various energies below the d+3H threshold, while analyzing powers manifest larger deviations from experiment for certain energies and angles.« less
Efficacy of the SU(3) scheme for ab initio large-scale calculations beyond the lightest nuclei
NASA Astrophysics Data System (ADS)
Dytrych, T.; Maris, P.; Launey, K. D.; Draayer, J. P.; Vary, J. P.; Langr, D.; Saule, E.; Caprio, M. A.; Catalyurek, U.; Sosonkina, M.
2016-10-01
We report on the computational characteristics of ab initio nuclear structure calculations in a symmetry-adapted no-core shell model (SA-NCSM) framework. We examine the computational complexity of the current implementation of the SA-NCSM approach, dubbed LSU3shell, by analyzing ab initio results for 6Li and 12C in large harmonic oscillator model spaces and SU3-selected subspaces. We demonstrate LSU3shell's strong-scaling properties achieved with highly-parallel methods for computing the many-body matrix elements. Results compare favorably with complete model space calculations and significant memory savings are achieved in physically important applications. In particular, a well-chosen symmetry-adapted basis affords memory savings in calculations of states with a fixed total angular momentum in large model spaces while exactly preserving translational invariance.
Ab initio Study of Naptho-Homologated DNA Bases
Sumpter, Bobby G; Vazquez-Mayagoitia, Alvaro; Huertas, Oscar; Fuentes-Cabrera, Miguel A; Orozco, Modesto; Luque, Javier
2008-01-01
Naptho-homologated DNA bases have been recently used to build a new type of size expanded DNA known as yyDNA. We have used theoretical techniques to investigate the structure, tautomeric preferences, base-pairing ability, stacking interactions, and HOMO-LUMO gaps of the naptho-bases. The structure of these bases is found to be similar to that of the benzo-fused predecessors (y-bases) with respect to the planarity of the aromatic rings and amino groups. Tautomeric studies reveal that the canonical-like form of naptho-thymine (yyT) and naptho-adenine (yyA) are the most stable tautomers, leading to hydrogen-bonded dimers with the corresponding natural nucleobases that mimic the Watson-Crick pairing. However, the canonical-like species of naptho-guanine (yyG) and naptho-cytosine (yyC) are not the most stable tautomers, and the most favorable hydrogen-bonded dimers involve wobble-like pairings. The expanded size of the naphto-bases leads to stacking interactions notably larger than those found for the natural bases, and they should presumably play a dominant contribution in modulating the structure of yyDNA duplexes. Finally, the HOMO-LUMO gap of the naptho-bases is smaller than that of their benzo-base counterparts, indicating that size-expansion of DNA bases is an efficient way of reducing their HOMO-LUMO gap. These results are examined in light of the available experimental evidence reported for yyT and yyC.
Ab initio theory of iron-based superconductors
NASA Astrophysics Data System (ADS)
Essenberger, F.; Sanna, A.; Buczek, P.; Ernst, A.; Sandratskii, L.; Gross, E. K. U.
2016-07-01
We report a first-principles study of the superconducting critical temperature and other properties of Fe-based superconductors taking into account, on equal footing, phonon, charge, and spin-fluctuation mediated Cooper pairing. We show that in FeSe this leads to a modulated s ± gap symmetry and that the antiferromagnetic paramagnons are the leading mechanism for superconductivity in FeSe, overcoming the strong repulsive effect of both phonons and charge pairing.
Kostadinova, O.; Chrissanthopoulos, A.; Petkova, T.; Petkov, P.; Yannopoulos, S.N.
2011-02-15
We report an investigation of the structure and vibrational modes of (AgI){sub x} (AsSe){sub 100-x}, bulk glasses using Raman spectroscopy and first principles calculations. The short- and medium-range structural order of the glasses was elucidated by analyzing the reduced Raman spectra, recorded at off-resonance conditions. Three distinct local environments were revealed for the AsSe glass including stoichiometric-like and As-rich network sub-structures, and cage-like molecules (As{sub 4}Se{sub n}, n=3, 4) decoupled from the network. To facilitate the interpretation of the Raman spectra ab initio calculations are employed to study the geometric and vibrational properties of As{sub 4}Se{sub n} molecular units that are parts of the glass structure. The incorporation of AgI causes appreciable structural changes into the glass structure. AgI is responsible for the population reduction of molecular units and for the degradation of the As-rich network-like sub-structure via the introduction of As-I terminal bonds. Ab initio calculations of mixed chalcohalide pyramids AsSe{sub m}I{sub 3-m} provided useful information augmenting the interpretation of the Raman spectra. -- Graphical abstract: Raman scattering and ab initio calculations are employed to study the structure of AgI-AsSe superionic glasses. The role of mixed chalcohalide pyramidal units as illustrated in the figure is elucidated. Display Omitted Research highlights: {yields} Doping binary As-Se glasses with AgI cause dramatic changes in glass structure. {yields} Raman scattering and ab initio calculations determine changes in short- and medium-range order. {yields} Three local environments exist in AsSe glass including a network sub-structure and cage-like molecules. {yields} Mixed chalcohalide pyramids AsSe{sub m}I{sub 3-m} dominate the AgI-doped glass structure.
Ab initio calculation of Ti NMR shieldings for titanium oxides and halides
NASA Astrophysics Data System (ADS)
Tossell, J. A.
Titanium NMR shielding constants have been calculated using ab initio coupled Hartree-Fock perturbation theory and polarized double-zeta basis sets for TiF 4, TiF 62-, TiCI 4, Ti(OH) 4, Ti(OH 2) 64+, Ti(OH) 4O, and Ti(OH) 3O -. In all cases the calculations were performed at Hartree-Fuck energy-optimized geometries. For Ti(OH) 4 a S4-symmetry geometry with nonlinear ∠ TiOH was employed. Relative shieldings are in reasonable agreement with experiment for TiF 62-, TiCI 4, and Ti(OR) 4, where R = H or alkyl. Ti(OH 2) 64+ is predicted to be more highly shielded than Ti(OH) 4 by about 340 ppm. The five-coordinate complex Ti(OH) 4O, whose calculated structure matches well that measured by extended X-ray absorption fine structure in K 2O · TiO 2 · SiO 2 glass, is actually deshielded compared to Ti(OH) 4 by about 40 ppm. X-ray absorption-near-edge spectral energies have also been calculated for TiF 4, TiCI 4, Ti(OH) 4, and Ti(OH) 4O using an equivalent ionic core virtual-orbital method and the observed reduction in term energy for the five-coordinate species compared to Ti(OH) 4 has been reproduced. Replacement of the H atoms in Ti(OH) 4 by point charges has only a slight effect upon σTi, suggesting a possible means of incorporating second-neighbor effects in NMR calculations for condensed phases.
Ab Initio Molecular Dynamics Simulations and GIPAW NMR Calculations of a Lithium Borate Glass Melt.
Ohkubo, Takahiro; Tsuchida, Eiji; Takahashi, Takafumi; Iwadate, Yasuhiko
2016-04-14
The atomic structure of a molten 0.3Li2O-0.7B2O3 glass at 1250 K was investigated using ab initio molecular dynamics (AIMD) simulations. The gauge including projector augmented wave (GIPAW) method was then employed for computing the chemical shift and quadrupolar coupling constant of (11)B, (17)O, and (7)Li from 764 AIMD derived structures. The chemical shift and quadrupolar coupling constant distributions were directly estimated from the dynamical structure of the molten glass. (11)B NMR parameters of well-known structural units such as the three-coordinated ring, nonring, and four-coordinated tetrahedron were found to be in good agreement with the experimental results. In this study, more detailed classification of B units was presented based on the number of O species bonded to the B atoms. This highlights the limitations of (11)B NMR sensitivity for resolving (11)B local environment using the experimentally obtained spectra only. The (17)O NMR parameter distributions can theoretically resolve the bridging and nonbridging O atoms with different structural units such as nonring, single boroxol ring, and double boroxol ring. Slight but clear differences in the number of bridging O atoms surrounding Li that have not been reported experimentally were observed in the theoretically obtained (7)Li NMR parameters. PMID:27010637
NASA Astrophysics Data System (ADS)
Simic-Milosevic, Violeta; Bocquet, Marie-Laure; Morgenstern, Karina
2009-08-01
Dissociative adsorption of doubly substituted benzene molecules leads to a molecule with two missing hydrogen atoms. We use scanning tunnelling microscopy at 5 K and density functional theory to investigate these benzyne molecules on Cu(1 1 1). Benzyne is either imaged as a depression, as a ring-shaped protrusion, or as a circular protrusion at different tunnelling parameters. Submolecular resolution and ab initio calculations give information on the adsorption properties about the in-situ formed biradical species.
Acido-base behavior of hydroxamic acids: experimental and ab initio studies on hydroxyureas.
Vrcek, Ivana Vinković; Kos, Ivan; Weitner, Tin; Birus, Mladen
2008-11-20
The values of Ka, DeltaSa, and DeltaHa for deprotonation of hydroxyurea (HU) and N-methylhydroxyurea (NMHU), as targeted compounds, and for betainohydroxamic acid, were potentiometrically determined. Although NMHU has two and HU even three deprotonation sites, the measurements confirm that they behave as weak acids with a single pK a approximately 10. Comparison with analogous thermodynamic parameters previously determined for series of monohydroxamic acids reveals deviations from a DeltaSa, vs DeltaHa plot for HU and NMHU, raising the question of the dissociation site of hydroxureas in water. In addition to the deprotonation of the hydroxyl oxygen, ab initio calculations performed at the MP2/6-311++G(d,p) level of theory for these two compounds indicate a notable participation of the nitrogen deprotonation site in HU. The calculations for the isolated, monohydrate, trihydrate, and decahydrate molecular and anionic forms of hydroxyureas support the importance of hydrogen bonding in the gas and aqueous phases. The hydroxylamino nitrogen in HU is the most acidic site in water, contributing approximately 94% to the overall deprotonation process at 25 degrees C. On the contrary, the hydroxylamino oxygen is by far the most favored deprotonation site in NMHU, contributing almost 100% in aqueous medium. The predicted participations of two deprotonation sites in HU, calculated at the MP2/6-311++G(d,p) level of theory, combined with the calculated relative reaction enthalpy and entropy for the deprotonation, satisfactorily explain the observed deviation from linearity of DeltaHa vs DeltaSa, plot. There is no such a simple explanation for acid-base behavior of NMHU.
Hegde, Ganesh Bowen, R. Chris
2015-10-15
The accuracy of a single s-orbital representation of Cu towards enabling multi-thousand atom ab initio calculations of electronic structure is evaluated in this work. If an electrostatic compensation charge of 0.3 electron per atom is used in this basis representation, the electronic transmission in bulk and nanocrystalline Cu can be made to compare accurately to that obtained with a Double Zeta Polarized basis set. The use of this representation is analogous to the use of single band effective mass representation for semiconductor electronic structure. With a basis of just one s-orbital per Cu atom, the representation is extremely computationally efficient and can be used to provide much needed ab initio insight into electronic transport in nanocrystalline Cu interconnects at realistic dimensions of several thousand atoms.
Rotational study of the CH4-CO complex: Millimeter-wave measurements and ab initio calculations.
Surin, L A; Tarabukin, I V; Panfilov, V A; Schlemmer, S; Kalugina, Y N; Faure, A; Rist, C; van der Avoird, A
2015-10-21
The rotational spectrum of the van der Waals complex CH4-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110-145 GHz. Newly observed and assigned transitions belong to the K = 2-1 subband correlating with the rotationless jCH4 = 0 ground state and the K = 2-1 and K = 0-1 subbands correlating with the jCH4 = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH4-CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH4-CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH4 face closest to the CO subunit and binding energy De = 177.82 cm(-1). The bound rovibrational levels of the CH4-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 91.32, 94.46, and 104.21 cm(-1) for A (jCH4 = 0), F (jCH4 = 1), and E (jCH4 = 2) nuclear spin modifications of CH4-CO, respectively.
Rotational study of the CH4-CO complex: Millimeter-wave measurements and ab initio calculations
NASA Astrophysics Data System (ADS)
Surin, L. A.; Tarabukin, I. V.; Panfilov, V. A.; Schlemmer, S.; Kalugina, Y. N.; Faure, A.; Rist, C.; van der Avoird, A.
2015-10-01
The rotational spectrum of the van der Waals complex CH4-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110-145 GHz. Newly observed and assigned transitions belong to the K = 2-1 subband correlating with the rotationless jCH4 = 0 ground state and the K = 2-1 and K = 0-1 subbands correlating with the jCH4 = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH4-CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH4-CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH4 face closest to the CO subunit and binding energy De = 177.82 cm-1. The bound rovibrational levels of the CH4-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 91.32, 94.46, and 104.21 cm-1 for A (jCH4 = 0), F (jCH4 = 1), and E (jCH4 = 2) nuclear spin modifications of CH4-CO, respectively.
Rotational study of the NH3-CO complex: millimeter-wave measurements and ab initio calculations.
Surin, L A; Potapov, A; Dolgov, A A; Tarabukin, I V; Panfilov, V A; Schlemmer, S; Kalugina, Y N; Faure, A; van der Avoird, A
2015-03-21
The rotational spectrum of the van der Waals complex NH3-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 112-139 GHz. Newly observed and assigned transitions belong to the K = 0-0, K = 1-1, K = 1-0, and K = 2-1 subbands correlating with the rotationless (jk)NH3 = 00 ground state of free ortho-NH3 and the K = 0-1 and K = 2-1 subbands correlating with the (jk)NH3 = 11 ground state of free para-NH3. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. Some of these transitions are continuations to higher J values of transition series observed previously [C. Xia et al., Mol. Phys. 99, 643 (2001)], the other transitions constitute newly detected subbands. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the ortho-NH3-CO and para-NH3-CO complexes. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of NH3-CO has been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations and an augmented correlation-consistent triple zeta basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the N atom closest to the CO subunit and binding energy De = 359.21 cm(-1). The bound rovibrational levels of the NH3-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 210.43 and 218.66 cm(-1) for ortho-NH3-CO and para-NH3-CO, respectively.
Rotational study of the CH4-CO complex: Millimeter-wave measurements and ab initio calculations.
Surin, L A; Tarabukin, I V; Panfilov, V A; Schlemmer, S; Kalugina, Y N; Faure, A; Rist, C; van der Avoird, A
2015-10-21
The rotational spectrum of the van der Waals complex CH4-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110-145 GHz. Newly observed and assigned transitions belong to the K = 2-1 subband correlating with the rotationless jCH4 = 0 ground state and the K = 2-1 and K = 0-1 subbands correlating with the jCH4 = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH4-CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH4-CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH4 face closest to the CO subunit and binding energy De = 177.82 cm(-1). The bound rovibrational levels of the CH4-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 91.32, 94.46, and 104.21 cm(-1) for A (jCH4 = 0), F (jCH4 = 1), and E (jCH4 = 2) nuclear spin modifications of CH4-CO, respectively. PMID:26493903
Treatment of dilute clusters of methanol and water by ab initio quantum mechanical calculations.
Ruckenstein, Eli; Shulgin, Ivan L; Tilson, Jeffrey L
2005-02-10
Large molecular clusters can be considered as intermediate states between gas and condensed phases, and information about them can help us understand condensed phases. In this paper, ab initio quantum mechanical methods have been used to examine clusters formed of methanol and water molecules. The main goal was to obtain information about the intermolecular interactions and the structure of methanol/water clusters at the molecular level. The large clusters (CH(4)O...(H(2)O)(12) and H(2)O...(CH(4)O)(10)) containing one molecule of one component (methanol or water) and many (12, 10) molecules of the other component were considered. Møller-Plesset perturbation theory (MP2) was used in the calculations. Several representative cluster geometries were optimized, and nearest-neighbor interaction energies were calculated for the geometries obtained in the first step. The results of the calculations were compared to the available experimental information regarding the liquid methanol/water mixtures and to the molecular dynamics and Monte Carlo simulations, and good agreement was found. For the CH(4)O...(H(2)O)(12) cluster, it was shown that the molecules of water can be subdivided into two classes: (i) H bonded to the central methanol molecule and (ii) not H bonded to the central methanol molecule. As expected, these two classes exhibited striking energy differences. Although they are located almost the same distance from the carbon atom of the central methanol molecule, they possess very different intermolecular interaction energies with the central molecule. The H bonding constitutes a dominant factor in the hydration of methanol in dilute aqueous solutions. For the H(2)O...(CH(4)O)(10) cluster, it was shown that the central molecule of water has almost three H bonds with the methanol molecules; this result differs from those in the literature that concluded that the average number of H bonds between a central water molecule and methanol molecules in dilute solutions of
Input/Output of ab-initio nuclear structure calculations for improved performance and portability
Laghave, Nikhil
2010-01-01
Many modern scientific applications rely on highly computation intensive calculations. However, most applications do not concentrate as much on the role that input/output operations can play for improved performance and portability. Parallelizing input/output operations of large files can significantly improve the performance of parallel applications where sequential I/O is a bottleneck. A proper choice of I/O library also offers a scope for making input/output operations portable across different architectures. Thus, use of parallel I/O libraries for organizing I/O of large data files offers great scope in improving performance and portability of applications. In particular, sequential I/O has been identified as a bottleneck for the highly scalable MFDn (Many Fermion Dynamics for nuclear structure) code performing ab-initio nuclear structure calculations. We develop interfaces and parallel I/O procedures to use a well-known parallel I/O library in MFDn. As a result, we gain efficient I/O of large datasets along with their portability and ease of use in the down-stream processing. Even situations where the amount of data to be written is not huge, proper use of input/output operations can boost the performance of scientific applications. Application checkpointing offers enormous performance improvement and flexibility by doing a negligible amount of I/O to disk. Checkpointing saves and resumes application state in such a manner that in most cases the application is unaware that there has been an interruption to its execution. This helps in saving large amount of work that has been previously done and continue application execution. This small amount of I/O provides substantial time saving by offering restart/resume capability to applications. The need for checkpointing in optimization code NEWUOA has been identified and checkpoint/restart capability has been implemented in NEWUOA by using simple file I/O.
Schnitzler, Elijah G; Jäger, Wolfgang
2014-02-14
The pure rotational, high-resolution spectrum of the benzoic acid-water complex was measured in the range of 4-14 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. In all, 40 a-type transitions and 2 b-type transitions were measured for benzoic acid-water, and 12 a-type transitions were measured for benzoic acid-D2O. The equilibrium geometry of benzoic acid-water was determined with ab initio calculations, at the B3LYP, M06-2X, and MP2 levels of theory, with the 6-311++G(2df,2pd) basis set. The experimental rotational spectrum is most consistent with the B3LYP-predicted geometry. Narrow splittings were observed in the b-type transitions, and possible tunnelling motions were investigated using the B3LYP/6-311++G(d,p) level of theory. Rotation of the water moiety about the lone electron pair hydrogen-bonded to benzoic acid, across a barrier of 7.0 kJ mol(-1), is the most likely cause for the splitting. Wagging of the unbound hydrogen atom of water is barrier-less, and this large amplitude motion results in the absence of c-type transitions. The interaction and spectroscopic dissociation energies calculated using B3LYP and MP2 are in good agreement, but those calculated using M06-2X indicate excess stabilization, possibly due to dispersive interactions being over-estimated. The equilibrium constant of hydration was calculated by statistical thermodynamics, using ab initio results and the experimental rotational constants. This allowed us to estimate the changes in percentage of hydrated benzoic acid with variations in the altitude, region, and season. Using monitoring data from Calgary, Alberta, and the MP2-predicted dissociation energy, a yearly average of 1% of benzoic acid is expected to be present in the form of benzoic acid-water. However, this percentage depends sensitively on the dissociation energy. For example, when using the M06-2X-predicted dissociation energy, we find it increases to 18%.
Schnitzler, Elijah G; Jäger, Wolfgang
2014-02-14
The pure rotational, high-resolution spectrum of the benzoic acid-water complex was measured in the range of 4-14 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. In all, 40 a-type transitions and 2 b-type transitions were measured for benzoic acid-water, and 12 a-type transitions were measured for benzoic acid-D2O. The equilibrium geometry of benzoic acid-water was determined with ab initio calculations, at the B3LYP, M06-2X, and MP2 levels of theory, with the 6-311++G(2df,2pd) basis set. The experimental rotational spectrum is most consistent with the B3LYP-predicted geometry. Narrow splittings were observed in the b-type transitions, and possible tunnelling motions were investigated using the B3LYP/6-311++G(d,p) level of theory. Rotation of the water moiety about the lone electron pair hydrogen-bonded to benzoic acid, across a barrier of 7.0 kJ mol(-1), is the most likely cause for the splitting. Wagging of the unbound hydrogen atom of water is barrier-less, and this large amplitude motion results in the absence of c-type transitions. The interaction and spectroscopic dissociation energies calculated using B3LYP and MP2 are in good agreement, but those calculated using M06-2X indicate excess stabilization, possibly due to dispersive interactions being over-estimated. The equilibrium constant of hydration was calculated by statistical thermodynamics, using ab initio results and the experimental rotational constants. This allowed us to estimate the changes in percentage of hydrated benzoic acid with variations in the altitude, region, and season. Using monitoring data from Calgary, Alberta, and the MP2-predicted dissociation energy, a yearly average of 1% of benzoic acid is expected to be present in the form of benzoic acid-water. However, this percentage depends sensitively on the dissociation energy. For example, when using the M06-2X-predicted dissociation energy, we find it increases to 18%. PMID:24366483
NASA Astrophysics Data System (ADS)
Kiyono, Hajime; Kuze, Nobuhiko; Fujiwara, Hideo; Takeuchi, Hiroshi; Egawa, Toru; Konaka, Shigehiro
1996-02-01
The molecular structure of methyl isonicotinate was studied by gas phase electron diffraction combined with ab initio calculations. The molecular skeleton was assumed to be planar. The determined values of principal structure parameters ( rg and ∠ α) are as follows: r( NC) = 1.343(5) Å, r( C…C) ring = 1.401(3) Å, r( Cγ C) = 1.499(9) Å, r( C O) = 1.205(5) Å, r( C( O) O) = 1.331(8) Å, r( OC Me) = 1.430(8) Å,
Microsolvation of LiBO2 in water: anion photoelectron spectroscopy and ab initio calculations.
Zeng, Zhen; Hou, Gao-Lei; Song, Jian; Feng, Gang; Xu, Hong-Guang; Zheng, Wei-Jun
2015-04-14
The microsolvation of LiBO2 in water was investigated by conducting anion photoelectron spectroscopy and ab initio studies on the LiBO2(H2O)n(-) (n = 0-5) clusters. By comparing calculations with experiments, the structures of these clusters and their corresponding neutrals were assigned, and their structural evolutions were revealed. During the anionic structural evolution with n increasing to 5, hydroxyborate and metaborate channels were identified and the metaborate channel is more favorable. For the hydroxyborate structures, the anionic Li(+)-BO2(-) ion pair reacts with a water molecule to produce the LiBO(OH)2(-) moiety and three water molecules tend to dissolve this moiety. In the metaborate channel, two types of solvent-separated ion pair (SSIP) geometries were determined as the ring-type and linear-type. The transition from the contact ion pair (CIP) to the ring-type of SSIP starts at n = 3, while that to the linear-type of SSIP occurs at n = 4. In neutral LiBO2(H2O)n clusters, the first water molecule prefers to react with the Li(+)-BO2(-) ion pair to generate the LiBO(OH)2 moiety, analogous to the bulk crystal phase of α-LiBO2 with two O atoms substituted by two OH groups. The Li-O distance in the LiBO(OH)2 moiety increases with the increasing number of water molecules and elongates abruptly at n = 4. Our studies provide new insight into the initial dissolution of LiBO2 salt in water at the molecular level and may be correlated to the bulk state.
The ab initio calculation of spectra of open shell diatomic molecules
NASA Astrophysics Data System (ADS)
Tennyson, Jonathan; Lodi, Lorenzo; McKemmish, Laura K.; Yurchenko, Sergei N.
2016-05-01
The spectra (rotational, rotation-vibrational or electronic) of diatomic molecules due to transitions involving only closed-shell (1Σ ) electronic states follow very regular, simple patterns and their theoretical analysis is usually straightforward. On the other hand, open-shell electronic states lead to more complicated spectral patterns and, moreover, often appear as a manifold of closely lying electronic states, leading to perturbed spectra of even greater complexity. This is especially true when at least one of the atoms is a transition metal. Traditionally these complex cases have been analysed using approaches based on perturbation theory, with semi-empirical parameters determined by fitting to spectral data. Recently the needs of two rather diverse scientific areas have driven the demand for improved theoretical models of open-shell diatomic systems based on an ab initio approach; these areas are ultracold chemistry and the astrophysics of ‘cool’ stars, brown dwarfs and most recently extrasolar planets. However, the complex electronic structure of these molecules combined with the accuracy requirements of high-resolution spectroscopy render such an approach particularly challenging. This review describes recent progress in developing methods for directly solving the effective Schrödinger equation for open-shell diatomic molecules, with a focus on molecules containing a transtion metal. It considers four aspects of the problem: (i) the electronic structure problem; (ii) non-perturbative treatments of the curve couplings; (iii) the solution of the nuclear motion Schrödinger equation; (iv) the generation of accurate electric dipole transition intensities. Examples of applications are used to illustrate these issues.
The ab initio calculation of spectra of open shell diatomic molecules
NASA Astrophysics Data System (ADS)
Tennyson, Jonathan; Lodi, Lorenzo; McKemmish, Laura K.; Yurchenko, Sergei N.
2016-05-01
The spectra (rotational, rotation–vibrational or electronic) of diatomic molecules due to transitions involving only closed-shell (1Σ ) electronic states follow very regular, simple patterns and their theoretical analysis is usually straightforward. On the other hand, open-shell electronic states lead to more complicated spectral patterns and, moreover, often appear as a manifold of closely lying electronic states, leading to perturbed spectra of even greater complexity. This is especially true when at least one of the atoms is a transition metal. Traditionally these complex cases have been analysed using approaches based on perturbation theory, with semi-empirical parameters determined by fitting to spectral data. Recently the needs of two rather diverse scientific areas have driven the demand for improved theoretical models of open-shell diatomic systems based on an ab initio approach; these areas are ultracold chemistry and the astrophysics of ‘cool’ stars, brown dwarfs and most recently extrasolar planets. However, the complex electronic structure of these molecules combined with the accuracy requirements of high-resolution spectroscopy render such an approach particularly challenging. This review describes recent progress in developing methods for directly solving the effective Schrödinger equation for open-shell diatomic molecules, with a focus on molecules containing a transtion metal. It considers four aspects of the problem: (i) the electronic structure problem; (ii) non-perturbative treatments of the curve couplings; (iii) the solution of the nuclear motion Schrödinger equation; (iv) the generation of accurate electric dipole transition intensities. Examples of applications are used to illustrate these issues.
Nonadiabatic ab initio dynamics of a model protonated Schiff base of 9-cis retinal.
Chung, Wilfredo Credo; Nanbu, Shinkoh; Ishida, Toshimasa
2010-08-19
The dynamics of the photoisomerization of a model protonated Schiff base of 9-cis retinal in isorhodopsin is investigated using nonadiabatic molecular dynamics simulation combined with ab initio quantum chemical calculations on-the-fly. The quantum chemical part is treated at the complete-active space self-consistent field level for six electrons in six active pi orbitals with the 6-31G basis set (CASSCF(6,6)/6-31G). The probabilities of nonadiabatic transitions between the S(1) ((1)pipi*) and S(0) states are estimated in light of the Zhu-Nakamura theory. The photoinduced cis-trans isomerization of 9-cis retinal proceeds slower than that of its 11-cis analogue and at a lower quantum yield, confirming experimental observations. An energetic barrier in the excited state impedes the elongation and twist of the C(9)=C(10) stretch and torsion coordinates, respectively, resulting in the trapping of trajectories before transition. Consequently, the isomerization takes longer time and the transition more often occurs at smaller twist angle of =C(8)-C(9)=C(10)-C(11)=, which leads to regeneration of the 9-cis reactant. Thus, neither the smaller twist observed in the X-ray crystal nor the slower movement of nuclei in the transition region would be the main reason for the longer reaction time and lower yield. A well-known space-saving asynchronous bicycle pedal or crankshaft photoisomerization mechanism is found to be operational in 9-cis retinal. The simulation in vacuo suggests that the excited-state barrier and the photoisomerization itself are intrinsic properties of the visual chromophore and not triggered mainly by the protein environment that surrounds the chromophore. PMID:20666503
Infrared and Raman spectra, ab initio calculations and conformational studies of ethyl iodosilane
NASA Astrophysics Data System (ADS)
Aleksa, Valdemaras; Powell, David L.; Gruodis, Alytis; Hassler, Karl; Hummeltenberg, Reinhard; Herzog, Klaus; Salzer, Reiner; Klaeboe, Peter; Nielsen, Claus J.
2003-01-01
Ethyl iodosilane (CH 3CH 2-SiH 2I) was synthesized for the first time. Infrared spectra were recorded in the vapour, amorphous and crystalline solid phases in MIR and FIR regions. Additional MIR spectra of the compound isolated in argon and nitrogen matrices were obtained at 5 K. Raman spectra of the liquid, excited by argon and by Nd 3+ YAG lasers, were recorded at room temperature including polarization measurements. The spectra were studied in an extended temperature range 173-353 K and a Δ H value of 1.2±0.3 kJ mol -1 was obtained with gauche being the low energy conformer. Spectra of the amorphous and crystalline solids were obtained at liquid nitrogen temperature. Ethyl iodosilane exists in an equilibrium between anti and gauche conformers, in the vapour, liquid and amorphous states. After careful annealing the amorphous solid on a cold Cu finger (Raman) or on a CsI or Si window (infrared) to 160 K a partly crystalline solid was formed. A number of IR and Raman bands were reduced in intensity after annealing, although they did not vanish completely. From comparison between the observed and calculated vibrational modes it was apparent that the gauche conformer was present in the crystal. The sample was mixed with argon and nitrogen in a ratio 1:1000, deposited on a window at 5 or 10 K and annealed to temperatures between 5 and 36 K (argon) and 5-30 K (nitrogen). IR bands attributed to the anti and gauche conformers were reduced and increased in intensities, respectively. Thus, the gauche conformer was the low energy conformer in the matrices and probably also in the vapour phase. Ab initio calculations were performed at the RHF/3-21 G* and 6-311G* B3LYPs and gave optimized geometries, IR and Raman intensities and vibrational frequencies for the anti and gauche conformers. An enthalpy difference of 0.9 kJ mol -1 was obtained from the calculations with gauche being the low energy conformer. After scaling, a reasonably good agreement between the experimental
NASA Astrophysics Data System (ADS)
Buck, Henk
In an effort to overcome a significant difference between high-level ab initio calculations and X-ray data of DNA duplexes, Fonseca Guerra et al. 9-11 studied a number of model systems for A-T and G-C basepairs at various levels of nonlocal Density Functional Theory. There was an excellent agreement with the gas-phase experimental bond enthalpies for the A-T and G-C basepairs. On the other hand the hydrogen bond lengths between the bases differ from the X-ray results. After introduction of a molecular environment as local water and Na+ ions, the agreement between theory and experiment was excellent. However, careful analysis shows that this picture is far from correct. In fact, the model was constructed as a backbone-modified DNA duplex in which the nonbonding oxygens of the phosphate linkages are completely shielded by proton addition. Experimental results with respect to backbone-modified DNAs clearly show that changes in the backbone focused on phosphate shielding result in DNA duplexes with a variety in conformational behavior. In addition to an analysis of the aforementioned contradiction, we also give molecular mechanics calculations which show that the A-T and G-C bond enthalpies are of the same order as the corresponding results of Fonseca Guerra et al. under the condition of complete anionic shielding of the nonbonded oxygens in the phosphate linkages.
NASA Astrophysics Data System (ADS)
Mozafari, E.; Shulumba, N.; Steneteg, P.; Alling, B.; Abrikosov, Igor A.
2016-08-01
We present a theoretical scheme to calculate the elastic constants of magnetic materials in the high-temperature paramagnetic state. Our approach is based on a combination of disordered local moments picture and ab initio molecular dynamics (DLM-MD). Moreover, we investigate a possibility to enhance the efficiency of the simulations of elastic properties using the recently introduced method: symmetry imposed force constant temperature-dependent effective potential (SIFC-TDEP). We have chosen cubic paramagnetic CrN as a model system. This is done due to its technological importance and its demonstrated strong coupling between magnetic and lattice degrees of freedom. We have studied the temperature-dependent single-crystal and polycrystalline elastic constants of paramagentic CrN up to 1200 K. The obtained results at T = 300 K agree well with the experimental values of polycrystalline elastic constants as well as the Poisson ratio at room temperature. We observe that the Young's modulus is strongly dependent on temperature, decreasing by ˜14 % from T = 300 K to 1200 K. In addition we have studied the elastic anisotropy of CrN as a function of temperature and we observe that CrN becomes substantially more isotropic as the temperature increases. We demonstrate that the use of Birch law may lead to substantial errors for calculations of temperature induced changes of elastic moduli. The proposed methodology can be used for accurate predictions of mechanical properties of magnetic materials at temperatures above their magnetic order-disorder phase transition.
Body, Monique; Silly, Gilles; Legein, Christophe; Buzaré, Jean-Yves
2005-05-26
(19)F NMR isotropic chemical shift (delta(iso)) calculations are performed in crystallized compounds using the GIAO method with the B3LYP hybrid functional at DFT level. Clusters centered on the studied fluorine atoms mimic the crystalline structures. The 6-311+G(d) basis set is chosen for the central fluorine atom, and the LanL2DZ basis set for the others. The metal atoms are described by the 3-21G(2d) basis set or, when not available, by the CRENBL basis set with the corresponding ECP, and augmented with 2d polarization functions when existing. First, for high-symmetry systems (MF, MF(2), and MF(3) compounds), a systematization of the cluster building up from coordination spheres is proposed, generalized to fluoroperovskites and fluoroaluminates KAlF(4) and RbAlF(4). When applied to rather low symmetry systems such as barium fluorometalates BaMgF(4), BaZnF(4), and Ba(2)ZnF(6), the definition of the coordination spheres is far from easy. Then, for structures built up from a MF(6) octahedron network, we may define different "starting clusters": [FM(2)F(8)] for the shared fluorine atoms, [FMF(4)] for the unshared ones, and [FBa(4)](7+) for the "free" ones. Analogous "starting clusters" are then tested on compounds from the NaF-AlF(3), BaF(2)-AlF(3), and CaF(2)-AlF(3) binary systems and for alpha-BaCaAlF(7) that are also built up from a MF(6) octahedron network. For each of these corresponding fluorine sites, delta(iso) values are calculated with the "starting clusters" and several larger clusters and compared to the experimental delta(iso) values. For the barium-containing clusters, the RMS deviation is equal to 51 ppm. It is suggested that this result may be related to the poor quality of the barium basis sets for which no polarization functions are available for the moment. In total, chemical shifts were calculated for 122 fluorine sites, in a various range of compounds. For the clusters without barium, the ab initio method leads to a RMS equal to 22 ppm, which is
NASA Astrophysics Data System (ADS)
Auer, Alexander A.
2009-01-01
In this contribution high-level ab-initio calculations of the chemical shifts of methanol including zero-point vibrational and temperature corrections are presented. For the first time, secondary isotope effects have been calculated via second order vibrational perturbation theory. In comparison with recent experimental gas-phase data and in contrast to other quantum-chemical methods the results are consistent and in very good agreement with the experimental 13C, 17O and 1H chemical shifts reported by Makulski [W. Makulski, J. Mol. Struct. 872 (2008) 81]. Secondary isotope effects can be calculated with remarkable accuracy of a few hundredths of a ppm in comparison to experiment.
NASA Astrophysics Data System (ADS)
Kawashima, Yoshiyuki; Iwano, Sakae; Hirota, Eizi
2016-06-01
This paper presents an extension of the preceding talk on the FTMW spectroscopy of N_2-ES (ethylene sulfide), namely the results on N_2-DMS (dimethyl sulfide). We have previously investigated two N_2 complexes: N_2-DME (dimethyl ether), for which we reported a prelimanary result, and N_2-EO (ethylene oxide). We have observed the ground-state rotational spectrum of the N_2-DMS complex, i.e. c-type transitions in the frequency region from 5 to 24 GHz, which we assigned to the normal, 15N_2-DMS, and 15NN-DMS species of the N_2-DMS. We have found both the ortho and para states for the 14N_2-DMS and 15N_2-DMS species. In the case of the 15N_2-DMS, some transitions with Ka = 2 and 3 were observed slightly split by the internal rotation of the two methyl tops of the DMS. The observed spectra of the 15N_2-DMS were analyzed by using the XIAM program. In the case of the para state of the 15N_2-DMS, three rotational and five centrifugal distortion constants with the V3 barrier to the methyl group internal rotation, whereas, in the case of the ortho state of the 15N_2-DMS, two more centrifugal distortion constants, ΦJK and ΦKJ, were needed to reproduce the observed spectra. For the N_2-DMS complex, we concluded that the N_2 moiety was located in a plane perpendicular to the C-S-C plane and bisecting the CSC angle of the DMS. We have carried out ab initio molecular orbital calculations at the level of MP2 with basis sets 6-311++G(d, p), aug-cc-pVDZ, and aug-cc-pVTZ, to complement the information on the intracomplex motions obtained from the observed rotational spectra. We have applied a natural bond orbital (NBO) analysis to the N_2-DMS and N_2-ES to calculate the stabilization energy CT (=Δ Eσσ*), which was closely correlated with the binding energy EB, as found for other related complexes. Y. Kawashima, Y. Tatamitani, Y. Morita, and E. Hirota, 61st International Symposium on Molecular Spectroscopy, TE10 (2006) Y. Kawashima and E. Hirota, J. Phys. Chem. A 2013 117, 13855
Ab initio based investigation of interstitial interactions and Snoek relaxation in Nb-O
NASA Astrophysics Data System (ADS)
Dmitriev, V. V.; Blanter, M. S.; Ruban, A. V.; Johansson, B.
2012-02-01
Chemical and strain-induced effective pair interactions of interstitial oxygen atoms in bcc Nb have been determined in supercell first-principles calculations using Vienna ab initio simulation package (VASP). The strain-induced interactions are in reasonable agreement with those obtained earlier within a phenomenological microscopic Krivoglaz-Kanzaki-Khachaturyan model (KKKM). At the same time, the chemical interactions, which have been considered to be small in earlier theoretical considerations, turned out to be dominating at the first several coordination shells. The obtained interactions have been used in calculations of the concentration- and temperature-dependence of the internal friction Snoek peak. The theoretical results are found to be in good agreement with the existing experimental data.
NASA Astrophysics Data System (ADS)
Titantah, John T.; Karttunen, Mikko
2016-05-01
Electronic and optical properties of silver clusters were calculated using two different ab initio approaches: (1) based on all-electron full-potential linearized-augmented plane-wave method and (2) local basis function pseudopotential approach. Agreement is found between the two methods for small and intermediate sized clusters for which the former method is limited due to its all-electron formulation. The latter, due to non-periodic boundary conditions, is the more natural approach to simulate small clusters. The effect of cluster size is then explored using the local basis function approach. We find that as the cluster size increases, the electronic structure undergoes a transition from molecular behavior to nanoparticle behavior at a cluster size of 140 atoms (diameter ~1.7 nm). Above this cluster size the step-like electronic structure, evident as several features in the imaginary part of the polarizability of all clusters smaller than Ag147, gives way to a dominant plasmon peak localized at wavelengths 350 nm ≤ λ ≤ 600 nm. It is, thus, at this length-scale that the conduction electrons' collective oscillations that are responsible for plasmonic resonances begin to dominate the opto-electronic properties of silver nanoclusters.
2013-01-01
Ab initio second-order algebraic diagrammatic construction (ADC(2)) calculations using the resolution of the identity (RI) method have been performed on poly-(p-phenylenevinylene) (PPV) oligomers with chain lengths up to eight phenyl rings. Vertical excitation energies for the four lowest π–π* excitations and geometry relaxation effects for the lowest excited state (S1) are reported. Extrapolation to infinite chain length shows good agreement with analogous data derived from experiment. Analysis of the bond length alternation (BLA) based on the optimized S1 geometry provides conclusive evidence for the localization of the defect in the center of the oligomer chain. Torsional potentials have been computed for the four excited states investigated and the transition densities divided into fragment contributions have been used to identify excitonic interactions. The present investigation provides benchmark results, which can be used (i) as reference for lower level methods and (ii) give the possibility to parametrize an effective Frenkel exciton Hamiltonian for quantum dynamical simulations of ultrafast exciton transfer dynamics in PPV type systems. PMID:23427902
NASA Astrophysics Data System (ADS)
Chudoba, C.; Kummrow, A.; Dreyer, J.; Stenger, J.; Nibbering, E. T. J.; Elsaesser, T.; Zachariasse, K. A.
1999-08-01
Combining femtosecond transient vibrational spectroscopy and high-level calculations is a powerful tool in the determination of excited-state structures. Striking differences in the experimental vibrational pattern of the locally excited states of 4-(dimethylamino)benzonitrile (DMABN) and 4-aminobenzonitrile (ABN) are explained on the basis of molecular structures obtained from ab initio complete-active-space self-consistent-field (CASSCF) calculations, giving evidence for a strong sensitivity of the molecular structure on modest changes in the substituents. The 4.0 ps charge-transfer time for DMABN in acetonitrile is resolved for the first time by tracking the downshifted CN stretching mode.
Wang, Yi-Siang; Yin, Chih-Chien; Chao, Sheng D.
2014-10-07
We perform an ab initio computational study of molecular complexes with the general formula CF{sub 3}X—B that involve one trifluorohalomethane CF{sub 3}X (X = Cl or Br) and one of a series of Lewis bases B in the gas phase. The Lewis bases are so chosen that they provide a range of electron-donating abilities for comparison. Based on the characteristics of their electron pairs, we consider the Lewis bases with a single n-pair (NH{sub 3} and PH{sub 3}), two n-pairs (H{sub 2}O and H{sub 2}S), two n-pairs with an unsaturated bond (H{sub 2}CO and H{sub 2}CS), and a single π-pair (C{sub 2}H{sub 4}) and two π-pairs (C{sub 2}H{sub 2}). The aim is to systematically investigate the influence of the electron pair characteristics and the central atom substitution effects on the geometries and energetics of the formed complexes. The counterpoise-corrected supermolecule MP2 and coupled-cluster single double with perturbative triple [CCSD(T)] levels of theory have been employed, together with a series of basis sets up to aug-cc-pVTZ. The angular and radial configurations, the binding energies, and the electrostatic potentials of the stable complexes have been compared and discussed as the Lewis base varies. For those complexes where halogen bonding plays a significant role, the calculated geometries and energetics are consistent with the σ-hole model. Upon formation of stable complexes, the C–X bond lengths shorten, while the C–X vibrational frequencies increase, thus rendering blueshifting halogen bonds. The central atom substitution usually enlarges the intermolecular bond distances while it reduces the net charge transfers, thus weakening the bond strengths. The analysis based on the σ-hole model is grossly reliable but requires suitable modifications incorporating the central atom substitution effects, in particular, when interaction components other than electrostatic contributions are involved.
Zhang, D X; Shen, B; Zheng, Y X; Wang, S Y; Zhang, J B; Yang, S D; Zhang, R J; Chen, L Y; Wang, C Z; Ho, K M
2014-03-24
The temperature dependent optical properties of tin film from solid to liquid were studied by spectroscopic ellipsometry and ab initio molecular dynamics simulations. The dielectric function of liquid Sn was different from solid, and an interband transition near 1.5 eV was easily observed in solid while it apparently disappeared upon melting. From the evolution of optical properties with temperature, an optical measurement to acquire the melting point by ellipsometry was presented. From first principles calculation, we show that the local structure difference in solid and liquid is responsible for this difference in the optical properties observed in experiment.
NASA Astrophysics Data System (ADS)
Ji, Pengfei; Zhang, Yuwen
2016-03-01
On the basis of ab initio quantum mechanics (QM) calculation, the obtained electron heat capacity is implemented into energy equation of electron subsystem in two temperature model (TTM). Upon laser irradiation on the copper film, energy transfer from the electron subsystem to the lattice subsystem is modeled by including the electron-phonon coupling factor in molecular dynamics (MD) and TTM coupled simulation. The results show temperature and thermal melting difference between the QM-MD-TTM integrated simulation and pure MD-TTM coupled simulation. The successful construction of the QM-MD-TTM integrated simulation provides a general way that is accessible to other metals in laser heating.
AB INITIO Modeling of Thermomechanical Properties of Mo-Based Alloys for Fossil Energy Conversion
Ching, Wai-Yim
2013-12-31
In this final scientific/technical report covering the period of 3.5 years started on July 1, 2011, we report the accomplishments on the study of thermo-mechanical properties of Mo-based intermetallic compounds under NETL support. These include computational method development, physical properties investigation of Mo-based compounds and alloys. The main focus is on the mechanical and thermo mechanical properties at high temperature since these are the most crucial properties for their potential applications. In particular, recent development of applying ab initio molecular dynamic (AIMD) simulations to the T1 (Mo{sub 5}Si{sub 3}) and T2 (Mo{sub 5}SiB{sub 2}) phases are highlighted for alloy design in further improving their properties.
Xu, Dong; Zhang, Yang
2012-01-01
Ab initio protein folding is one of the major unsolved problems in computational biology due to the difficulties in force field design and conformational search. We developed a novel program, QUARK, for template-free protein structure prediction. Query sequences are first broken into fragments of 1–20 residues where multiple fragment structures are retrieved at each position from unrelated experimental structures. Full-length structure models are then assembled from fragments using replica-exchange Monte Carlo simulations, which are guided by a composite knowledge-based force field. A number of novel energy terms and Monte Carlo movements are introduced and the particular contributions to enhancing the efficiency of both force field and search engine are analyzed in detail. QUARK prediction procedure is depicted and tested on the structure modeling of 145 non-homologous proteins. Although no global templates are used and all fragments from experimental structures with template modeling score (TM-score) >0.5 are excluded, QUARK can successfully construct 3D models of correct folds in 1/3 cases of short proteins up to 100 residues. In the ninth community-wide Critical Assessment of protein Structure Prediction (CASP9) experiment, QUARK server outperformed the second and third best servers by 18% and 47% based on the cumulative Z-score of global distance test-total (GDT-TS) scores in the free modeling (FM) category. Although ab initio protein folding remains a significant challenge, these data demonstrate new progress towards the solution of the most important problem in the field. PMID:22411565
Zhang, Yang
2014-02-01
We develop and test a new pipeline in CASP10 to predict protein structures based on an interplay of I-TASSER and QUARK for both free-modeling (FM) and template-based modeling (TBM) targets. The most noteworthy observation is that sorting through the threading template pool using the QUARK-based ab initio models as probes allows the detection of distant-homology templates which might be ignored by the traditional sequence profile-based threading alignment algorithms. Further template assembly refinement by I-TASSER resulted in successful folding of two medium-sized FM targets with >150 residues. For TBM, the multiple threading alignments from LOMETS are, for the first time, incorporated into the ab initio QUARK simulations, which were further refined by I-TASSER assembly refinement. Compared with the traditional threading assembly refinement procedures, the inclusion of the threading-constrained ab initio folding models can consistently improve the quality of the full-length models as assessed by the GDT-HA and hydrogen-bonding scores. Despite the success, significant challenges still exist in domain boundary prediction and consistent folding of medium-size proteins (especially beta-proteins) for nonhomologous targets. Further developments of sensitive fold-recognition and ab initio folding methods are critical for solving these problems.
NASA Technical Reports Server (NTRS)
Lee, Timothy J.; Langhoff, Stephen R. (Technical Monitor)
1996-01-01
Due to advances in quantum mechanical methods over the last few years, it is now possible to determine ab initio potential energy surfaces in which fundamental vibrational frequencies are accurate to within +/- 8 cm(sup -1) on average, and molecular bond distances are accurate to within +/- 0.001-0.003 A, depending on the nature of the bond. That is, the potential energy surfaces have not been scaled or empirically adjusted in any way, showing that theoretical methods have progressed to the point of being useful in analyzing spectra that are not from a tightly controlled laboratory environment, such as rovibrational spectra from the interstellar medium. Some recent examples demonstrating this accuracy win be presented and discussed. These include the HNO, CH4, C2H4, and ClCN molecules. The HNO molecule is interesting due to the very large H-N anharmonicity, while ClCN has a very large Fermi resonance. The ab initio studies for the CH4 and C2H4 molecules present the first accurate full quartic force fields of any kind (i.e., whether theoretical or empirical) for a five-atom and six-atom system, respectively.
Ab initio calculation of the ro-vibrational spectrum of H2F+
NASA Astrophysics Data System (ADS)
Kyuberis, Aleksandra A.; Lodi, Lorenzo; Zobov, Nikolai F.; Polyansky, Oleg L.
2015-10-01
An ab initio study of the rotation-vibrational spectrum of the electronic ground state of the (gas-phase) fluoronium ion H2F+ is presented. A new potential energy surface (PES) and a new dipole moment surface (DMS) were produced and used to compute rotation-vibrational energy levels, line positions and line intensities. Our computations achieve an accuracy of 0.15 cm-1 for the fundamental vibrational frequencies, which is about 50 times more accurate than previous ab initio results. The computed room-temperature line list should facilitate the experimental observations of new H2F+ lines, in particular of yet unobserved overtone transitions. The H2F+ molecular ion, which is isoelectronic to water, has a non-linear equilibrium geometry but a low-energy barrier to linearity at about 6000 cm-1. As a result the effects of so-called quantum monodromy become apparent already at low bending excitations. An analysis of excited bends in terms of quantum monodromy is presented.
Ab Initio Potential Energy Surfaces and the Calculation of Accurate Vibrational Frequencies
NASA Technical Reports Server (NTRS)
Lee, Timothy J.; Dateo, Christopher E.; Martin, Jan M. L.; Taylor, Peter R.; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
Due to advances in quantum mechanical methods over the last few years, it is now possible to determine ab initio potential energy surfaces in which fundamental vibrational frequencies are accurate to within plus or minus 8 cm(exp -1) on average, and molecular bond distances are accurate to within plus or minus 0.001-0.003 Angstroms, depending on the nature of the bond. That is, the potential energy surfaces have not been scaled or empirically adjusted in any way, showing that theoretical methods have progressed to the point of being useful in analyzing spectra that are not from a tightly controlled laboratory environment, such as vibrational spectra from the interstellar medium. Some recent examples demonstrating this accuracy will be presented and discussed. These include the HNO, CH4, C2H4, and ClCN molecules. The HNO molecule is interesting due to the very large H-N anharmonicity, while ClCN has a very large Fermi resonance. The ab initio studies for the CH4 and C2H4 molecules present the first accurate full quartic force fields of any kind (i.e., whether theoretical or empirical) for a five-atom and six-atom system, respectively.
NASA Astrophysics Data System (ADS)
Kowalski, P.; Jahn, S.; Wunder, B.
2011-12-01
Stable isotopes are excellent geochemical tracers widely used in petrology. Among them the isotopes of light elements such as Li or B strongly fractionate between minerals and aqueous fluids during fluid-rock interaction processes, which makes them excellent tracers of mass transfer processes in the subduction cycle. In order to use the full power of isotopes tracing methods the isotope fractionation mechanisms and fractionation factors between minerals and fluids of interest must be well known and characterized. One of the most important mechanisms leading to the formation of isotopic signatures is the equilibrium isotope fractionation, which nowadays can be modeled on the atomic scale by modern computational methods. However, due to high computational requirements the current works have been limited to calculations of simple materials only. In order to overcome these limitations we develop an efficient ab initio based computational approach for prediction of the equilibrium isotope fractionation factors between high pressure and temperature materials, including fluids, which would allow for efficient calculations of the isotope fractionation factors of complex minerals and fluids containing even hundreds of atoms in the supercell. We will show our results for the Li and B stable equilibrium isotope fractionation factors between complex Li/B-bearing crystalline solids (staurolite, spodumene, tourmaline, olenite and micas) and aqueous fluids. The fractionation factors were obtained in an efficient way by simplifying the consideration to calculations of the properties of fractionating atoms only. The comparison of the calculated fractionation factors, on the qualitative and quantitative levels, with the existing experimental data show the comparable to the in situ experimental techniques, predictive power of the computations. We show that with the atomistic scale modelling we are able to reproduce correctly the experimental isotope fractionation sequences
Mishin, Y.; Mehl, M. J.; Papaconstantopoulos, D. A.; Voter, A. F.; Kress, J. D.
2001-06-01
We evaluate the ability of the embedded-atom method (EAM) potentials and the tight-binding (TB) method to predict reliably energies and stability of nonequilibrium structures by taking Cu as a model material. Two EAM potentials are used here. One is constructed in this work by using more fitting parameters than usual and including ab initio energies in the fitting database. The other potential was constructed previously using a traditional scheme. Excellent agreement is observed between ab initio, TB, and EAM results for the energies and stability of several nonequilibrium structures of Cu, as well as for energies along deformation paths between different structures. We conclude that not only TB calculations but also EAM potentials can be suitable for simulations in which correct energies and stability of different atomic configurations are essential, at least for Cu. The bcc, simple cubic, and diamond structures of Cu were identified as elastically unstable, while some other structures (e.g., hcp and 9R) are metastable. As an application of this analysis, nonequilibrium structures of epitaxial Cu films on (001)-oriented fcc or bcc substrates are evaluated using a simple model and atomistic simulations with an EAM potential. In agreement with experimental data, the structure of the film can be either deformed fcc or deformed hcp. The bcc structure cannot be stabilized by epitaxial constraints.
The constrained space orbital variation analysis for periodic ab initio calculations
Cruz Hernandez, N.; Zicovich-Wilson, Claudio Marcelo; Fdez Sanz, Javier
2006-05-21
The constrained space orbital variation (CSOV) method for the analysis of the interaction energy has been implemented in the periodic ab initio CRYSTAL03 code. The method allows for the partition of the energy of two interacting chemical entities, represented in turn by periodic models, into contributions which account for electrostatic effects, mutual polarization and charge transfer. The implementation permits one to carry out the analysis both at the Hartree-Fock and density functional theory levels, where in the latter the most popular exchange-correlation functionals can be used. As an illustrating example, the analysis of the interaction between CO and the MgO (001) surface has been considered. As expected by the almost fully ionic character of the support, our periodic CSOV results, in general agree with those previously obtained using the embedded cluster approach, showing the reliability of the present implementation.
M Kondo; T Mates; D Fischer; F Wudl; E Kramer
2011-12-31
Interfaces between phenylacetylene (PA) monolayers and two silicon surfaces, Si(111) and Si(100), are probed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and the results are analyzed using ab initio molecular orbital calculations. The monolayer systems are prepared via the surface hydrosilylation reaction between PA and hydrogen-terminated silicon surfaces. The following spectral features are obtained for both of the PA-Si(111) and PA-Si(100) systems: a broad {pi}-{pi}* shakeup peak at 292 eV (XPS), a broad first ionization peak at 3.8 eV (UPS), and a low-energy C 1s {yields} {pi}* resonance peak at 284.3 eV (NEXAFS). These findings are ascribed to a styrene-like {pi}-conjugated molecular structure at the PA-Si interface by comparing the experimental data with theoretical analysis results. A conclusion is drawn that the vinyl group can keep its {pi}-conjugation character on the hydrogen-terminated Si(100) [H:Si(100)] surface composed of the dihydride (SiH{sub 2}) groups as well as on hydrogen-terminated Si(111) having the monohydride (SiH) group. The formation mechanism of the PA-Si(100) interface is investigated within cluster ab initio calculations, and the possible structure of the H:Si(100) surface is discussed based on available data.
NASA Astrophysics Data System (ADS)
Aryal, Sita Ram
The alumino-silicate solid solution series (Al 4+2xSi2-2 xO10-x) is an important class of ceramics. Except for the end member (x=0), Al2 SiO5 the crystal structures of the other phases, called mullite, have partially occupied sites. Stoichiometric supercell models for the four mullite phases 3Al2O 3 · 2SiO2 · 2Al 2O3 · SiO2, 4 Al2O3· SiO 2, 9Al2O3 · SiO2, and iota-Al2 O3 (iota-alumina) are constructed starting from experimentally reported crystal structures. A large number of models were built for each phase and relaxed using the Vienna ab initio simulation package (VASP) program. The model with the lowest total energy for a given x was chosen as the representative structure for that phase. Electronic structure and mechanical properties of mullite phases were studied via first-principles calculations. Of the various phases of transition alumina, iota-Al 2O3 is the least well known. In addition structural details have not, until now, been available. It is the end member of the aluminosilicate solid solution series with x=1. Based on a high alumina content mullite phase, a structural model for iota- Al2O3 is constructed. The simulated x-ray diffraction (XRD) pattern of this model agrees well with a measured XRD pattern. The iota-Al2 O3 is a highly disordered ultra-low-density phase of alumina with a theoretical density of 2854kg/m3. Using this theoretically constructed model, elastic, thermodynamic, electronic, and spectroscopic properties of iota-Al2 O3 have been calculated and compared it with those of alpha- Al2O3 and gamma- Al2O3. Boron carbide (B4C) undergoes an amorphization under high velocity impacts. The mechanism of amorphization is not clear. Ab initio methods are used to carry out large-scale uniaxial compression simulations on two polytypes of stoichiometric boron carbide (B4C), B 11C-CBC, and B12- CCC where B11C or B12 is the 12-atom icosahedron and CBC or CCC is the three-atom chain. The simulations were performed on large supercells of 180 atoms
Full-dimensional vibrational calculations for H5O2+ using an ab initio potential energy surface
NASA Astrophysics Data System (ADS)
McCoy, Anne B.; Huang, Xinchuan; Carter, Stuart; Landeweer, Marc Y.; Bowman, Joel M.
2005-02-01
We report quantum diffusion Monte Carlo (DMC) and variational calculations in full dimensionality for selected vibrational states of H5O2+ using a new ab initio potential energy surface [X. Huang, B. Braams, and J. M. Bowman, J. Chem. Phys. 122, 044308 (2005)]. The energy and properties of the zero-point state are focused on in the rigorous DMC calculations. OH-stretch fundamentals are also calculated using "fixed-node" DMC calculations and variationally using two versions of the code MULTIMODE. These results are compared with infrared multiphoton dissociation measurements of Yeh et al. [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)]. Some preliminary results for the energies of several modes of the shared hydrogen are also reported.
NASA Astrophysics Data System (ADS)
Rey, Michael; Nikitin, Andrei; Bezard, Bruno; Rannou, Pascal; Coustenis, Athena; Tyuterev, Vladimir
2016-06-01
Knowledge of intensities of spectral transitions in various temperature ranges including very low-T conditions is essential for the modeling of optical properties of planetary atmospheres and for other astrophysical applications. The temperature dependence of spectral features is crucial, but quantified experimental information in a wide spectral range is generally missing. A significant progress has been recently achieved in first principles quantum mechanical predictions (ab initio electronic structure + variational nuclear motion calculations) of rotationally resolved spectra for hydrocarbon molecules such as methane , ethylene and their isotopic species [1,2] . We have recently reported the TheoReTS information system (theorets.univ-reims.fr, theorets.tsu.ru) for theoretical spectra based on variational predictions from molecular potential energy and dipole moment surfaces [3] that permits online simulation of radiative properties including low-T conditions of cold planets. In this work, we apply ab initio predictions of the spectra of methane isotopologues down to T=80 K for the modeling of the transmittance in the atmosphere of Titan, Saturn's largest satellite explored by the Cassini-Huygens space mission. A very good agreement over the whole infrared range from 6,000 to 11,000 cm-1 compared with observations obtained by the Descent Imager / Spectral Radiometer (DISR) on the Huygens probe [4,5] at various altitudes will be reported.
NASA Astrophysics Data System (ADS)
Seijo, Luis
1995-05-01
Presented in this paper, is a practical implementation of the use of the Wood-Boring Hamiltonian [Phys. Rev. B 18, 2701 (1978)] in atomic and molecular ab initio core model potential calculations (AIMP), as a means to include spin-orbit relativistic effects, in addition to the mass-velocity and Darwin operators, which were already included in the spin-free version of the relativistic AIMP method. Calculations on the neutral and singly ionized atoms of the halogen elements and sixth-row p-elements Tl-Rn are presented, as well as on the one or two lowest lying states of the diatomic molecules HX, HX+, (X=F, Cl, Br, I, At) TlH, PbH, BiH, and PoH. The calculated spin-orbit splittings and bonding properties show a stable, good quality, of the size of what can be expected from an effective potential method.
Direct molecular simulation of nitrogen dissociation based on an ab initio potential energy surface
Valentini, Paolo Schwartzentruber, Thomas E. Bender, Jason D. Nompelis, Ioannis Candler, Graham V.
2015-08-15
The direct molecular simulation (DMS) approach is used to predict the internal energy relaxation and dissociation dynamics of high-temperature nitrogen. An ab initio potential energy surface (PES) is used to calculate the dynamics of two interacting nitrogen molecules by providing forces between the four atoms. In the near-equilibrium limit, it is shown that DMS reproduces the results obtained from well-established quasiclassical trajectory (QCT) analysis, verifying the validity of the approach. DMS is used to predict the vibrational relaxation time constant for N{sub 2}–N{sub 2} collisions and its temperature dependence, which are in close agreement with existing experiments and theory. Using both QCT and DMS with the same PES, we find that dissociation significantly depletes the upper vibrational energy levels. As a result, across a wide temperature range, the dissociation rate is found to be approximately 4–5 times lower compared to the rates computed using QCT with Boltzmann energy distributions. DMS calculations predict a quasi-steady-state distribution of rotational and vibrational energies in which the rate of depletion of high-energy states due to dissociation is balanced by their rate of repopulation due to collisional processes. The DMS approach simulates the evolution of internal energy distributions and their coupling to dissociation without the need to precompute rates or cross sections for all possible energy transitions. These benchmark results could be used to develop new computational fluid dynamics models for high-enthalpy flow applications.
NASA Astrophysics Data System (ADS)
Fonseca, Leonardo
2005-03-01
Future reduction of transistor dimensions in line with historical trends cannot be achieved with the current SiO2/polysilicon technology due to limited effective oxide thickness (EOT) scalability and excessive power consumption caused by high gate leakage current. Among the proposed solutions, the high permissivity dielectric (high-K)/metal combination seems to be a promising route. While considerable progress has been made towards identifying a favorable high-K dielectric, with HfO2 and its silicates and nitrides as the leading candidates, n- and p-type metal gates with appropriate work functions still lack. A rough, ``first order'' metal gate screening can be performed with considerable confidence through measurement and calculation of metal vacuum work functions (WFs). However, charge exchange at metal/dielectric interfaces cause the metal effective WF on a particular dielectric to differ from its vacuum value, sometimes by as much as 1 eV [1]. For that reason, metal effective WF ab-initio calculations using interface models are of primary importance if theory is to be used as a guide for correctly identifying metal gates. In this talk I will discuss the role of interface states on the pinning of metal Fermi levels and show results for model HfO2/Si and Al2O3/Si interfaces that correctly reproduce experimental data with polysilicon as the gate metal [2]. Next I will describe results of theoretical metal screening for polysilicon replacement. We have found that while vacuum WF calculations can be quite accurate, hence useful as a predictive tool, metal/dielectric interface calculations are severely limited in accuracy by the lack of experimental information on the atomistic structure of the interfaces and possibly by an unexpected and still unclear drawback of density functional theory (DFT) within the local density approximation (LDA) [3]. Improvements based on empirical scaling of the DFT/LDA calculated metal/dielectric valence band offset and on bulk GW
Serralheiro, C; Duflot, D; da Silva, F Ferreira; Hoffmann, S V; Jones, N C; Mason, N J; Mendes, B; Limão-Vieira, P
2015-08-27
The electronic spectroscopy of isolated toluene in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 4.0-10.8 eV energy range, with absolute cross-section measurements derived. We present the first set of ab initio calculations (vertical energies and oscillator strengths), which we use in the assignment of valence and Rydberg transitions of the toluene molecule. The spectrum reveals several new features not previously reported in the literature, with particular relevance to 7.989 and 8.958 eV, which are here tentatively assigned to the π*(17a') ← σ(15a') and 1π*(10a″) ← 1π(14a') transitions, respectively. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of toluene in the upper stratosphere (20-50 km).
Serralheiro, C; Duflot, D; da Silva, F Ferreira; Hoffmann, S V; Jones, N C; Mason, N J; Mendes, B; Limão-Vieira, P
2015-08-27
The electronic spectroscopy of isolated toluene in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 4.0-10.8 eV energy range, with absolute cross-section measurements derived. We present the first set of ab initio calculations (vertical energies and oscillator strengths), which we use in the assignment of valence and Rydberg transitions of the toluene molecule. The spectrum reveals several new features not previously reported in the literature, with particular relevance to 7.989 and 8.958 eV, which are here tentatively assigned to the π*(17a') ← σ(15a') and 1π*(10a″) ← 1π(14a') transitions, respectively. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of toluene in the upper stratosphere (20-50 km). PMID:26244250
Phonon spectra of elpasolites Cs2NaRF6 (R=Y,Yb): Ab initio calculations
NASA Astrophysics Data System (ADS)
Chernyshev, Vladimir; Petrov, Vladislav; Nikiforov, Anatoliy; Zakiryanov, Dmitriy
2015-12-01
The influence of hydrostatic pressure on structure and dynamics of a crystal lattice of elpasolites Cs2NaYbF6 and Cs2NaYF6 (S.G. 225) within ab initio approach is investigated. Frequencies and irreducible representations (irreps) of phonon modes are determined. Elastic constants are calculated. The calculations are carried out within MO LCAO approach using DFT method with hybrid functionalities of B3LYP and PBE0 in CRYSTAL09 periodic code. For the description of rare earth ion the pseudopotential replacing internal orbitals including 4f orbitals was used. External 5s and 5p orbitals defining chemical bond were described by valence basis sets.
Phonon spectra of elpasolites Cs{sub 2}NaRF{sub 6} (R=Y,Yb): Ab initio calculations
Chernyshev, Vladimir Petrov, Vladislav; Nikiforov, Anatoliy; Zakiryanov, Dmitriy
2015-12-07
The influence of hydrostatic pressure on structure and dynamics of a crystal lattice of elpasolites Cs{sub 2}NaYbF{sub 6} and Cs{sub 2}NaYF{sub 6} (S.G. 225) within ab initio approach is investigated. Frequencies and irreducible representations (irreps) of phonon modes are determined. Elastic constants are calculated. The calculations are carried out within MO LCAO approach using DFT method with hybrid functionalities of B3LYP and PBE0 in CRYSTAL09 periodic code. For the description of rare earth ion the pseudopotential replacing internal orbitals including 4f orbitals was used. External 5s and 5p orbitals defining chemical bond were described by valence basis sets.
NASA Astrophysics Data System (ADS)
Alipour, Mojtaba; Mohajeri, Afshan
2011-08-01
We have employed conventional ab initio and density functional theory methods to study the electronic properties such as the mean static dipole polarizability, α¯, anisotropy of the polarizability, Δ α, and dipole moment, μ, of yttrium bromide. The bond length dependence of properties is determined at different levels of theory and appropriate expansions around experimental internuclear distance have been presented. Moreover, the first and second geometrical derivatives for each property are quantified and their level of theory dependence has been analyzed. To study the effect of molecular rotation and vibration on the electronic properties, the rovibrational corrections have also been carried out. It is found that these corrections are less pronounced for considered properties of YBr. In all calculations, the electron correlation effects have been considered and discussed. The obtained results show that the electron correlation is more significant in the calculation of the mean and the anisotropy of dipole polarizability.
NASA Astrophysics Data System (ADS)
Cortez, Enriqueta; Laane, Jaan
1995-02-01
Infrared and Raman spectra of the vapor, liquid, and solid phases of 1,3-dioxole have been recorded and analyzed. Much of the spectra can be interpreted assuming C2v symmetry. However, several combination bands with the ring-puckering vibration along with the observation of an otherwise inactive mode confirm the non-planarity of this molecule. The observed frequencies are compared with predicted values from molecular mechanics (MM3) and ab initio (STO3-21G∗) calculations. These calculated values provide useful estimates but about half of them differ from the observed values by more than 50 cm -1. Several predicted values disagree by more than 200 cm -1.
NASA Astrophysics Data System (ADS)
Pagès, O.; Hussein, R. Hajj; Torres, V. J. B.
2013-07-01
We formalize within the percolation scheme that operates along the linear chain approximation, i.e., at one dimension (1D), an intrinsic ability behind Raman scattering to achieve a quantitative insight into local clustering/anticlustering in an alloy, using GeSi as a case study. For doing so, we derive general expressions of the individual fractions of the six GeSi percolation-type oscillators [1×(Ge-Ge), 3×(Ge-Si), 2×(Si-Si)], which monitor directly the Raman intensities, via a relevant order parameter κ. This is introduced by adapting to the 1D oscillators of the GeSi-diamond version of the 1D-percolation scheme, i.e., along a fully consistent 1D treatment, the approach originally used by Verleur and Barker for the three-dimensional (3D) oscillators of their 1D-cluster scheme applying to zincblende alloys [H. W. Verleur and A. S. Barker, Phys. Rev. 149, 715 (1966)], a somehow problematic one in fact, due to its 3D-1D ambivalence. Predictive κ-dependent intensity-interplays between the Ge0.5Si0.5 Raman lines are confronted with existing experimental data and with ab initio Raman spectra obtained by using (32-atom) disordered supercells matching the required κ values, with special attention to the Ge-Si triplet and to the Si-Si doublet, respectively.
Efficient ab-initio thermodynamic calculations at high pressure and temperature
NASA Astrophysics Data System (ADS)
Wilson, Hugh
2014-03-01
Prediction of solubility properties and phase diagrams under conditions of high temperature and pressure requires the computation of the Gibbs free energies of materials, a property not directly accessible from molecular dynamics trajectories. Two-step coupling constant integration methods have previously achieved success in the computation of free energies of fluid, solid, and superionic phases of materials by connecting the ab-initio system of interest to a non-interacting reference system via a series of thermodynamic integration steps. These methods, however, require a series of time-consuming and computationally awkward integrations over molecular dynamics trajectories, limiting the utility of the method. Here we propose and demonstrate a method for more efficiently carrying out the same thermodynamic integration without the need for separate molecular dynamics runs, and show how it may be used to carry out the integration up to an order of magnitude more efficiently, in a massively parallel manner, and without the need for code modification. Applications of thermodynamic integration including core solubility in Jupiter and Saturn, and superionic-to-superionic phase transitions in Uranus and Neptune, will be discussed.
Guirgis, Gamil A; Klaassen, Joshua J; Deodhar, Bhushan S; Sawant, Dattatray K; Panikar, Savitha S; Dukes, Horace W; Wyatt, Justin K; Durig, James R
2012-12-01
The infrared spectra (3500-220 cm(-1)) of cyclobutylgermane, c-C(4)H(7)GeH(3) have been recorded of the gas. Also variable temperature (-65 to -100 °C) studies of the infrared spectra (3500-400 cm(-1)) of the sample dissolved in liquid xenon were recorded and both the equatorial and axial conformers were identified. The enthalpy difference has been determined from 10 band pairs 8 temperatures to give 112 ± 11 cm(-1) (1.34 ± 0.13 kJ mol(-1)) with the equatorial conformer the more stable form. The percentage of the axial conformer present at ambient temperature is estimated to be 37 ± 1%. From ab initio calculations conformational stabilities have been predicted from both MP2(full) and density functional theory calculations from a variety of basic sets. The r(0) structure parameters have been obtained for both conformers from the previously reported rotational constants from the three isotopologues. The determined heavy atom distances for the equatorial [axial] form are (Å) Ge-C(α)=1.952(3) [1.950(3)], [Formula: see text] , [Formula: see text] [1.551(3)] and angles in degrees (°) ∠GeC(α)C(β)=118.6(5) [113.4(5)], [Formula: see text] , ∠C(α)C(β)C(γ)=87.8(5) [88.8(5)], [Formula: see text] and a puckering angle of 29.1(5) [25.1(5)]. Data from ab initio calculations were used to predict vibrational harmonic force constants, fundamental wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are compared to the corresponding properties of some related molecules.
Durig, James R; Panikar, Savitha S; Groner, Peter; Nanaie, Hossein; Bürger, Hans; Moritz, Peter
2010-04-01
The microwave spectra of seven isotopomers of fluoromethylsilane, CH(2)FSiH(3), in the ground vibrational state were measured and analyzed in the frequency range 18-40 GHz. The rotational and centrifugal distortion constants were evaluated by the least-squares treatment of the observed frequencies of a- and b-type R- and b-type Q-transitions. The values for the components of the dipole moment were obtained from the measurements of Stark effects from both a- and b-type transitions and the determined values are: |mu(a)| = 1.041(5), |mu(b)| = 1.311(6), and |mu(t)| = 1.674(4) D. Structural parameters have been determined and the heavy atom distances (r(0)) in Angstroms are: Si-C = 1.8942(57) and C-F = 1.4035(55) and the angle in degree, angleSiCF = 109.58(14). A semi-experimental r(e) structure was also determined from experimental ground state rotational constants and vibration-rotation constants derived from ab initio force fields. The internal torsional fundamental, SiH(3), was observed at 149.2 cm(-1) with two accompanying hot bands at 138.8 and 127.5 cm(-1). The barrier to internal rotation was obtained as 717.3(16) cm(-1) (2.051(46) kcal mol(-1)) by combining the analysis of the microwave A and E splittings and the torsional fundamental and hot band frequencies. Ab initio calculations have been carried out with full electron correlation by the second-order perturbation method with several different basis sets up to MP2/6-311+G(d,p) to obtain geometrical parameters, barriers to internal rotation, and centrifugal distortion constants. Adjusted r(0) structural parameters have been obtained by combining the ab initio MP2/6-311+G(d,p) predicted values with the determined rotational constants for the fluoride as well as with the previously reported microwave data for the chloro- and bromo- compounds. These experimental results are compared to the corresponding parameters for the carbon analogues.
González-Cataldo, F.; Wilson, Hugh F.; Militzer, B.
2014-05-20
By combining density functional molecular dynamics simulations with a thermodynamic integration technique, we determine the free energy of metallic hydrogen and silica, SiO{sub 2}, at megabar pressures and thousands of degrees Kelvin. Our ab initio solubility calculations show that silica dissolves into fluid hydrogen above 5000 K for pressures from 10 and 40 Mbars, which has implications for the evolution of rocky cores in giant gas planets like Jupiter, Saturn, and a substantial fraction of known extrasolar planets. Our findings underline the necessity of considering the erosion and redistribution of core materials in giant planet evolution models, but they also demonstrate that hot metallic hydrogen is a good solvent at megabar pressures, which has implications for high-pressure experiments.
Ab initio calculation of x-ray absorption of iron up to 3 Mbar and 8000 K
NASA Astrophysics Data System (ADS)
Mazevet, S.; Recoules, V.; Bouchet, J.; Guyot, F.; Harmand, M.; Ravasio, A.; Benuzzi-Mounaix, A.
2014-03-01
Using ab initio simulations within the generalized gradient approximation, we calculate x-ray absorption near edge spectra (XANES) at the iron K edge throughout the high-pressure phase diagram and up to extreme density and temperature conditions that are representative of the Earth's inner core (up to 3 Mbar and 8000 K). We show that XANES spectra near the Fe K edge exhibit clear signatures for the different high-temperature, high-pressure phases of iron. This suggests that XANES spectroscopy might be used to resolve ongoing controversies regarding both the high-pressure melting curve of iron and the nature of the solid phases undergoing melting up to several Mbar. In contrast to diffraction measurements, it also offers a severe constraint for density functional theory predictions of the transport properties of iron by providing direct information on the electronic structure of iron at these extreme conditions.
Orlando, Roberto Erba, Alessandro; Dovesi, Roberto; De La Pierre, Marco; Zicovich-Wilson, Claudio M.
2014-09-14
Use of symmetry can dramatically reduce the computational cost (running time and memory allocation) of self-consistent-field ab initio calculations for molecular and crystalline systems. Crucial for running time is symmetry exploitation in the evaluation of one- and two-electron integrals, diagonalization of the Fock matrix at selected points in reciprocal space, reconstruction of the density matrix. As regards memory allocation, full square matrices (overlap, Fock, and density) in the Atomic Orbital (AO) basis are avoided and a direct transformation from the packed AO to the symmetry adapted crystalline orbital basis is performed, so that the largest matrix to be handled has the size of the largest sub-block in the latter basis. Quantitative examples, referring to the implementation in the CRYSTAL code, are given for high symmetry families of compounds such as carbon fullerenes and nanotubes.
Mizukami, Wataru; Kurashige, Yuki; Yanai, Takeshi
2010-09-01
An investigation into spin structures of poly(m-phenylenecarbene), a prototype of magnetic organic molecules, is presented using the ab initio density matrix renormalization group method. It is revealed by achieving large-scale multireference calculations that the energy differences between high-spin and low-spin states (spin-gaps) of polycarbenes decrease with increasing the number of carbene sites. This size-dependency of the spin-gaps strikingly contradicts the predictions with single-reference methods including density functional theory. The wave function analysis shows that the low-spin states are beyond the classical spin picture, namely, much of multireference character, and thus are manifested as strongly correlated quantum states. The size dependence of the spin-gaps involves an odd-even oscillation, which cannot be explained by the integer-spin Heisenberg model with a single magnetic-coupling constant.
Ab-initio calculation study on the formation mechanism of boron-oxygen complexes in c-Si
Yu, Xuegong; Chen, Peng; Chen, Xianzi; Liu, Yong; Yang, Deren
2015-07-15
Boron-oxygen (B-O) complex in crystalline silicon (c-Si) solar cells is responsible for the light-induced efficiency degradation of solar cell. However, the formation mechanism of B-O complex is not clear yet. By Ab-initio calculation, it is found that the stagger-type oxygen dimer (O{sub 2i}{sup st}) should be the component of B-O complex, whose movement occurs through its structure reconfiguration at low temperature, instead of its long-distance diffusion. The O{sub 2i}{sup st} can form two stable “latent centers” with the B{sub s}, which are recombination-inactive. The latent centers can be evolved into the metastable recombination centers via their structure transformation in the presence of excess carriers. These results can well explain the formation behaviors of B-O complexes in c-Si.
Walsh, Ian; Baù, Davide; Martin, Alberto JM; Mooney, Catherine; Vullo, Alessandro; Pollastri, Gianluca
2009-01-01
Background Prediction of protein structures from their sequences is still one of the open grand challenges of computational biology. Some approaches to protein structure prediction, especially ab initio ones, rely to some extent on the prediction of residue contact maps. Residue contact map predictions have been assessed at the CASP competition for several years now. Although it has been shown that exact contact maps generally yield correct three-dimensional structures, this is true only at a relatively low resolution (3–4 Å from the native structure). Another known weakness of contact maps is that they are generally predicted ab initio, that is not exploiting information about potential homologues of known structure. Results We introduce a new class of distance restraints for protein structures: multi-class distance maps. We show that Cα trace reconstructions based on 4-class native maps are significantly better than those from residue contact maps. We then build two predictors of 4-class maps based on recursive neural networks: one ab initio, or relying on the sequence and on evolutionary information; one template-based, or in which homology information to known structures is provided as a further input. We show that virtually any level of sequence similarity to structural templates (down to less than 10%) yields more accurate 4-class maps than the ab initio predictor. We show that template-based predictions by recursive neural networks are consistently better than the best template and than a number of combinations of the best available templates. We also extract binary residue contact maps at an 8 Å threshold (as per CASP assessment) from the 4-class predictors and show that the template-based version is also more accurate than the best template and consistently better than the ab initio one, down to very low levels of sequence identity to structural templates. Furthermore, we test both ab-initio and template-based 8 Å predictions on the CASP7 targets
Stephens, Susanna L; Tew, David P; Walker, Nicholas R; Legon, Anthony C
2016-07-28
The new compound H3PAgI has been synthesized in the gas phase by means of the reaction of laser-ablated silver metal with a pulse of gas consisting of a dilute mixture of ICF3 and PH3 in argon. Ground-state rotational spectra were detected and assigned for the two isotopologues H3P(107)AgI and H3P(109)AgI in their natural abundance by means of a chirped-pulse, Fourier-transform, microwave spectrometer. Both isotopologues exhibit rotational spectra of the symmetric-top type, analysis of which led to accurate values of the rotational constant B0, the quartic centrifugal distortion constants DJ and DJK, and the iodine nuclear quadrupole coupling constant χaa(I) = eQqaa. Ab initio calculations at the explicitly-correlated level of theory CCSD(T)(F12*)/aug-cc-pVDZ confirmed that the atoms PAg-I lie on the C3 axis in that order. The experimental rotational constants were interpreted to give the bond lengths r0(PAg) = 2.3488(20) Å and r0(Ag-I) = 2.5483(1) Å, in good agreement with the equilibrium lengths of 2.3387 Å and 2.5537 Å, respectively, obtained in the ab initio calculations. Measures of the strength of the interaction of PH3 and AgI (the dissociation energy De for the process H3PAgI = H3P + AgI and the intermolecular stretching force constant FPAg) are presented and are interpreted to show that the order of binding strength is H3PHI < H3PICl < H3PAgI for these metal-bonded molecules and their halogen-bonded and hydrogen-bonded analogues. PMID:27354204
NASA Astrophysics Data System (ADS)
Music, Denis; Schneider, Jochen M.
2008-05-01
We have studied the correlation between chemical composition, structure, chemical bonding and elastic properties of amorphous B6O based solids using ab initio molecular dynamics. These solids are of different chemical compositions, but the elasticity data appear to be a function of density. This is in agreement with previous experimental observations. As the density increases from 1.64 to 2.38 g cm-3, the elastic modulus increases from 74 to 253 GPa. This may be understood by analyzing the cohesive energy and the chemical bonding of these compounds. The cohesive energy decreases from -7.051 to -7.584 eV/atom in the elastic modulus range studied. On the basis of the electron density distributions, Mulliken analysis and radial distribution functions, icosahedral bonding is the dominating bonding type. C and N promote cross-linking of icosahedra and thus increase the density, while H hinders the cross-linking by forming OH groups. The presence of icosahedral bonding is independent of the density.
NASA Astrophysics Data System (ADS)
Campetella, Marco; Bodo, Enrico; Montagna, Maria; De Santis, Serena; Gontrani, Lorenzo
2016-03-01
We have explored by means of ab initio molecular dynamics the homologue series of 11 different ionic liquids based on the combination of the cholinium cation with deprotonated amino acid anions. We present a structural analysis of the liquid states of these compounds as revealed by accurate ab initio computations of the forces. We highlight the persistent structural motifs that see the ionic couple as the basic building block of the liquid whereby a strong hydrogen bonding network substantially determines the short range structural behavior of the bulk state. Other minor docking features of the interaction network are also discovered and described. Special cases along the series such as Cysteine and Phenylalanine are discussed in the view of their peculiar properties due to zwitterion formation and additional long-range structural organization.
Dane Morgan
2010-06-10
The project began March 13, 2006, allocated for three years, and received a one year extension from March 13, 2009 to March 12, 2010. It has now completed 48 of 48 total months. The project was focused on using ab initio methods to gain insights into radiation induced segregation (RIS) in Ni-Fe-Cr alloys. The project had the following key accomplishments • Development of a large database of ab initio energetics that can be used by many researchers in the future for increased understanding of this system. For example, we have the first calculations showing a dramatic stabilization effect of Cr-Cr interstitial dumbbells in Ni. • Prediction of both vacancy and interstitial diffusion constants for Ni-Cr and Ni-Fe for dilute Cr and Fe. This work included generalization of widely used multifrequency models to make use of ab initio derived energetics and thermodynamics. • Prediction of qualitative trends of RIS from vacancy and interstitial mechanisms, suggesting the two types of defect fluxes drive Cr RIS in opposite directions. • Detailed kinetic Monte Carlo modeling of diffusion by vacancy mechanism in Ni-Cr as a function of Cr concentration. The results demonstrate that Cr content can have a significant effect on RIS. • Development of a quantitative RIS transport model, including models for thermodynamic factors and boundary conditions.
Wang, Yimin; Braams, Bastiaan J; Bowman, Joel M; Carter, Stuart; Tew, David P
2008-06-14
Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1 kcalmol, in excellent agreement with the reported ab initio value. Model one-dimensional and "exact" full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased "fixed-node" diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6 cm(-1) in Cartesian coordinates and 22.6 cm(-1) in normal coordinates, with an uncertainty of 2-3 cm(-1). This splitting is also calculated based on a model which makes use of the exact single-well zero-point energy (ZPE) obtained with the MULTIMODE code and DMC ZPE and this calculation gives a tunneling splitting of 21-22 cm(-1). The corresponding computed splittings for the D-atom transfer are 3.0, 3.1, and 2-3 cm(-1). These calculated tunneling splittings agree with each other to within less than the standard uncertainties obtained with the DMC method used, which are between 2 and 3 cm(-1), and agree well with the experimental values of 21.6 and 2.9 cm(-1) for the H and D transfer, respectively.
Chen, Ziqiu; van Wijngaarden, Jennifer
2012-09-27
Rotationally resolved vibrational spectra of the four-membered heterocycle 3-oxetanone (c-C(3)H(4)O(2)) have been investigated in the 360-720 cm(-1) region with a resolution of 0.000 959 cm(-1) using synchrotron radiation from the Canadian Light Source. The observed bands correspond to motions best described as C═O deformation out-of-plane (ν(20)) at 399.6 cm(-1), C═O deformation in-plane (ν(16)) at 448.2 cm(-1), and the ring deformation (ν(7)) at 685.0 cm(-1). Infrared ground state combination differences along with previously reported pure rotational transitions were used to obtain the ground state spectroscopic parameters. Band centers, rotational and centrifugal distortion constants for the ν(7), ν(16), and ν(20) vibrational excited states were accurately determined by fitting a total of 10,319 assigned rovibrational transitions in a global analysis. The two adjacent carbonyl deformation bands, ν(16) and ν(20), were found to be mutually perturbed through a first-order a-type Coriolis interaction which was accounted for in the multiband analysis. The band centers agree within 3% of the ab initio estimates using DFT theory.
Computer simulation of acetonitrile and methanol with ab initio-based pair potentials
NASA Astrophysics Data System (ADS)
Hloucha, M.; Sum, A. K.; Sandler, S. I.
2000-10-01
This study address the adequacy of ab initio pair interaction energy potentials for the prediction of macroscopic properties. Recently, Bukowski et al. [J. Phys. Chem. A 103, 7322 (1999)] performed a comprehensive study of the potential energy surfaces for several pairs of molecules using symmetry-adapted perturbation theory. These ab initio energies were then fit to an appropriate site-site potential form. In an attempt to bridge the gap between ab initio interaction energy information and macroscopic properties prediction, we performed Gibbs ensemble Monte Carlo (GEMC) simulations using their developed pair potentials for acetonitrile and methanol. The simulations results show that the phase behavior of acetonitrile is well described by just the pair interaction potential. For methanol, on the other hand, pair interactions are insufficient to properly predict its vapor-liquid phase behavior, and its saturated liquid density. We also explored simplified forms for representing the ab initio interaction energies by refitting a selected range of the data to a site-site Lennard-Jones and to a modified Buckingham (exponential-6) potentials plus Coulombic interactions. These were also used in GEMC simulations in order to evaluate the quality and computational efficiency of these different potential forms. It was found that the phase behavior prediction for acetonitrile and methanol are highly dependent on the details of the interaction potentials developed.
Huang, Lei; Roux, Benoît
2013-08-13
Classical molecular dynamics (MD) simulations based on atomistic models are increasingly used to study a wide range of biological systems. A prerequisite for meaningful results from such simulations is an accurate molecular mechanical force field. Most biomolecular simulations are currently based on the widely used AMBER and CHARMM force fields, which were parameterized and optimized to cover a small set of basic compounds corresponding to the natural amino acids and nucleic acid bases. Atomic models of additional compounds are commonly generated by analogy to the parameter set of a given force field. While this procedure yields models that are internally consistent, the accuracy of the resulting models can be limited. In this work, we propose a method, General Automated Atomic Model Parameterization (GAAMP), for generating automatically the parameters of atomic models of small molecules using the results from ab initio quantum mechanical (QM) calculations as target data. Force fields that were previously developed for a wide range of model compounds serve as initial guess, although any of the final parameter can be optimized. The electrostatic parameters (partial charges, polarizabilities and shielding) are optimized on the basis of QM electrostatic potential (ESP) and, if applicable, the interaction energies between the compound and water molecules. The soft dihedrals are automatically identified and parameterized by targeting QM dihedral scans as well as the energies of stable conformers. To validate the approach, the solvation free energy is calculated for more than 200 small molecules and MD simulations of 3 different proteins are carried out.
NASA Astrophysics Data System (ADS)
Zhu, X. L.; Zeng, X. C.; Lei, Y. A.; Pan, B.
2004-05-01
Ab initio all-electron molecular-orbital calculations are carried out to study the structures and relative stability of low-energy silicon clusters (Sin,n=12-20). Selected geometric isomers include those predicted by Ho et al. [Nature (London) 392, 582 (1998)] based on an unbiased search with tight-binding/genetic algorithm, as well as those found by Rata et al. [Phys. Rev. Lett. 85, 546 (2000)] based on density-functional tight-binding/single-parent evolution algorithm. These geometric isomers are optimized at the Møller-Plesset (MP2) MP2/6-31G(d) level. The single-point energy at the coupled-cluster single and double substitutions (including triple excitations) [CCSD(T)] CCSD(T)/6-31G(d) level for several low-lying isomers are further computed. Harmonic vibrational frequency analysis at the MP2/6-31G(d) level of theory is also undertaken to assure that the optimized geometries are stable. For Si12-Si17 and Si19 the isomer with the lowest-energy at the CCSD(T)/6-31G(d) level is the same as that predicted by Ho et al., whereas for Si18 and Si20, the same as predicted by Rata et al. However, for Si14 and Si15, the vibrational frequency analysis indicates that the isomer with the lowest CCSD(T)/6-31G(d) single-point energy gives rise to imaginary frequencies. Small structural perturbation onto the Si14 and Si15 isomers can remove the imaginary frequencies and results in new isomers with slightly lower MP2/6-31G(d) energy; however the new isomers have a higher single-point energy at the CCSD(T)/6-31G(d) level. For most Sin (n=12-18,20) the low-lying isomers are prolate in shape, whereas for Si19 a spherical-like isomer is slightly lower in energy at the CCSD(T)/6-31G(d) level than low-lying prolate isomers.
Akin-Ojo, Omololu; Song, Yang; Wang, Feng
2008-08-14
A new method called adaptive force matching (AFM) has been developed that is capable of producing high quality force fields for condensed phase simulations. This procedure involves the parametrization of force fields to reproduce ab initio forces obtained from condensed phase quantum-mechanics/molecular-mechanics (QM/MM) calculations. During the procedure, the MM part of the QM/MM is iteratively improved so as to approach ab initio quality. In this work, the AFM method has been tested to parametrize force fields for liquid water so that the resulting force fields reproduce forces calculated using the ab initio MP2 and the Kohn-Sham density functional theory with the Becke-Lee-Yang-Parr (BLYP) and Becke three-parameter LYP (B3LYP) exchange correlation functionals. The AFM force fields generated in this work are very simple to evaluate and are supported by most molecular dynamics (MD) codes. At the same time, the quality of the forces predicted by the AFM force fields rivals that of very expensive ab initio calculations and are found to successfully reproduce many experimental properties. The site-site radial distribution functions (RDFs) obtained from MD simulations using the force field generated from the BLYP functional through AFM compare favorably with the previously published RDFs from Car-Parrinello MD simulations with the same functional. Technical aspects of AFM such as the optimal QM cluster size, optimal basis set, and optimal QM method to be used with the AFM procedure are discussed in this paper.
Niu, Zhen-Wei; Zeng, Zhao-Yi; Hu, Cui-E; Cai, Ling-Cang; Chen, Xiang-Rong
2015-01-07
The thermodynamic properties of CeO{sub 2} have been reevaluated by a simple but accurate scheme. All our calculations are based on the self-consistent ab initio lattice dynamical (SCAILD) method that goes beyond the quasiharmonic approximation. Through this method, the effects of phonon-phonon interactions are included. The obtained thermodynamic properties and phonon dispersion relations are in good agreement with experimental data when considering the correction of phonon-phonon interaction. We find that the correction of phonon-phonon interaction is equally important and should not be neglected. At last, by comparing with quasiharmonic approximation, the present scheme based on SCAILD method is probably more suitable for high temperature systems.
NASA Astrophysics Data System (ADS)
Li, Xiaoping; Harrison, James F.; Gustafsson, Magnus; Wang, Fei; Abel, Martin; Frommhold, Lothar; Hunt, Katharine L. C.
2012-12-01
New ab initio results are reported for the interaction-induced changes in the dipole moments and polarizabilities of pairs of hydrogen molecules, computed using finite-field coupled-cluster methods in MOLPRO 2000 and GAMESS, with an aug-cc-pV5Z (spdf) basis set. Earlier work by X. Li, C. Ahuja, J. F. Harrison, and K. L. C. Hunt, J. Chem. Phys. 126, 214302 (2007), on collision-induced polarizabilities Δα has been extended with 170 additional geometrical configurations of the H2 pairs. In calculations of Δα, we have used a "random field" technique, with up to 120 different field strengths, having components that range from 0.001 to 0.01 a.u. Numerical tests show that the pair dipoles Δμ can be obtained accurately from calculations limited to 6 values of the field in each direction, so this approach has been used to compute Δμ by X. Li, K. L. C. Hunt, F. Wang, M. Abel, and L. Frommhold, Int. J. Spectroscopy 2010, 371201 (2010). We have evaluated the collision-induced dipoles of H2 pairs for 28 combinations of bond lengths (ranging from 0.942 a.u. to 2.801 a.u.), 7 intermolecular separations R, and 17 different relative orientations. In our work on Δα, the bond lengths are fixed at 1.449 a.u. Our results agree well with the previous ab initio work of W. Meyer, A. Borysow, and L. Frommhold, Phys. Rev. A 40, 6931 (1989), and of Y. Fu, C. G. Zheng and A. Borysow, J. Quant. Spectroscopy and Rad. Transfer, 67, 303 (2000)-where those data exist-for Δμ of H2 pairs. For Δα, our results agree well with the CCSD(T) results obtained by G. Maroulis, J. Phys. Chem. A 104, 4772 (2000) for two pair orientations and fixed R. The pair polarizability anisotropies also agree well with the small-basis self-consistent field results of D. G. Bounds, Mol. Phys. 38, 2099 (1979), although the trace of the polarizability differs by factors of 2 or more from Bounds' results. We have determined the expansion coefficients for Δμ and Δα, expressed as series in the spherical
NASA Astrophysics Data System (ADS)
Rajesh, C.; Majumder, C.; Rajan, M. G. R.; Kulshreshtha, S. K.
2005-12-01
The geometric and electronic structure of the Pbn clusters (n=2-15) has been calculated to elucidate its structural evolution and compared with other group-IV elemental clusters. The search for several low-lying isomers was carried out using the ab initio molecular dynamics simulations under the framework of the density functional theory formalism. The results suggest that unlike Si, Ge, and Sn clusters, which favor less compact prolate shape in the small size range, Pb clusters favor compact spherical structures consisting of fivefold or sixfold symmetries. The difference in the growth motif can be attributed to their bulk crystal structure, which is diamond-like for Si, Ge, and Sn but fcc for Pb. The relative stability of Pbn clusters is analyzed based on the calculated binding energies and second difference in energy. The results suggest that n=4 , 7, 10, and 13 clusters are more stable than their respective neighbors, reflecting good agreement with experimental observation. Based on the fragmentation pattern it is seen that small clusters up to n=12 favor monomer evaporation, larger ones fragment into two stable daughter products. The experimental observation of large abundance for n=7 and lowest abundance of n=14 have been demonstrated from their fragmentation pattern. Finally a good agreement of our theoretical results with that of the experimental findings reported earlier implies accurate predictions of the ground state geometries of these clusters.
Chalupský, Jakub Yanai, Takeshi
2013-11-28
The derivation, implementation, and validation of a new approximation to the two-electron spin–orbit coupling (SOC) terms is reported. The approximation, referred to as flexible nuclear screening spin–orbit, is based on the effective one-electron spin–orbit operator and accounts for two-electron SOC effects by screening nuclear charges. A highly flexible scheme for the nuclear screening is developed, mainly using parameterization based on ab initio atomic SOC calculations. Tabulated screening parameters are provided for contracted and primitive Gaussian-type basis functions of the ANO-RCC basis set for elements from H to Cm. The strategy for their adaptation to any other Gaussian basis set is presented and validated. A model to correct for the effect of splitting of transition metal d orbitals on their SOC matrix elements is introduced. The method is applied to a representative set of molecules, and compared to exact treatment and other approximative approaches at the same level of relativistic theory. The calculated SOC matrix elements are in very good agreement with their “exact” values; deviation below 1% is observed on average. The presented approximation is considered to be generally applicable, simple to implement, highly efficient, and accurate.
The Anharmonic Force Field of Ethylene, C2H4, by Means of Accurate Ab Initio Calculations
NASA Technical Reports Server (NTRS)
Martin, Jan M. L.; Lee, Timothy J.; Taylor, Peter R.; Francois, Jean-Pierre; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
The quartic force field of ethylene, C2H4, has been calculated ab initio using augmented coupled cluster, CCSD(T), methods and correlation consistent basis sets of spdf quality. For the C-12 isotopomers C2H4, C2H3D, H2CCD2, cis-C2H2D2, trans-C2H2D2, C2HD3, and C2D4, all fundamentals could be reproduced to better than 10 per centimeter, except for three cases of severe Fermi type 1 resonance. The problem with these three bands is identified as a systematic overestimate of the Kiij Fermi resonance constants by a factor of two or more; if this is corrected for, the predicted fundamentals come into excellent agreement with experiment. No such systematic overestimate is seen for Fermi type 2 resonances. Our computed harmonic frequencies suggest a thorough revision of the accepted experimentally derived values. Our computed and empirically corrected re geometry differs substantially from experimentally derived values: both the predicted rz geometry and the ground-state rotational constants are, however, in excellent agreement with experiment, suggesting revision of the older values. Anharmonicity constants agree well with experiment for stretches, but differ substantially for stretch-bend interaction constants, due to equality constraints in the experimental analysis that do not hold. Improved criteria for detecting Fermi and Coriolis resonances are proposed and found to work well, contrary to the established method based on harmonic frequency differences that fails to detect several important resonances for C2H4 and its isotopomers. Surprisingly good results are obtained with a small spd basis at the CCSD(T) level. The well-documented strong basis set effect on the v8 out-of-plane motion is present to a much lesser extent when correlation-optimized polarization functions are used. Complete sets of anharmonic, rovibrational coupling, and centrifugal distortion constants for the isotopomers are available as supplementary material to the paper.
NASA Astrophysics Data System (ADS)
Martin, Jan M. L.
The quartic force fields of a number of small polyatomic molecules (specifically, rm H _2O, NH_2, NH_3, CH_4, BH_3, BeH_2, H_2CO, N_2O, CO_2, CS_2, OCS, H_2S, FNO, ClNO, and H_2CS) have been computed ab initio using large basis sets and augmented coupled cluster methods. It has been established throughout that harmonic and fundamental frequencies can consistently be reproduced to within about 10 cm^{ -1} of experimental using spdf basis sets, except in such inherently problematic cases as the umbrella motion in NH_3; such problems are solved by recomputing the harmonic frequencies with an spdf g basis set. Coupled cluster frequencies using small basis sets of spd quality agree surprisingly well with experiment (mean absolute error of 26 cm^ {-1}), but bond distances are generally seriously overestimated. Using spdf basis sets, they are consistently overestimated by 0.002 and 0.006-7 A for single and multiple bonds, respectively; for spdf g basis sets this drops to 0.001 and 0.003-4 A, respectively. Geometries and harmonic frequencies for highly polar fluorine compounds such as HF and FNO are qualitatively wrong unless special anion functions are added to the fluorine basis set. Anharmonicity, rovibrational coupling, and centrifugal distortion constants are consistently predicted well; the anharmonic portions of the computed force fields are probably more reliable than their experimental counterparts in many cases. Remaining errors in the computed geometries and harmonic frequencies are shown to be almost entirely due to a combination of core correlation and residual deficiencies in the electron correlation treatment. A 3-term correction for remaining basis set incompleteness to computed total atomization energies is proposed by the author, and is shown to result in mean absolute errors of as little as 0.5 kcal/mol for spdf g basis sets. Example applications on rm HCO^+, HOC^+, B_2C, BCN, and BNC testify to the predictive power of the methods used in this work.
NASA Technical Reports Server (NTRS)
Davidson, E. R.; Ellenbogen, J. C.; Langhoff, S. R.
1980-01-01
The spin dipole-dipole and spin-orbit contributions to the zero-field splitting of the 3A-double prime state of formaldehyde have been evaluated at the excited state experimental geometry. Ab initio CI wave functions were generated from a Dunning double zeta plus polarization bases set using 3A-double prime rhf orbitals. Twelve states of each symmetry were used to evaluate the second-order spin-orbit effect. The resulting values of D and E were 0.19 and 0.03 kayser with the principal magnetic axes rotated 36 deg from the CO bond. The values of alpha and beta relative to the inertial axes were calculated to be 0.03 and 0.01 kayser compared to the experimental values of 0.05 plus or minus 0.01 and 0.02 plus or minus 0.02 kayser.
NASA Astrophysics Data System (ADS)
Sathiyanarayanan, Rajesh; Einstein, T. L.
2009-08-01
We have parameterized the various interactions between Cu adatoms on Cu(1 1 0) using density-functional theory based ab-initio calculations. Our results indicate that in addition to pair interactions, 3-adatom and 4-adatom interactions of significant strengths are present in this system. This further stresses the importance of multi-site interactions in constructing a complete lattice-gas picture. Even though adding these multi-site interactions leads to good convergence in interaction energies, we find that some multi-site interactions are very sensitive to adatom relaxations. This makes the application of a simple lattice-gas picture inadequate for such surfaces. We also parameterize adatom interactions on this surface using the recently developed connector model. The connector model parameterization is as efficient as the parameterization using lattice-gas model. Further, we present diffusion barriers for nearest-neighbor (NN) and next-nearest-neighbor (NNN) hops on this surface.
Orellana, Walter
2014-07-14
The stability, electronic, and optical properties of (6,5) single-walled carbon nanotubes (CNTs) functionalized with free-base tetraphenylporphyrin (TPP) molecules through π-stacking interactions are studied by ab-initio calculations. The stability and optical response of the CNT-TPP compounds for increasing CNT-surface coverage are investigated. Our results show that four TPP molecules forming a ring around the CNT is the most stable configuration, showing strong binding energies of about 2.5 eV/TPP. However, this binding energy can increase even more after additional molecules assemble side by side along the CNT, favoring the formation of a full single layer of TPP, as experimentally suggested. The strong π-π attractive forces induce molecular distortions that move the TPP higher-occupied molecular orbital levels inside the CNT bandgap, changing the optical response of the TPP molecules stacked on the CNT.
NASA Technical Reports Server (NTRS)
Gregurick, Susan K.; Chaban, Galina M.; Gerber, R. Benny; Kwak, Dochou (Technical Monitor)
2001-01-01
The second-order Moller-Plesset ab initio electronic structure method is used to compute points for the anharmonic mode-coupled potential energy surface of N-methylacetamide (NMA) in the trans(sub ct) configuration, including all degrees of freedom. The vibrational states and the spectroscopy are directly computed from this potential surface using the Correlation Corrected Vibrational Self-Consistent Field (CC-VSCF) method. The results are compared with CC-VSCF calculations using both the standard and improved empirical Amber-like force fields and available low temperature experimental matrix data. Analysis of our calculated spectroscopic results show that: (1) The excellent agreement between the ab initio CC-VSCF calculated frequencies and the experimental data suggest that the computed anharmonic potentials for N-methylacetamide are of a very high quality; (2) For most transitions, the vibrational frequencies obtained from the ab initio CC-VSCF method are superior to those obtained using the empirical CC-VSCF methods, when compared with experimental data. However, the improved empirical force field yields better agreement with the experimental frequencies as compared with a standard AMBER-type force field; (3) The empirical force field in particular overestimates anharmonic couplings for the amide-2 mode, the methyl asymmetric bending modes, the out-of-plane methyl bending modes, and the methyl distortions; (4) Disagreement between the ab initio and empirical anharmonic couplings is greater than the disagreement between the frequencies, and thus the anharmonic part of the empirical potential seems to be less accurate than the harmonic contribution;and (5) Both the empirical and ab initio CC-VSCF calculations predict a negligible anharmonic coupling between the amide-1 and other internal modes. The implication of this is that the intramolecular energy flow between the amide-1 and the other internal modes may be smaller than anticipated. These results may have
Gounev, T K; Guirgis, G A; Zhen, P; Durig, J R
2000-11-15
The infrared (3,200-30 cm(-1) spectra of gaseous and solid 1-bromosilacyclobutane, c-C3H6SiBrH, have been recorded. Additionally, the Raman spectra of the liquid (3,200- 30 cm(-1) with quantitative depolarization values and the solid have been recorded. Both the equatorial and the axial conformers have been identified in the fluid phases, Variable temperature ( - 105 to - 150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data the enthalpy difference has been determined to be 182 +/- 18 cm(-1) (2.18 +/- 0.22 kJ/mol) with the equatorial conformer the more stable rotamer and only conformer remaining in the annealing solid. At ambient temperature there is approximately 22% of the axial conformer present in the vapor phase. A complete vibrational assignment is proposed for both conformers based on infrared contours, relative intensities, depolarization values and group frequencies. The vibrational assignments are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations. From the frequencies of the Si-H stretch, the Si-H bond distance of 1.483 A has been determined for both the equatorial and the axial conformers. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing the 6-31G(d) and 6-311 +/- G(d,p) basis sets at levels of Hartree Fock (RHF) and/or Moller- Plesset with full electron correlation by the perturbation method to the second order (MP2). The results are discussed and compared to those obtained for some similar molecules.
NASA Astrophysics Data System (ADS)
Gróf, M.; Gatial, A.; Milata, V.; Prónayová, N.; Kožíšek, J.; Breza, M.; Matějka, P.
2008-11-01
The IR, Raman and NMR spectra of 3- N, N-dimethylhydrazino-2-methylsulfonyl propenenitrile (DMHSP) [(H 3C) 2N sbnd NH sbnd CH dbnd C(CN) (SO 2CH 3)] as a solid and in different solvents were measured. The spectra and X-ray analysis revealed that DMHSP was prepared as a pure E-isomer and E- syn conformer with the syn orientation of N, N-dimethylhydrazino group towards the C dbnd C double bond in the solid state. Due to the low barrier practically free isomerization process occurred in solutions at room temperature. DMHSP exists in more polar solvents as pure E-isomer in conformational equilibrium between E- syn and E- anti but in a less polar solvent the presence of Z-isomer was observed as well. From the IR and NMR temperature dependence spectra in polar solvents the energy difference between E- anti and E- syn of Δ H = 2.3 ± 0.9 kJ/mol and Δ H = 3.2 ± 0.4 kJ/mol, respectively, was estimated with the syn one being more stable. The geometries and relative energies of possible conformers of DMHSP were evaluated using ab initio MP2 and B3LYP density functional methods in 6-31G ∗∗ basis set and compared with the X-ray data. The interpretation of NMR spectra was supported by ab initio MP2 calculations. The influence of solvent polarity on the conformational equilibrium is discussed with respect to the SCRF solvent effect calculations using PCM model. In addition, the observed IR and Raman bands were compared also with harmonic vibrational frequencies, calculated on the same levels of theory, and assigned on the base of potential energy distribution.
Kishi, Ryohei; Fujii, Hiroaki; Minami, Takuya; Shigeta, Yasuteru; Nakano, Masayoshi
2015-01-22
In this study, we apply the ab initio molecular orbital - configuration interaction based quantum master equation (MOQME) approach to the calculation and analysis of the dynamic first hyperpolarizabilities (β) of asymmetric π-conjugated molecules. In this approach, we construct the excited state models by the ab initio configuration interaction singles method. Then, time evolutions of system reduced density matrix ρ(t) and system polarization p(t) are calculated by the QME approach. Dynamic β in the second harmonic generation is calculated based on the nonperturbative definition of nonlinear optical susceptibility, using the frequency domain system polarization p(ω). Spatial contributions of electrons to β are analyzed based on the dynamic hyperpolarizability density map, which visualizes the second-order response of charge density oscillating with a frequency of 2ω. We apply the present method to the calculation of the dynamic β of a series of donor/acceptor substituted polyene oligomers, and then discuss the applicability of the MOQME method to the calculation and analysis of dynamic NLO properties of molecular systems.
Ab initio calculation of electron-phonon coupling in monoclinic β-Ga2O3 crystal
NASA Astrophysics Data System (ADS)
Ghosh, Krishnendu; Singisetti, Uttam
2016-08-01
The interaction between electrons and vibrational modes in monoclinic β-Ga2O3 is theoretically investigated using ab-initio calculations. The large primitive cell of β-Ga2O3 gives rise to 30 phonon modes all of which are taken into account in transport calculation. The electron-phonon interaction is calculated under density functional perturbation theory and then interpolated using Wannier-Fourier interpolation. The long-range interaction elements between electrons and polar optical phonon (POP) modes are calculated separately using the Born effective charge tensor. The direction dependence of the long-range POP coupling in a monoclinic crystal is explored and is included in the transport calculations. Scattering rate calculations are done using the Fermi golden rule followed by solving the Boltzmann transport equation using the Rode's method to estimate low field mobility. A room temperature mobility of 115 cm2/V s is observed. Comparison with recent experimentally reported mobility is done for a wide range of temperatures (30 K-650 K). It is also found that the POP interaction dominates the electron mobility under low electric field conditions. The relative contribution of the different POP modes is analyzed and the mode 21 meV POP is found to have the highest impact on low field electron mobility at room temperature.
NASA Astrophysics Data System (ADS)
Hajj Hussein, R.; Pagès, O.; Polian, A.; Postnikov, A. V.; Dicko, H.; Firszt, F.; Strzałkowski, K.; Paszkowicz, W.; Broch, L.; Ravy, S.; Fertey, P.
2016-05-01
Near-forward Raman scattering combined with ab initio phonon and bond length calculations is used to study the ‘phonon-polariton’ transverse optical modes (with mixed electrical-mechanical character) of the II-VI ZnSe1-x S x mixed crystal under pressure. The goal of the study is to determine the pressure dependence of the poorly-resolved percolation-type Zn-S Raman doublet of the three oscillator [1 × (Zn-Se), 2 × (Zn-S)] ZnSe0.68S0.32 mixed crystal, which exhibits a phase transition at approximately the same pressure as its two end compounds (~14 GPa, zincblende → rocksalt), as determined by high-pressure x-ray diffraction. We find that the intensity of the lower Zn-S sub-mode of ZnSe0.68S0.32, due to Zn-S bonds vibrating in their own (S-like) environment, decreases under pressure (Raman scattering), whereas its frequency progressively converges onto that of the upper Zn-S sub-mode, due to Zn-S vibrations in the foreign (Se-like) environment (ab initio calculations). Ultimately, only the latter sub-mode survives. A similar ‘phonon freezing’ was earlier evidenced with the well-resolved percolation-type Be-Se doublet of Zn1-x Be x Se (Pradhan et al 2010 Phys. Rev. B 81 115207), that exhibits a large contrast in the pressure-induced structural transitions of its end compounds. We deduce that the above collapse/convergence process is intrinsic to the percolation doublet of a short bond under pressure, at least in a ZnSe-based mixed crystal, and not due to any pressure-induced structural transition.
NASA Astrophysics Data System (ADS)
Bürger, H.; Schneider, W.; Sommer, S.; Thiel, W.; Willner, H.
1991-10-01
Infrared spectra of the short-lived difluoroethyne molecule have been recorded in neon and argon matrices between 200 and 5000 cm-1. Fourier transform infrared spectra with a resolution of 0.004 cm-1 have been measured in the gas phase around 1350 cm-1 (ν3, ν2+ν4+ν5, hot bands) and 2150 cm-1 (ν2+ν3, ν1-ν5, hot bands). The high resolution study yields rotational parameters of the ground and all singly excited vibrational states. The interpretation of the experimental data has been guided by ab initio calculations at the SCF (self-consistent-field) level and the correlated MP2 level (Moller-Plesset second order perturbation theory) employing three different large basis sets. The theoretical calculations provide the SCF and MP2 harmonic fields as well as the SCF anharmonic force field of FCCF. The agreement between the available theoretical and experimental results is generally quite good, with the exception of the spectroscopic constants involving the trans-bending mode ν4 where more theoretical work is required. The combined use of theoretical and experimental information leads to an estimate of the equilibrium structure [D∞h, re(CC)=1.1865 Å, re(CF)=1.2832 Å] and to recommended ``best'' values for the wave numbers of all fundamental vibrations based on the matrix and high resolution infrared data and some ab initio anharmonicity constants. The present study demonstrates the advantages of a combined theoretical and experimental approach to the spectroscopy of short-lived molecules.
Ab initio Sternheimer-GW method for quasiparticle calculations using plane waves
NASA Astrophysics Data System (ADS)
Lambert, Henry; Giustino, Feliciano
2013-08-01
We report on the extension and implementation of the Sternheimer-GW method introduced by Giustino [Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.81.115105 81, 115105 (2010)] to the case of first-principles pseudopotential calculations based on a plane-waves basis. The Sternheimer-GW method consists of calculating the GW self-energy operator without resorting to the standard expansion over unoccupied Kohn-Sham electronic states. The Green's function is calculated by solving linear systems for frequencies along the real axis. The screened Coulomb interaction is calculated for frequencies along the imaginary axis by using the Sternheimer equation. Analytic continuation to the real axis is performed using Padé approximants. The generalized plasmon-pole approximation is avoided by performing explicit calculations at multiple frequencies using Frommer's multishift solver. We demonstrate our methodology by reporting tests on common insulators and semiconductors, including Si, diamond, LiCl, and SiC. Our calculated quasiparticle energies are in agreement with the results of fully converged calculations based on the sum-over-states approach. As the Sternheimer-GW method yields the complete self-energy Σ(r,r',ω) and not only its expectation values on Kohn-Sham states, this work opens the way to nonperturbative GW calculations and to direct calculations of spectral functions for angle-resolved photoemission spectroscopy. As an example of the capabilities of the method we calculate the G0W0 spectral functions of silicon and diamond.
NASA Astrophysics Data System (ADS)
Bucci, F.; Sanna, A.; Continenza, A.; Katrych, S.; Karpinski, J.; Gross, E. K. U.; Profeta, G.
2016-01-01
As a follow-up to the discovery of a new family of Fe-based superconductors, namely, the RE4Fe2As2Te1 -xO4 (42214) (RE = Pr, Sm, and Gd), we present a detailed ab initio study of these compounds highlighting the role of rare-earth (RE) atoms, external pressure, and Te content on their physical properties. Modifications of the structural, magnetic, and electronic properties of the pure (e.g., x =0.0 ) 42214 compounds and their possible correlations with the observed superconducting properties are calculated and discussed. The careful analysis of the results obtained shows that (i) changing the RE atoms allows one to tune the internal pressure acting on the As height with respect to the Fe planes; (ii) similarly to other Fe pnictides, the 42214 pure compounds show an antiferromagnetic-stripe magnetic ground state phase joined by an orthorhombic distortion (not experimentally found yet); (iii) smaller RE atoms increase the magnetic instability of the compounds possibly favoring the onset of the superconducting state; (iv) external pressure induces the vanishing of the magnetic order with a transition to the tetragonal phase and can be a possible experimental route towards higher superconducting critical temperature (Tc) ; and (v) Te vacancies act on the structural parameters, changing the As height and affecting the stability of the magnetic phase.
NASA Astrophysics Data System (ADS)
Tohme, Samir N.; Korek, Mahmoud; Awad, Ramadan
2015-03-01
Ab initio techniques have been applied to investigate the electronic structure of the LiYb molecule. The potential energy curves have been computed in the Born-Oppenheimer approximation for the ground and 29 low-lying doublet and quartet excited electronic states. Complete active space self-consistent field, multi-reference configuration interaction, and Rayleigh Schrödinger perturbation theory to second order calculations have been utilized to investigate these states. The spectroscopic constants, ωe, Re, Be, …, and the static dipole moment, μ, have been investigated by using the two different techniques of calculation with five different types of basis. The eigenvalues, Ev, the rotational constant, Bv, the centrifugal distortion constant, Dv, and the abscissas of the turning points, Rmin and Rmax, have been calculated by using the canonical functions approach. The comparison between the values of the present work, calculated by different techniques, and those available in the literature for several electronic states shows a very good agreement. Twenty-one new electronic states have been studied here for the first time.
Tohme, Samir N.; Korek, Mahmoud E-mail: fkorek@yahoo.com; Awad, Ramadan
2015-03-21
Ab initio techniques have been applied to investigate the electronic structure of the LiYb molecule. The potential energy curves have been computed in the Born–Oppenheimer approximation for the ground and 29 low-lying doublet and quartet excited electronic states. Complete active space self-consistent field, multi-reference configuration interaction, and Rayleigh Schrödinger perturbation theory to second order calculations have been utilized to investigate these states. The spectroscopic constants, ω{sub e}, R{sub e}, B{sub e}, …, and the static dipole moment, μ, have been investigated by using the two different techniques of calculation with five different types of basis. The eigenvalues, E{sub v}, the rotational constant, B{sub v}, the centrifugal distortion constant, D{sub v}, and the abscissas of the turning points, R{sub min} and R{sub max}, have been calculated by using the canonical functions approach. The comparison between the values of the present work, calculated by different techniques, and those available in the literature for several electronic states shows a very good agreement. Twenty-one new electronic states have been studied here for the first time.
Tohme, Samir N; Korek, Mahmoud; Awad, Ramadan
2015-03-21
Ab initio techniques have been applied to investigate the electronic structure of the LiYb molecule. The potential energy curves have been computed in the Born-Oppenheimer approximation for the ground and 29 low-lying doublet and quartet excited electronic states. Complete active space self-consistent field, multi-reference configuration interaction, and Rayleigh Schrödinger perturbation theory to second order calculations have been utilized to investigate these states. The spectroscopic constants, ωe, Re, Be, …, and the static dipole moment, μ, have been investigated by using the two different techniques of calculation with five different types of basis. The eigenvalues, Ev, the rotational constant, Bv, the centrifugal distortion constant, Dv, and the abscissas of the turning points, Rmin and Rmax, have been calculated by using the canonical functions approach. The comparison between the values of the present work, calculated by different techniques, and those available in the literature for several electronic states shows a very good agreement. Twenty-one new electronic states have been studied here for the first time. PMID:25796254
Z-Contrast STEM Imaging and Ab-Initio Calculations of Grain Boundaries in SrTiO
Kim, M.; Browning, N.D.; Pennyscook, S.J.; Sohlberg, K.; Pantelides, S.T.
1999-11-29
The understanding of electrical properties of grain boundaries in perovskites is essential for their application to capacitors, varistors and positive-temperature coefficient resistors. The origin of the electrical activity is generally attributed to the existence of charged defects in grain boundaries, usually assumed to be impurities, which set up a double Schottky barrier as they are screened by dopants in the adjacent bulk crystal. Microscopic understanding of the origin of the grain boundary charge, however, has not been achieved. It is not known yet if the charged grain boundary states are an intrinsic property of a stoichiometric grain boundary, arise from nonstoichiometry, or are caused by impurities. Here, the relation between atomic structure and electronic properties is studied by combining experiment with ab-initio calculations. The starting structures for theoretical calculations were obtained from Z-contrast images combined with electron energy loss spectroscopy to res olve the dislocation Core structures comprising the boundary. Dislocation core reconstructions are typical of all grain boundaries so far observed in this material. They avoid like-ion repulsion, and provide alternative sites for cation occupation in the grain boundaries. Optimized atomic positions are found by total energy calculations. Calculated differences in vacancy formation energies between the grain boundaries and the bulk suggest that vacancy segregation can account for the postulated grain boundary charge.
NASA Astrophysics Data System (ADS)
Gróf, M.; Polovková, J.; Gatial, A.; Milata, V.; Černuchová, P.; Prónayová, N.; Matějka, P.
2007-05-01
The isomers and conformers of two push-pull hydrazines: 3- N, N-dimethylhydrazino-2-acetyl propenenitrile [(H 3C) 2N sbnd NH sbnd CH dbnd C(CN)(COCH 3)] (DMHAP) and 3- N, N-dimethylhydrazino-2-methylsulfonyl propenenitrile [(H 3C) 2N sbnd NH sbnd CH dbnd C(CN)(SO 2CH 3)] (DMHSP) have been studied experimentally by NMR and vibrational spectroscopy and theoretically by the ab initio calculations at MP2 level in 6-31G** basis set. The IR and Raman spectra of both compounds as a solid and solute in various solvents have been recorded. The NMR spectra were obtained in chloroform and DMSO at room temperature. Both compounds have been prepared by the same way. NMR spectra revealed that DMHAP was prepared as a pure Z-isomer whereas in the case of DMHSP a pure E-isomer was obtained. Due to the low barrier for both compounds practically free isomerisation process occurred in the solutions but in opposite directions. Whereas DMHAP exists in the solid state and in the less polar solvent as Z-isomer, in more polar solvents the appearance of next two conformers of E-isomer was observed. On the contrary DMHSP exists in the solid state and in the more polar solvent as E-isomer only but in less polar solvent the presence of Z-isomer was observed as well. Conformational possibilities of both studied compounds are given by the rotation of dimethylhydrazino group with its anti- or syn-orientation towards the olefinic double bond. Moreover, by the rotation of the acetyl group with Z- and E-orientation of carbonyl bond towards olefinic double bond can occur in DMHAP. Vibrational and NMR spectra revealed the existence of single conformer with intramolecular hydrogen bond for Z-isomer in less polar solvent and next two conformers for E-isomer of DMHAP with Z-orientation of acetyl group and anti and syn orientation of dimethylhydrazine group in more polar solvents. For E-isomer of DMHSP two conformers with anti or syn orientations of dimethylhydrazino group have been also confirmed by NMR
NASA Astrophysics Data System (ADS)
Pigošová, J.; Gatial, A.; Milata, V.; Černuchová, P.; Prónayová, N.; Liptaj, T.; Matějka, P.
2005-06-01
The isomers and conformers of six push-pull enamines: 3-dimethylamino-, 3-methylamino- and 3-amino-2-acetyl propenenitrile [(H 3C) 2N-CH dbnd6 C(CN)(COCH 3), H 3C-NH-CH dbnd6 C(CN)(COCH 3) and H 2N-CH dbnd6 C(CN)(COCH 3)] and 3-dimethylamino-, 3-methylamino- and 3-amino-2-methylsulfonyl propenenitrile [(H 3C) 2N-CH dbnd6 C(CN)(SO 2CH 3), H 3C-NH-CH dbnd6 C(CN)(SO 2CH 3) and H 2N-CH dbnd6 C(CN)(SO 2CH 3)] have been studied experimentally by vibrational and NMR spectroscopy and theoretically by the ab initio calculations at MP2 level in 6-31G** basis set. The IR and Raman spectra of all compounds as a solid and solute in various solvents have been recorded in the region 4000-50 cm -1. The NMR spectra were obtained in chloroform and DMSO at room temperature. All six compounds have been prepared by the same way. NMR spectra revealed that both dimethylamino compounds were prepared as a pure E isomers whereas in the case of methylamino compounds the 3-methylamino-2-methylsulfonyl propenenitrile was prepared also as a pure E isomer but 3-methylamino-2-acetyl propenenitrile as a pure Z isomer. Also 3-amino-2-methylsulfonyl propenenitrile was obtained as a pure the E isomer, but 3-amino-2-acetyl propenenitrile as a mixture of both E and Z isomers. Confomational possibilities of studied compounds are given only by the rotation of the acetyl and methylamino groups. Vibrational spectra revealed existence of two conformers with Z and E orientation of acetyl group for 3-dimethylamino-2-acetyl propenenitrile. Two conformers with anti or syn orientation of methylamino group for 3-methylamino-2-methylsulfonyl propenenitrile have been confirmed by vibrational and NMR spectra, but only one conformer with anti orientation of methylamino group for 3-methylamino-2-acetyl propenenitrile in chloroform solution and in solid phase was found. For latter compound the additional isomer/conformer was detected in more polar solvents (acetonitrile/DMSO). These experimental findings have been
da Silva, F Ferreira; Duflot, D; Hoffmann, S V; Jones, N C; Rodrigues, F N; Ferreira-Rodrigues, A M; de Souza, G G B; Mason, N J; Eden, S; Limão-Vieira, P
2015-08-01
We present the first set of ab initio calculations (vertical energies and oscillator strengths) of the valence and Rydberg transitions of the anaesthetic compound halothane (CF3CHBrCl). These results are complemented by high-resolution vacuum ultraviolet photoabsorption measurements over the wavelength range 115-310 nm (10.8-4.0 eV). The spectrum reveals several new features that were not previously reported in the literature. Spin-orbit effects have been considered in the calculations for the lowest-lying states, allowing us to explain the broad nature of the 6.1 and 7.5 eV absorption bands assigned to σ*(C-Br) ← nBr and σ*(C-Cl) ← n(Cl) transitions. Novel absolute photoabsorption cross sections from electron scattering data were derived in the 4.0-40.0 eV range. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of halothane in the upper stratosphere (20-50 km).
da Silva, F Ferreira; Duflot, D; Hoffmann, S V; Jones, N C; Rodrigues, F N; Ferreira-Rodrigues, A M; de Souza, G G B; Mason, N J; Eden, S; Limão-Vieira, P
2015-08-01
We present the first set of ab initio calculations (vertical energies and oscillator strengths) of the valence and Rydberg transitions of the anaesthetic compound halothane (CF3CHBrCl). These results are complemented by high-resolution vacuum ultraviolet photoabsorption measurements over the wavelength range 115-310 nm (10.8-4.0 eV). The spectrum reveals several new features that were not previously reported in the literature. Spin-orbit effects have been considered in the calculations for the lowest-lying states, allowing us to explain the broad nature of the 6.1 and 7.5 eV absorption bands assigned to σ*(C-Br) ← nBr and σ*(C-Cl) ← n(Cl) transitions. Novel absolute photoabsorption cross sections from electron scattering data were derived in the 4.0-40.0 eV range. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of halothane in the upper stratosphere (20-50 km). PMID:26171941
NASA Technical Reports Server (NTRS)
Oh, B. K.; Kim, S. K.
1974-01-01
A model of helium adsorption on an argon crystal is built up from the premise that local adsorption predominates in the first layer and nonlocal adsorption in the second. Application of the virial expansion theorem to the second layer gives a series in which the first term represents the motion of a single molecule in the external potential field and the second a two-body interaction under this field. The thermodynamic functions of the adsorbed phase are calculated ab initio, the gas-solid interaction potential being derived from lattice summation and the partition function from an appropriate choice of a site-spacing polynomial to describe the periodic potential. The mutual interaction of adsorbed molecules is calculated with a two-dimensional Lennard-Jones potential. The second virial coefficient is calculated and its dependence on temperature and choice of potential is studied. It is found that the second virial coefficient is very well approximated by a two-dimensional gas in free space. The adsorption isotherm, isosteric heat, and specific heat are obtained and compared with the results of Ross and Steele, giving excellent agreement.
Knight, L.B. Jr.; Gregory, B.W.; Cobranchi, S.T.; Feller, D.; Davidson, E.R.
1987-06-10
The /sup 11/B/sub 2/ molecule has been trapped in neon and argon matrices at 5 +/- 1 K for ESR (electron spin resonance) investigations. Laser sputtering from solid boron was used to generate the B/sub 2/ molecule whose ground electronic state has been established experimentally for the first time to be /sup 3/..sigma../sub g//sup -/. The ESR nuclear hyperfine parameters for B/sub 2/ and B atoms have been compared with the results of ab initio CI theoretical calculations. Good agreement between theory and experiment is found for the dipolar component of the A tensor but not for the small and difficult to calculate isotropic hfs. Reasons that make the Fermi contact term especially difficult to calculate in this particular case are presented. The argon matrix ESR results yield the following magnetic parameters for /sup 11/B/sub 2/ in its ground electronic state: g/sub parallel/ = g/sub perpendicular/ = 2.0015 (4); A/sub parallel/ = +/- 11 (1) MHz and A/sub perpendicular/ = +/- 27.4 (3) MHz for /sup 11/B and the zero field parameter; and the D value is 3633 (3) MHz. As discussed, the most reasonable signs for the A values are A/sub parallel/ < O and A/sub perpendicular/ > 0. The neon matrix value of D was found to be 3840 (3) MHz and A/sub perpendicular/ in neon is +/- 26.6 (3) MHz.
Aruguete, Deborah A.; Marcus, Matthew A.; Li, Liang-shi; Williamson, Andrew; Fakra, Sirine; Gygi, Francois; Galli, Giulia; Alivisatos, A. Paul
2006-01-27
We report orientation-specific, surface-sensitive structural characterization of colloidal CdSe nanorods with extended X-ray absorption fine structure spectroscopy and ab-initio density functional theory calculations. Our measurements of crystallographically-aligned CdSe nanorods show that they have reconstructed Cd-rich surfaces. They exhibit orientation-dependent changes in interatomic distances which are qualitatively reproduced by our calculations. These calculations reveal that the measured interatomic distance anisotropy originates from the nanorod surface.
TRIQS/DFTTools: A TRIQS application for ab initio calculations of correlated materials
NASA Astrophysics Data System (ADS)
Aichhorn, Markus; Pourovskii, Leonid; Seth, Priyanka; Vildosola, Veronica; Zingl, Manuel; Peil, Oleg E.; Deng, Xiaoyu; Mravlje, Jernej; Kraberger, Gernot J.; Martins, Cyril; Ferrero, Michel; Parcollet, Olivier
2016-07-01
We present the TRIQS/DFTTools package, an application based on the TRIQS library that connects this toolbox to realistic materials calculations based on density functional theory (DFT). In particular, TRIQS/DFTTools together with TRIQS allows an efficient implementation of DFT plus dynamical mean-field theory (DMFT) calculations. It supplies tools and methods to construct Wannier functions and to perform the DMFT self-consistency cycle in this basis set. Post-processing tools, such as band-structure plotting or the calculation of transport properties are also implemented. The package comes with a fully charge self-consistent interface to the Wien2k band structure code, as well as a generic interface that allows to use TRIQS/DFTTools together with a large variety of DFT codes. It is distributed under the GNU General Public License (GPLv3).
NASA Astrophysics Data System (ADS)
Pask, J. E.; Sterne, P. A.
2004-03-01
The finite-element (FE) method is a general approach for the solution of partial differential equations. Like the planewave (PW) method, the FE method is a systematically improvable expansion approach. Unlike the PW method, however, its basis functions are strictly local in real space, which allows for variable resolution in real space and facilitates massively parallel implementation. We discuss the application of the FE method to ab initio electronic-structure calculations.(J.E. Pask, B.M. Klein, C.Y. Fong, and P.A. Sterne, Phys. Rev. B 59), 12352 (1999). In particular, we discuss the use of nonlocal pseudopotentials in bulk calculations, and the handling of long-range interactions in the construction of the Kohn-Sham effective potential and total energy. We show that the total energy converges variationally, and at the optimal theoretical rate consistent with the cubic completeness of the basis. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.
NASA Astrophysics Data System (ADS)
Chougale, Yashwant; Nath, Rejish
2016-07-01
We obtain ab initio the Hubbard parameters for Rydberg-dressed atoms in a one-dimensional (1D) sinusoidal optical lattice on the basis of maximally-localized Wannier states. Finite range, soft-core interatomic interactions become the trait of Rydberg admixed atoms, which can be extended over many neighboring lattice sites. In contrast to dipolar gases, where the interactions follow an inverse cubic law, the key feature of Rydberg-dressed interactions is the possibility of making neighboring couplings to the same magnitude as that of the onsite ones. The maximally-localized Wannier functions (MLWFs) are typically calculated via a spread-minimization procedure (Marzari N and Vanderbilt D 1997 Phys. Rev. B 56 12847) and are always found to be real functions apart from a trivial global phase when an isolated set of Bloch bands are considered. For an isolated single Bloch band, the above procedure reduces to a simple quasi-momentum-dependent unitary phase transformation. Here, instead of minimizing the spread, we employ a diagonal phase transformation which eliminates the imaginary part of the Wannier functions. The resulting Wannier states are found to be maximally localized and in exact agreement with those obtained via a spread-minimization procedure. Using these findings, we calculate the Hubbard couplings from the Rydberg admixed interactions, including dominant density-assisted tunneling (DAT) coefficients. Finally, we provide realistic lattice parameters for the state-of-the-art experimental Rydberg-dressed rubidium setup.
NASA Astrophysics Data System (ADS)
Dhayal, Suman; Rostovtsev, Yuri
2011-03-01
Recent progress in ultrashort, e.g. attosecond, laser technology allows to obtain ultra-strong fields which can be of the same order of magnitude as the electric field created by an atomic nucleus. Interaction of such strong and broadband field with atomic systems even under the action of a far-off resonance strong pulse of laser radiation should be revisited. As we have shown, such pulses can excite remarkable coherence on high frequency transitions. We have found and analyzed analitical solutions for various pulse shapes. We have developed new mechanisms of efficient atomic coherent excitation by using two-frequency laser pulses and via tunneling through electric fields. We have done ab initio calculations using TDDFT for several atoms and simple molecules interacting with strong optical fields. We compare efficiency generation with the efficiency of high harmonic generation approach, and discuss the CEP effects and possible applications of the results obtained to cooperative generation of XUV radiation. The efficiency of XUV generation is calculated for particular candidates for XUV radiation such as H (100 nm) and He (50 nm) atoms and H-like ions (Li 2+ (30 nm), as well as Ar 8+ and Xe 8+ (30-50 nm).
NASA Astrophysics Data System (ADS)
Gonzalez, Dayana; Mebel, Alexander
2016-03-01
It has been recently shown that Titan provides a unique perspective in our solar system: its atmosphere is comparable to a model of prebiotic Earth's. Provided the organic cationic and anionic molecular species identified by the Cassini spacecraft, this research characterizes reaction pathways for the reactions of methyl derivatives of the cyclopropenyl cation, the methyl cation with methyl- and dimethyl-acetylene, and reactions of resonance structures of protonated acrylonitrile with CH2NH. Isomerization and dissociation reactions involving methyl-cyclopropenyl cations, the perinaphthenyl cation and anion, and cations of pyrimidine and purine precursors of nucleobases will be examined to locate reaction pathways, intermediates, transition states, and products of the reactions. Gaussian '09 software is used for ab initio calculations to map out the PES. Geometry optimizations and vibrational frequency computations are preformed via the double-hybrid density functional B2PLYP-D3. Single-point energies are refined by use of the explicitly-correlated coupled-cluster CCSD(T)-F12 method. Rate constants are calculated using microcanonical RRKM theory, and pressure effects evaluated used the Master Equation approach; these allow for prediction of absolute rate constants and product branching ratios at different pressures and temperatures.
Ab initio calculations on the mechanism of charge transfer in Co-Fe Prussian-blue compounds
NASA Astrophysics Data System (ADS)
Kawamoto, T.; Asai, Y.; Abe, S.
1999-11-01
The mechanism of the heat-induced charge transfer observed in Prussian-blue compounds K1-2xCo1+xFe(CN)6 is studied theoretically in connection with photoinduced magnetism. In the case of x=0, our first-principles band calculation confirms that the band-gap excitation corresponds to charge transfer from Fe-dɛ to Co-dγ orbitals. By increasing the lattice constant (specifically, by elongating the Co-N bond in the crystal), the band gap decreases systematically due to the variation of the crystal field at each Co site. In the nonstoichiometric case (x≠0), we demonstrate on the basis of ab initio cluster calculations that a small increase in the Co-N distance is sufficient to cause charge transfer between Fe and Co atoms located near Fe vacancies accompanied by water molecules. This mechanism by electron-lattice coupling combined with disorder explains most of experimental findings on heat-induced changes and is suggested to be responsible also for the photoinduced magnetic transition.
NASA Astrophysics Data System (ADS)
Pham, Tuan Anh; Nguyen, Huy-Viet; Rocca, Dario; Galli, Giulia
2013-03-01
We recently developed a method for the calculation of quasiparticle energies within many body perturbation theory, at the GW level, which avoids costly summations over empty electronic states and does not require the use of plasmon-pole models. We present a comprehensive validation of this method, encompassing calculations of i) the vertical ionization energies of a set of over 80 molecules (containing from 14 to 424 valence electrons); ii) the relative position of energy levels of anions and water in hydrated sulfate and chloride clusters; iii) the band structure of a variety of semiconductors and (iv) the electronic properties of amorphous and liquid systems. The efficiency of our approach allowed us to compute quasiparticle energies of multiple configurations of liquid water, using samples with 64 molecules, selected over trajectories generated by ab initio molecular dynamics simulations. Work supported by DOE-BES Grant DE-FG02-06ER46262. Work at LLNL was performed under Contract DE-AC52-07NA27344.
Marinakis, Sarantos; Dean, Indigo Lily; Kłos, Jacek; Lique, François
2015-09-01
We present a new set of potential energy surfaces (PESs) for the CH(X(2)Π)-He van der Waals system. Ab initio calculations of the CH-He PES were carried out using the open-shell single- and double-excitation coupled cluster approach with non-iterative perturbational treatment of triple excitations [RCCSD(T)]. The augmented correlation-consistent polarized valence quadruple-zeta (aug-cc-pVQZ) basis set was employed augmented by mid-bond functions. Integral cross sections for the rotational excitation in CH-He collisions were calculated using the new PES and compared with available experimental results. The newly constructed PES reproduces the available experimental results for CH(X(2)Π, v = 0)-He collisions better than any previously available PES. Differential cross sections (DCS) are presented for the first time for this system and discussed within the context of rotational rainbows. Finally, our work provides the first rate thermal coefficients for this system that are crucially needed for astrochemical modelling and future anticipated experiments in CH(X(2)Π)-He collisions. PMID:26220835
NASA Astrophysics Data System (ADS)
Naumkin, F. Y.; McCourt, F. R. W.
1997-10-01
Two new ground state potential energy surfaces have been obtained for the ArCl2 Van der Waals complex, one purely ab initio, the other constructed from empirical ArCl potentials modified in terms of ab initio data for the ArCl2 and ArCl moities. The ab initio surface has a well for the linear conformer that is slightly deeper than that for the T-shaped conformer, but inclusion of the zero-point energies reverses the relative binding in the two configurations. The microwave spectrum has been calculated using the new potential surfaces, and a series of lines that can be associated with the linear conformer have been predicted. Possible reasons for their nonobservability under the usual experimental conditions are discussed. A simple modification which employs empirical information on the ArCl potentials used in modelling the ArCl2 potential surface transforms the well for the T-shaped conformer into the global minimum, with a dissociation energy that lies within 0.5% of the experimental value, and microwave transition energies that deviate from experimental values by no more than 1.3%. A final two-parameter scaling allows reproduction of both the experimental D0 value (within experimental uncertainty) and the positions of all observed microwave lines (within 0.02%) for each of the ab initio and empirically based potential surfaces.
Godet-Bar, T; Leprêtre, J-C; Le Bacq, O; Sanchez, J-Y; Deronzier, A; Pasturel, A
2015-10-14
Different N-substituted phenothiazines have been synthesized and their electrochemical behavior has been investigated in CH3CN in order to design the best polyphenothiazine based cathodic material candidate for lithium batteries. These compounds exhibit two successive reversible one-electron oxidation processes. Ab initio calculations demonstrate that the potential of the first process is a result of both the hybridization effects between the substituent and the phenothiazine unit as well as the change of conformation of the phenothiazine heterocycle during the oxidation process. More specifically, we show that an asymmetric molecular orbital spreading throughout an external cycle of the phenothiazine unit and the alkyl fragment is formed only if the alkyl fragment is long enough (from the methyl moiety onwards) and is at the origin of the bent conformation for N-substituted phenothiazines during oxidation. Electrochemical investigations supported by ab initio calculations allow the selection of a phenothiazinyl unit which is then polymerized by a Suzuki coupling strategy to avoid the common solubilization issue in carbonate-based liquid electrolytes of lithium cells. The first electrochemical measurements performed show that phenothiazine derivatives pave the way for a promising family of redox polymers intended to be used as organic positives for lithium batteries.
Alam, T.M.
1998-09-01
The influence of changes in the contracted Gaussian basis set used for ab initio calculations of nuclear magnetic resonance (NMR) phosphorous chemical shift anisotropy (CSA) tensors was investigated. The isotropic chemical shitl and chemical shift anisotropy were found to converge with increasing complexity of the basis set at the Hartree-Fock @IF) level. The addition of d polarization function on the phosphorous nucIei was found to have a major impact of the calculated chemical shi~ but diminished with increasing number of polarization fimctions. At least 2 d polarization fimctions are required for accurate calculations of the isotropic phosphorous chemical shift. The introduction of density fictional theory (DFT) techniques through tie use of hybrid B3LYP methods for the calculation of the phosphorous chemical shift tensor resulted in a poorer estimation of the NMR values, even though DFT techniques result in improved energy and force constant calculations. The convergence of the W parametem with increasing basis set complexity was also observed for the DFT calculations, but produced results with consistent large deviations from experiment. The use of a HF 6-31 l++G(242p) basis set represents a good compromise between accuracy of the simulation and the complexity of the calculation for future ab initio calculations of 31P NMR parameters in larger complexes.
NASA Astrophysics Data System (ADS)
Fähnle, Manfred; Drautz, Ralf; Lechermann, Frank; Singer, Reinhard; Diaz-Ortiz, Alejandro; Dosch, Helmut
2005-05-01
The cover picture from the Feature Article [1] depicts the calculated landscape of lowest formation energies for the ternary compound system Ni-Fe-Al. The figure shows for each composition the difference in the formation energy (in meV/atom) for the respective homogeneous configuration with lowest energy on the bcc and the fcc parent lattice. The phases on the fcc lattice dominate the Ni- and Al-rich regions of the Gibbs triangle.The first author Manfred Fähnle is Professor at the University of Stuttgart and member of the theory group of the department of Prof. Schütz at the Max-Planck-Institut für Metallforschung in Stuttgart. In 1980 he was awarded with the Otto-Hahn medal of the Max-Planck Society and in 1985 he received the Academy Award for Physics of the University of Göttingen. His present research interests are the static and dynamic properties of bulk and nanostructured magnetic systems, as well as the ab-initio statistical mechanics of alloys.
NASA Astrophysics Data System (ADS)
Durig, J. R.; Guirgis, G. A.; Eltayeb, S.
1994-07-01
The Raman (2500 to 30 cm -1) and infrared (2500 to 60 cm 1) spectra of gaseous and solid trifluoromethyl isocyanate, CF 3NCO, have been recorded. The Raman spectrum of the liquid has also been obtained and qualitative depolarization ratios have been measured. The CNC bend has been observed in the low-frequency Raman spectrum of the gas at 130 cm -1 as a relatively strong line which is split in the spectrum of the solid. However, the CF 3 torsional mode was not observed. The infrared spectrum of CF 3NCO dissolved in liquid xenon was also obtained and the observed spectrum indicates a very small barrier to internal rotation. With the exception of the torsion, a complete assignment of the vibrational fundamentals is proposed. The structural parameters, force constants, and vibrational frequencies have been determined from ab initio calculations utilizing a variety of basis sets and the theoretical results are compared to the experimental values when appropriate. These results have been compared with the corresponding quantities obtained for some similar molecules.
NASA Astrophysics Data System (ADS)
Ogitsu, Tadashi; Gygi, Francois; Galli, Giulia
2004-03-01
Boron is the only low-Z element in the periodic table whose atomic ground state structure has not yet been fully determined. For example, it is yet unclear whether perfectly pure elemental Boron is stable in an ordered crystalline form and the number of atoms in the unit cell (varying from 315 to about 325) is still the subject of debate. Using ab-initio calculations and supercells with 1260-1280 atoms, we have studied the physical properties of Boron at ambient conditions and under pressure (P). Results about the ionic and electronic structure will be presented, in particular the role of interstitial atoms and the presence of localized states right above the Fermi level will be discussed in detail. The computed equation of state under pressure is in agreement with recent experimental data. At about 120 GPa we observe amorphization, consistent with the results of Ref. [1] at l00 GPa. Amorphization occurs by random deformation of icosahedral units which remain intact; it is accompanied by a delocalization of states near the Fermi level yielding a poorly conducting system. This work was performed under the auspices of the U.S. Dept. of Energy at the University of California/ LLNL under contract no. W-7405-Eng-48. [1] Sanz et al. Phys. Rev. Lett. 89, 245501 (2002)
Nakanishi, Ryuzo; Kato, Shugo; Matsuyama, Yasushi; Nagata, Takashi
2016-09-01
Carbon sulfides are known as a class of binary compounds that can exist in various isomeric and/or polymeric forms. As for a sulfur-rich compound with the composition formula CS3, two possible constitutional isomers have been proposed experimentally or theoretically for the neutral species and its corresponding radical cation and anion. Although the previous studies claim that one isomer has a carbon trisulfide (CS3) C-centered configuration and the other has a carbon disulfide S-sulfide (SCSS) chain configuration, they have not yet been fully identified by a spectroscopic method. In this study, we have prepared the anions of those isomers in the gas phase by employing two types of reactions: dissociative electron attachment to 1,3-dithiole-2-thione for CS3(-) formation and the S(-) + CS2 ion-molecule reaction for SCSS(-). Photoelectron spectroscopic measurements reveal that the reactions result in the production of two anionic species that can be well distinguished by their vertical detachment energy. With the aid of ab initio calculations, they are identified distinctively as the anions of carbon trisulfide and carbon disulfide S-sulfide. PMID:27533492
Ab initio nuclear many-body perturbation calculations in the Hartree-Fock basis
NASA Astrophysics Data System (ADS)
Hu, B. S.; Xu, F. R.; Sun, Z. H.; Vary, J. P.; Li, T.
2016-07-01
Starting from realistic nuclear forces, the chiral N3LO and JISP16, we have applied many-body perturbation theory (MBPT) to the structure of closed-shell nuclei, 4He and 16O. The two-body N3LO interaction is softened by a similarity renormalization group transformation while JISP16 is adopted without renormalization. The MBPT calculations are performed within the Hartree-Fock (HF) bases. The angular momentum coupled scheme is used, which can reduce the computational task. Corrections up to the third order in energy and up to the second order in radius are evaluated. Higher-order corrections in the HF basis are small relative to the leading-order perturbative result. Using the antisymmetrized Goldstone diagram expansions of the wave function, we directly correct the one-body density for the calculation of the radius, rather than calculate corrections to the occupation probabilities of single-particle orbits as found in other treatments. We compare our results with other methods where available and find good agreement. This supports the conclusion that our methods produce reasonably converged results with these interactions. We also compare our results with experimental data.
Roemelt, Michael
2015-07-28
Spin Orbit Coupling (SOC) is introduced to molecular ab initio density matrix renormalization group (DMRG) calculations. In the presented scheme, one first approximates the electronic ground state and a number of excited states of the Born-Oppenheimer (BO) Hamiltonian with the aid of the DMRG algorithm. Owing to the spin-adaptation of the algorithm, the total spin S is a good quantum number for these states. After the non-relativistic DMRG calculation is finished, all magnetic sublevels of the calculated states are constructed explicitly, and the SOC operator is expanded in the resulting basis. To this end, spin orbit coupled energies and wavefunctions are obtained as eigenvalues and eigenfunctions of the full Hamiltonian matrix which is composed of the SOC operator matrix and the BO Hamiltonian matrix. This treatment corresponds to a quasi-degenerate perturbation theory approach and can be regarded as the molecular equivalent to atomic Russell-Saunders coupling. For the evaluation of SOC matrix elements, the full Breit-Pauli SOC Hamiltonian is approximated by the widely used spin-orbit mean field operator. This operator allows for an efficient use of the second quantized triplet replacement operators that are readily generated during the non-relativistic DMRG algorithm, together with the Wigner-Eckart theorem. With a set of spin-orbit coupled wavefunctions at hand, the molecular g-tensors are calculated following the scheme proposed by Gerloch and McMeeking. It interprets the effective molecular g-values as the slope of the energy difference between the lowest Kramers pair with respect to the strength of the applied magnetic field. Test calculations on a chemically relevant Mo complex demonstrate the capabilities of the presented method.
Luo, Ye Sorella, Sandro; Zen, Andrea
2014-11-21
We present a systematic study of a recently developed ab initio simulation scheme based on molecular dynamics and quantum Monte Carlo. In this approach, a damped Langevin molecular dynamics is employed by using a statistical evaluation of the forces acting on each atom by means of quantum Monte Carlo. This allows the use of an highly correlated wave function parametrized by several variational parameters and describing quite accurately the Born-Oppenheimer energy surface, as long as these parameters are determined at the minimum energy condition. However, in a statistical method both the minimization method and the evaluation of the atomic forces are affected by the statistical noise. In this work, we study systematically the accuracy and reliability of this scheme by targeting the vibrational frequencies of simple molecules such as the water monomer, hydrogen sulfide, sulfur dioxide, ammonia, and phosphine. We show that all sources of systematic errors can be controlled and reliable frequencies can be obtained with a reasonable computational effort. This work provides convincing evidence that this molecular dynamics scheme can be safely applied also to realistic systems containing several atoms.
NASA Astrophysics Data System (ADS)
Dacal, Luis C. O.; Cantarero, A.
2014-03-01
Most III-V semiconductors, which acquire the zinc-blende phase as bulk materials, adopt the metastable wurtzite phase when grown in the form of nanowires. These are new semiconductors with new optical properties, in particular, a different electronic band gap when compared with that grown in the zinc-blende phase. The electronic gap of wurtzite InAs at the \\Gamma -point of the Brillouin zone ({{E}_{0}} gap) has been recently measured, {{E}_{0}}=0.46 eV at low temperature. The electronic gap at the A-point of the Brillouin zone (equivalent to the L-point in the zinc-blende structure, {{E}_{1}}) has also been obtained recently based on a resonant Raman scattering experiment. In this work, we calculate the band structure of InAs in the zinc-blende and wurtzite phases, using the full potential linearized augmented plane wave method, including spin-orbit interaction. The electronic band gap has been improved through the modified Becke-Johnson exchange-correlation potential. Both the {{E}_{0}} and {{E}_{1}} gaps agree very well with the experiment. From the calculations, a crystal field splitting of 0.122 eV and a spin-orbit splitting of 0.312 eV (the experimental value in zinc-blende InAs is 0.4 eV) has been obtained. Finally, we calculate the dielectric function of InAs in both the zinc-blende and wurtzite phases and a comparative discussion is given.
Ab initio calculations of optical constants from ultraviolet to X-rays
NASA Astrophysics Data System (ADS)
Rivas, Gildardo
We present a theory of x-ray absorption based on the real space Green's function formalism implemented in the FEFF x-ray spectroscopy code, and test it from UV to the hard x-ray limits. The one-electron theory includes self-consistent scattering potentials, core-hole and self-energy effects, and full or high order multiple scattering. From it we derive the total photo-absorption cross section summed over all edges. In contrast to standard atomic tables, the calculations include important solid-state corrections, e.g., the x-ray absorption fine structure, inelastic losses, lifetime broadening, etc. We further show that the same theory can be used to calculate the imaginary part of the complex dielectric function, from which other optical constants can be derived, e.g., the real part of the dielectric function, the complex index of refraction, and energy loss spectra. Typical results for elemental solids are presented and compared with standard theoretical tables and with experiment.
AB INITIO CALCULATIONS OF ELASTIC CONSTANTS OF BCC V-NB SYSTEM AT HIGH PRESSURES
Landa, A; Klepeis, J; Soderlind, P; Naumov, I; Velikokhatnyi, O; Vitos, L; Ruban, A
2005-05-02
First-principles total energy calculation based on the exact muffin-tin orbital and full potential linear muffin-tin orbital methods were used to calculate the equation of state and shear elastic constants of bcc V, Nb, and the V{sub 95}Nb{sub 05} disordered alloy as a function of pressure up to 6 Mbar. We found a mechanical instability in C{sub 44} and a corresponding softening in C at pressures {approx} 2 Mbar for V. Both shear elastic constants show softening at pressures {approx} 0.5 Mbar for Nb. Substitution of 5 at. % of V with Nb removes the instability of V with respect to trigonal distortions in the vicinity of 2 Mbar pressure, but still leaves the softening of C{sub 44} in this pressure region. We argue that the pressure induced shear instability (softening) of V (Nb) originates from the electronic system and can be explained by a combination of the Fermi surface nesting, electronic topological transition, and band Jahn-Teller effect.
Ab initio calculation of thermodynamic potentials and entropies for superionic water
French, Martin; Desjarlais, Michael P.; Redmer, Ronald
2016-02-25
We construct thermodynamic potentials for two superionic phases of water [with body-centered cubic (bcc) and face-centered cubic (fcc) oxygen lattice] using a combination of density functional theory (DFT) and molecular dynamics simulations (MD). For this purpose, a generic expression for the free energy of warm dense matter is developed and parametrized with equation of state data from the DFT-MD simulations. A second central aspect is the accurate determination of the entropy, which is done using an approximate two-phase method based on the frequency spectra of the nuclear motion. The boundary between the bcc superionic phase and the ices VII andmore » X calculated with thermodynamic potentials from DFT-MD is consistent with that directly derived from the simulations. As a result, differences in the physical properties of the bcc and fcc superionic phases and their impact on interior modeling of water-rich giant planets are discussed.« less
Ab initio calculation of thermodynamic potentials and entropies for superionic water.
French, Martin; Desjarlais, Michael P; Redmer, Ronald
2016-02-01
We construct thermodynamic potentials for two superionic phases of water [with body-centered cubic (bcc) and face-centered cubic (fcc) oxygen lattice] using a combination of density functional theory (DFT) and molecular dynamics simulations (MD). For this purpose, a generic expression for the free energy of warm dense matter is developed and parametrized with equation of state data from the DFT-MD simulations. A second central aspect is the accurate determination of the entropy, which is done using an approximate two-phase method based on the frequency spectra of the nuclear motion. The boundary between the bcc superionic phase and the ices VII and X calculated with thermodynamic potentials from DFT-MD is consistent with that directly derived from the simulations. Differences in the physical properties of the bcc and fcc superionic phases and their impact on interior modeling of water-rich giant planets are discussed. PMID:26986321
Ab initio calculation of thermodynamic potentials and entropies for superionic water
NASA Astrophysics Data System (ADS)
French, Martin; Desjarlais, Michael P.; Redmer, Ronald
2016-02-01
We construct thermodynamic potentials for two superionic phases of water [with body-centered cubic (bcc) and face-centered cubic (fcc) oxygen lattice] using a combination of density functional theory (DFT) and molecular dynamics simulations (MD). For this purpose, a generic expression for the free energy of warm dense matter is developed and parametrized with equation of state data from the DFT-MD simulations. A second central aspect is the accurate determination of the entropy, which is done using an approximate two-phase method based on the frequency spectra of the nuclear motion. The boundary between the bcc superionic phase and the ices VII and X calculated with thermodynamic potentials from DFT-MD is consistent with that directly derived from the simulations. Differences in the physical properties of the bcc and fcc superionic phases and their impact on interior modeling of water-rich giant planets are discussed.
Ab initio calculation of the non-relativistic free-free Gaunt factor incorporating plasma screening
NASA Astrophysics Data System (ADS)
Armstrong, G. S. J.; Colgan, J.; Kilcrease, D. P.; Magee, N. H.
2014-03-01
We present calculations of Gaunt factors for free-free absorption over a wide range of temperatures and densities. The calculations employ a partial wave expansion approach, which is able to account for plasma screening within the calculation of the free-free Gaunt factor. Much of the existing Gaunt factor data pertains to hydrogenic systems, and plasma screening is often incorporated in opacity calculations using approximate methods. The use of a more accurate method allows us to determine the accuracy of such approximations in calculations of the free-free monochromatic and mean opacities.
Alam, Todd
2013-07-29
The impacts of local polymer chain conformations on the methine and carbonyl 13C-NMR chemical shifts for polyethylene acrylic acid p(E-AA) copolymers were predicted using ab initio methods. Using small molecular cluster models, the magnitude and sign of the γ-gauche torsional angle effect, along with the impact of local tetrahedral structure distortions near the carbonyl group, on the 13C-NMR chemical shifts were determined. These 13C-NMR chemical shift variations were compared to the experimental trends observed for precise p(E-AA) copolymers as a function acid group spacing and degree of zinc-neutralization in the corresponding p(E-AA) ionomers. These ab initio calculations address the future ability of 13C-NMR chemical shift variations to provide information about the local chain conformations in p(E-AA) copolymer materials.
Ab Initio Many-Body Calculations Of n-3H, n-4He, p-3,4He, And n-10Be Scattering
Quaglioni, S; Navratil, P
2008-03-26
We develop a new ab initio many-body approach capable of describing simultaneously both bound and scattering states in light nuclei, by combining the resonating-group method with the use of realistic interactions, and a microscopic and consistent description of the nucleon clusters. This approach preserves translational symmetry and Pauli principle. We present phase shifts for neutron scattering on {sup 3}H, {sup 4}He and {sup 10}Be and proton scattering on {sup 3,4}He, using realistic nucleon-nucleon potentials. Our A = 4 scattering results are compared to earlier ab initio calculations. We demonstrate that a proper treatment of the coupling to the n-{sup 10}Be continuum is essential to explain the parity-inverted ground state in {sup 11}Be.
Surin, L. A.; Tarabukin, I. V.; Panfilov, V. A.; Schlemmer, S.; Kalugina, Y. N.; Faure, A.; Rist, C.; Avoird, A. van der
2015-10-21
The rotational spectrum of the van der Waals complex CH{sub 4}–CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110–145 GHz. Newly observed and assigned transitions belong to the K = 2–1 subband correlating with the rotationless j{sub CH4} = 0 ground state and the K = 2–1 and K = 0–1 subbands correlating with the j{sub CH4} = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH{sub 4}–CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH{sub 4}–CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH{sub 4} face closest to the CO subunit and binding energy D{sub e} = 177.82 cm{sup −1}. The bound rovibrational levels of the CH{sub 4}–CO complex were calculated for total angular momentum J = 0–6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D{sub 0} are 91.32, 94.46, and 104.21 cm{sup −1} for A (j{sub CH4} = 0), F (j{sub CH4} = 1), and E (j{sub CH4} = 2) nuclear spin modifications of CH{sub 4}–CO, respectively.
Ab-Initio Description and Prediction of Properties of Carbon-Based and Other Non-Metallic Materials
NASA Technical Reports Server (NTRS)
Bagayoko, D.; Zhao, G. L.; Hasan, S.
2001-01-01
We have resolved the long-standing problem consisting of 30%-50% theoretical underestimates of the band gaps of non-metallic materials. We describe the Bagayoko, Zhao, and Williams (BZW) method that rigorously circumvents the basis-set and variational effect presumed to be a cause of these underestimates. We present ab-initio, computational results that are in agreement with experiment for diamond (C), silicon (Si), silicon carbides (3C-SiC and 4H-SiC), and other semiconductors (GaN, BaTiO3, AlN, ZnSe, ZnO). We illustrate the predictive capability of the BZW method in the case of the newly discovered cubic phase of silicon nitride (c-Si3N4) and of selected carbon nanotabes [(10,0), and (8,4)]. Our conclusion underscores the inescapable need for the BZW method in ab-initio calculations that employ a basis set in a variational approach. Current nanoscale trends amplify this need. We estimate that the potential impact of applications of the BZW method in advancing our understanding of nonmetallic materials, in informing experiment, and particularly in guiding device design and fabrication is simply priceless.
Ab initio calculations on collisions of low energy electrons with polyatomic molecules
Rescigno, T.N.
1991-08-01
The Kohn variational method is one of simplest, and oldest, techniques for performing scattering calculations. Nevertheless, a number of formal problems, as well as practical difficulties associated with the computation of certain required matrix elements, delayed its application to electron--molecule scattering problems for many years. This paper will describe the recent theoretical and computational developments that have made the complex'' Kohn variational method a practical tool for carrying out calculations of low energy electron--molecule scattering. Recent calculations on a number of target molecules will also be summarized. 41 refs., 7 figs.
A compilation of ab-initio calculations of embrittling potencies in binary metallic alloys
Gibson, Michael A.; Schuh, Christopher A.
2015-01-01
Segregation-induced changes in interfacial cohesion often control the mechanical properties of metals. The change in the work of separation of an interface upon segregation of a solute to the interface, termed the embrittling potency, is an atomic-level quantity used to predict and understand embrittlement phenomena. We present a compilation of calculations of embrittling potencies, along with references for these calculations. A discussion of this data is made in a separate article (Gibson and Schuh, 2016 [1]). PMID:26858979
Theoretical prediction of high pressure phase transition in ScC and YC: Ab initio calculations
NASA Astrophysics Data System (ADS)
Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.
2013-08-01
The structural stability of ScC and YC has been analyzed under hydrostatic compression employing the first-principles calculations using the plane-wave pseudopotential method. The comparison of theoretically calculated enthalpies of rocksalt type (B1), primitive orthorhombic (Pmmn), and CsCl type (B2) structures as a function of pressure suggests that the B1 structure transforms to Pmmn phase instead of transforming to B2 phase that predicted by Soni et al. [J. Phys. Chem. Solids 72, 810 (2011)]. The pressure for B1 to Pmmn transition predicted for ScC and YC are ˜80 GPa and ˜30 GPa, respectively. To further substantiate the outcomes of our static lattice calculations, we have performed lattice dynamic calculations also. Our lattice dynamic calculations correctly demonstrate that the B1 phase is dynamically stable structure at ambient condition. Further, for both the carbides, we find that the Pmmn structure becomes dynamically stable around the transition pressure whereas the B2 structure remains unstable, supporting the B1 to Pmmn phase transition predicted by our static lattice calculations.
Ab-initio calculation of ZnGeAs{sub 2} semiconductor
Tripathy, S. K. Kumar, V.
2014-04-24
The structural, electronic, optical and elastic properties of ZnGeAs{sub 2} semiconductor have been investigated using pseudopotential plane wave method within the density functional theory (DFT). The optimized lattice constants, energy gap and crystal field splitting parameter are calculated. The optical properties such as dielectric function, optical reflectivity,, extinction coefficient, absorption spectra, refractive index and electron energy loss spectrum have been studied. The values of bulk modulus (B), elastic constants (C{sub ij}), Young’s modulus (Y), Zener anisotropic factor (A), Poisson’s ratio (ν) and Debye temperature (Θ{sub D}) have been calculated. The calculated values of all these parameters are compared with the available experimental values and the values reported by different workers. A fairly good agreement has been found between them.
Ab Initio Calculations of Spin-Orbit Coupling for Heavy-Metal Containing Radicals
NASA Astrophysics Data System (ADS)
Cheng, Lan
2016-06-01
The perturbative treatment of spin-orbit coupling (SOC) on top of scalar-relativistic calculations is a cost-effective alternative to rigorous fully relativistic calculations. In this work the applicability of the perturbative scheme in the framework of spin-free exact two-component theory is demonstrated with calculations of SO splittings and SOC contributions to molecular properties in small heavy-metal containing radicals, including AuO, AuS, and ThO^+. The equation of motion coupled cluster techniques have been used to accurately account for the electron-correlation effects in these radicals, and basis-set effects are carefully analyzed. The computed results are compared with experimental measurements for SO splittings and dipole moments when available.
Sun, J.; Pohl, K.; Mikkelsen, A.; Fuglsang Jensen, M.; Hofmann, Ph.; Koroteev, Y. M.; Bihlmayer, G.; Chulkov, E. V.
2006-12-15
The surface structure of Bi(110) has been investigated by low-energy electron diffraction intensity analysis and by first-principles calculations. Diffraction patterns at a sample temperature of 110 K and normal incidence reveal a bulk truncated (1x1) surface without indication of any structural reconstruction despite the presence of dangling bonds on the surface layer. Good agreement is obtained between the calculated and measured diffraction intensities for this surface containing only one mirror-plane symmetry element and a buckled bilayer structure. No significant interlayer spacing relaxations are found. The Debye temperature for the surface layer is found to be lower than in the bulk, which is indicative of larger atomic vibrational amplitudes at the surface. Meanwhile, the second layer shows a Debye temperature close to the bulk value. The experimental results for the relaxations agree well with those of our first-principles calculation.
Abs-initio, Predictive Calculations for Optoelectronic and Advanced Materials Research
NASA Astrophysics Data System (ADS)
Bagayoko, Diola
2010-10-01
Most density functional theory (DFT) calculations find band gaps that are 30-50 percent smaller than the experimental ones. Some explanations of this serious underestimation by theory include self-interaction and the derivative discontinuity of the exchange correlation energy. Several approaches have been developed in the search for a solution to this problem. Most of them entail some modification of DFT potentials. The Green function and screened Coulomb approximation (GWA) is a non-DFT formalism that has led to some improvements. Despite these efforts, the underestimation problem has mostly persisted in the literature. Using the Rayleigh theorem, we describe a basis set and variational effect inherently associated with calculations that employ a linear combination of atomic orbitals (LCAO) in a variational approach of the Rayleigh-Ritz type. This description concomitantly shows a source of large underestimation errors in calculated band gaps, i.e., an often dramatic lowering of some unoccupied energies on account of the Rayleigh theorem as opposed to a physical interaction. We present the Bagayoko, Zhao, and Williams (BZW) method [Phys. Rev. B 60, 1563 (1999); PRB 74, 245214 (2006); and J. Appl. Phys. 103, 096101 (2008)] that systematically avoids this effect and leads (a) to DFT and LDA calculated band gaps of semiconductors in agreement with experiment and (b) theoretical predictions of band gaps that are confirmed by experiment. Unlike most calculations, BZW computations solve, self-consistently, a system of two coupled equations. DFT-BZW calculated effective masses and optical properties (dielectric functions) also agree with measurements. We illustrate ten years of success of the BZW method with its results for GaN, C, Si, 3C-SIC, 4H-SiC, ZnO, AlAs, Ge, ZnSe, w-InN, c-InN, InAs, CdS, AlN and nanostructures. We conclude with potential applications of the BZW method in optoelectronic and advanced materials research.
Ab initio spur size calculation in liquid water at room temperature
NASA Astrophysics Data System (ADS)
Muroya, Yusa; Mozumder, Asokendu
2016-07-01
An attempt was made to calculate the spur size in liquid water at room temperature from fundamental interactions. Electron trapping, elastic scattering, and positive-ion back attraction undergone in sub-excitation and sub-vibrational stages in the 100 fs time scale for thermalization were considered and included in the model. Overall diffusional broadening was estimated to be 41.2 Å, attended by the positive-ion pull back of 24.0 Å, resulting in a calculated spur size of 17.2 Å. Electron trapping is seen in competition with thermalization in the ultimate stage, which results in the trapped electron position distribution as a sum of Gaussians.
{ital Ab Initio} Calculation of Crystalline Electric Fields and Kondo Temperatures in Ce Compounds
Han, J.; Alouani, M.; Cox, D.; Han, J.; Cox, D.; Alouani, M.
1997-02-01
We have calculated the band-f hybridizations for Ce{sub x}La{sub 1-x}M{sub 3} compounds (x=1 and x{r_arrow}0; M=Pb,In,Sn,Pd) within the local density approximation and fed this into a noncrossing approximation for the Anderson impurity model applied to both dilute and concentrated limits. Our calculations produce crystalline electric field splittings and Kondo temperatures with trends in good agreement with experiment and demonstrate the need for detailed electronic structure information on hybridization to describe the diverse behaviors of these Ce compounds. {copyright} {ital 1997} {ital The American Physical Society}
Vancoillie, Steven; Chalupský, Jakub; Ryde, Ulf; Solomon, Edward I.; Pierloot, Kristine; Neese, Frank; Rulíšek, Lubomír
2010-01-01
EPR spectroscopy has proven to be an indispensable tool in elucidating the structure of metal sites in proteins. In recent years, experimental EPR data have been complemented by theoretical calculations, which have become a standard tool of many quantum chemical packages. However, there have only been a few attempts to calculate EPR g tensors for exchange-coupled systems with more than two spins. In this work, we present a quantum chemical study of structural, electronic, and magnetic properties of intermediates in the reaction cycle of multicopper oxidases and of their inorganic models. All these systems contain three copper(II) ions bridged by hydroxide or O2− anions and their ground states are antiferromagnetically coupled doublets. We demonstrate that only multireference methods, such as CASSCF/CASPT2 or MRCI can yield qualitatively correct results (compared to the experimental values) and consider the accuracy of the calculated EPR g tensors as the current benchmark of quantum chemical methods. By decomposing the calculated g tensors into terms arising from interactions of the ground state with the various excited states, the origin of the zero-field splitting is explained. The results of the study demonstrate that a truly quantitative prediction of the g tensors of exchange-coupled systems is a great challenge to contemporary theory. The predictions strongly depend on small energy differences that are difficult to predict with sufficient accuracy by any quantum chemical method that is applicable to systems of the size of our target systems. PMID:20469875
Water dissolution in albite melts: - Constraints from ab initio NMR calculations
NASA Astrophysics Data System (ADS)
Liu, Yun; Nekvasil, Hanna; Long, Hongbo
2002-12-01
Hartree-Fock and B3LYP NMR calculations were performed at the 6-311+G(2df,p) level on cluster models representing albite glasses using B3LYP/6 to 31G* optimized geometries. Calculation results on several well-known crystalline materials, such as low albite and KHSi 2O 5, were used to check the accuracy of the calculation methods. Calculated 29Si-NMR results on clusters that model protonation of Al-O-Si linkages and the replacement of Na + by H + indicate a major increase in Si-O(H) bond length and a 5 ppm difference in δ iso for 29Si compared to that for anhydrous albite glass. The calculated δ iso of 27Al in such linkages agrees with the experimental data, but shows an increase in C q that cannot be fully diminished by H-bonding to additional water molecules. This protonation model is consistent with both experimental 17O NMR data and the major peak of 1H-NMR spectra. It cannot readily explain the existence of the small peak in the experimental 1H spectra around 1.5 ppm. Production of the depolymerized units Al [Q 3]-O-H upon the dissolution of water is not consistent with 27Al, 1H, or 17O NMR experimental results. Production of Si [Q 3]-O-H is consistent with all of the experimental 17O and 1H-NMR data; such units can produce both the major peak at 3.5 ppm and the small peak at 1.5 ppm in 1H spectra, either with or without hydrogen bonding. This species, however, cannot produce the main features of 29Si spectra. It is concluded that although neither protonation nor the production of Si [Q 3]-O-H alone is consistent with the available experimental data, the combination of these two processes is consistent with available experimental NMR data.
Ab Initio calculations of [CoY6-nXn]2+ complexes
NASA Astrophysics Data System (ADS)
Rulís̆ek, Lubomír; Havlas, Zdenĕk
2000-01-01
The complete active space self-consistent field (CASSCF) and multireference second order perturbation theory (CASPT2) calculations of [CoF6]4-, [Co(H2O)6]2+, [Co(NH3)6]2+, [Co(H2O)5X]2+, and [Co(H2O)4X2]2+ complexes (X=CH3OH, CH3SH, CH3NH2) are reported. The potential energy surfaces of 10 lowest quartet states of [Co(H2O)5X]2+ complexes near the equilibrium geometry are calculated and splitting of triple-degenerate 4T1g(F), 4T2g(F), and 4T1g(P) electronic states of [Co(H2O)6]2+ complex induced by the substitution of one or two water ligands is characterized and quantified. It is shown that the energy differences between originally degenerate states are almost invariant to the changes of metal-ligand distances, and despite their proximity, the crossing does not occur. The coefficients of leading configuration of multireference wave functions of [Co(H2O)5X]2+ and [Co(H2O)4X2]2+ complexes are shown to approach unity and the usage of single-reference methods is justified. As a consequence, interaction energies of the studied functional groups with Co2+ are computed also at the HF (Hartree-Fock), DFT (density functional theory), and MP2 (second-order Moller-Plesset) levels. They are compared to CASSCF calculations and to the equivalent calculations done for Zn2+ and Ni2+ ions. The computational methodology for the accurate calculations of various cobalt (II) ionic complexes is described and the implications for the theoretical investigation of interactions of chemically and biologically important functional groups with Co2+ are discussed.
Thermodynamic and mechanical properties of TiC from ab initio calculation
Dang, D. Y.; Fan, J. L.; Gong, H. R.
2014-07-21
The temperature-dependent thermodynamic and mechanical properties of TiC are systematically investigated by means of a combination of density-functional theory, quasi-harmonic approximation, and thermal electronic excitation. It is found that the quasi-harmonic Debye model should be pertinent to reflect thermodynamic properties of TiC, and the elastic properties of TiC decease almost linearly with the increase of temperature. Calculations also reveal that TiC possesses a pronounced directional pseudogap across the Fermi level, mainly due to the strong hybridization of Ti 3d and C 2p states. Moreover, the strong covalent bonding of TiC would be enhanced (reduced) with the decrease (increase) of temperature, while the change of volume (temperature) should have negligible effect on density of states at the Fermi level. The calculated results agree well with experimental observations in the literature.
Pressure induced structural phase transition in actinide mono-bismuthides: Ab initio calculations
NASA Astrophysics Data System (ADS)
Pataiya, J.; Makode, C.; Aynyas, M.; Sanyal, Sankar P.
2013-06-01
The structural and electronic properties of mono-bismuthides of Plutonium and Americium have been investigated using tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). From present study with the help of total energy calculations it is found that PuBi and AmBi are stable in NaCl - type structure under ambient pressure. The structure stability of PuBi and AmBi changes under the application of pressure. We predict a structural phase transition from NaCl-type (B1-phase) structure to CsCl-type (B2-phase) structure for these phospides in the pressure range of 45 - 4.5 GPa for PuBi and AmBi respectively. The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.
Pressure Induced Structural Phase Transition in Actinide Monophospides: Ab Initio Calculations
NASA Astrophysics Data System (ADS)
Makode, Chandrabhan; Sanyal, Sankar P.
2011-07-01
The structural and electronic properties of monophospides of Thorium, Uranium and Neptunium have been investigated using tight binding linear muffin-in-orbital (TB-LMTO) method within the local density approximation (LDA). From present study with the help of total energy calculations it is found that ThP, UP and NpP are stable in NaCl- type structure under ambient pressure. The structure stability of ThP, UP and NpP changes under the application of pressure. We predict a structural phase transition from NaCl-type (B1-phase) structure to CsCl-type (B2-phase) structure for these phospides in the pressure range of 37.0-24.0 GPa (ThP to NpP). The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.
NASA Astrophysics Data System (ADS)
Wang, Ying; Sessions, Alex L.; Nielsen, Robert J.; Goddard, William A., III
2009-12-01
Carbon-bound hydrogen in sedimentary organic matter can undergo exchange over geologic timescales, altering its isotopic composition. Studies investigating the natural abundance distribution of 1H and 2H in such molecules must account for this exchange, which in turn requires quantitative knowledge regarding the endpoint of exchange, i.e., the equilibrium isotopic fractionation factor ( α eq). To date, relevant data have been lacking for molecules larger than methane. Here we describe an experimental method to measure α eq for C-bound H positions adjacent to carbonyl group (H α) in ketones. H at these positions equilibrates on a timescale of days as a result of keto-enol tautomerism, allowing equilibrium 2H/ 1H distributions to be indirectly measured. Molecular vibrations for the same ketone molecules are then computed using Density Functional Theory at the B3LYP/6-311G∗∗ level and used to calculate α eq values for H α. Comparison of experimental and computational results for six different straight and branched ketones yields a temperature-dependent linear calibration curve with slope = 1.081-0.00376 T and intercept = 8.404-0.387 T, where T is temperature in degrees Celsius. Since the dominant systematic error in the calculation (omission of anharmonicity) is of the same size for ketones and C-bound H in most other linear compounds, we propose that this calibration can be applied to analogous calculations for a wide variety of organic molecules with linear carbon skeletons for temperatures below 100 °C. In a companion paper ( Wang et al., 2009) we use this new calibration dataset to calculate the temperature-dependent equilibrium isotopic fractionation factors for a range of linear hydrocarbons, alcohols, ethers, ketones, esters and acids.
Chemisorption of a molecular oxygen on the UN(0 0 1) surface: Ab initio calculations
NASA Astrophysics Data System (ADS)
Zhukovskii, Yu. F.; Bocharov, D.; Kotomin, E. A.
2009-09-01
The results of DFT GGA calculations on oxygen molecules adsorbed upon the (0 0 1) surface of uranium mononitride (UN) are presented and discussed. We demonstrate that O 2 molecules oriented parallel to the substrate can dissociate either (i) spontaneously when the molecular center lies above the surface hollow site or atop N ion, (ii) with the activation barrier when a molecule sits atop the surface U ion. This explains fast UN oxidation in air.
Constrained LDA ab-initio calculation of screening of charging energy in C60
NASA Astrophysics Data System (ADS)
Sau, Jay; Neaton, Jeffrey; Khoo, K. H.; Choi, Hyoung; Louie, Steven; Cohen, Marvin
2006-03-01
Recent measurements and theoretical calculations of the electronic properties of C60 on metal substrates have shown that the electron-electron repulsion parameter U, which determines the coulomb blockade transport properties, is strongly screened in the presence of a metal susbtrate. Since standard Density Functional Theory calculations treat this charging energy in a mean field sense, it ignores the discreteness of the charge on the C60 that is critical to coulomb blockade. To account for the effect of the screened U in transport experiments we calculate the charging energy of C60 in a few environments using a constrained LDA approach and explore the implications for coulomb blockade transport phenomena. This work was supported by National Science Foundation Grant No. DMR04-39768 and by the Director, Office of Science, Office of Basic Energy Sciences, Division of Material Sciences and Engineering, U. S Department of Energy under Contract No. DE-AC03-76SF00098. Computational resources have been provided by DOE at the National Energy Research Scientific Computing Center(NERSC)
Davis, Sergio; Gutiérrez, Gonzalo
2011-12-14
First-principles molecular dynamics calculations of the structural, elastic, vibrational and electronic properties of amorphous Al(2)O(3), in a system consisting of a supercell of 80 atoms, are reported. A detailed analysis of the interatomic correlations allows us to conclude that the short-range order is mainly composed of AlO(4) tetrahedra, but, in contrast with previous results, also an important number of AlO(6) octahedra and AlO(5) units are present. The vibrational density of states presents two frequency bands, related to bond-bending and bond-stretching modes. It also shows other recognizable features present in similar amorphous oxides. We also present the calculation of elastic properties (bulk modulus and shear modulus). The calculated electronic structure of the material, including total and partial electronic density of states, charge distribution, electron localization function and the ionicity for each species, gives evidence of correlation between the ionicity and the coordination for each Al atom.
Ab-initio Calculations of Electronic Properties of Calcium Fluoride (CaF2)
NASA Astrophysics Data System (ADS)
Bohara, Bir; Franklin, Lashounda; Malozovsky, Yuriy; Bagayoko, Diola
We have performed first principle, local density approximation (LDA) calculations of electronic and related properties of cubic calcium fluorite (CaF2) . Our non-relativistic computations employed the Ceperley and Alder LDA potential and the linear combination of atomic orbitals (LCAO) formalism. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). We discuss the electronic energy bands, including the large band gap, total and partial density of states, electron and hole effective masses, and the bulk modulus. Our calculated, indirect (X- Γ) band gap is 12.98 eV; it is 1 eV above an experimental value of 11.8 eV. The calculated bulk modulus (82.89 GPA) is excellent agreement with the experimental result of 82.0 +/-0.7. Our predicted equilibrium lattice constant is 5.42Å. Acknowledgments: This work is funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR], and NSF HRD-1002541, the US Department of Energy, National, Nuclear Security Administration (NNSA) (Award No. DE-NA-0002630), LaSPACE, and LONI-SUBR.
The H + CO ⇌ HCO reaction studied by ab initio benchmark calculations
NASA Astrophysics Data System (ADS)
Peters, Phillip S.; Duflot, Denis; Wiesenfeld, Laurent; Toubin, Céline
2013-10-01
The title reaction has been calculated using complete active space self-consistent field and internally contracted multi-reference configuration interaction, including Davidson correction, calculations. Dunning's correlation consistent atomic basis sets, together with several complete basis set extrapolation schemes, were employed. Core-valence and scalar relativistic effects were also taken into account, as well as anharmonicity of the vibrational modes. Core-valence correlation appears to have a large impact on the calculated frequencies, spectroscopic constants, and on the energetics. In particular, the best estimate for the HCO (DCO) formation barrier height at 0 K, 4.54 ± 0.14 (4.43 ± 0.14) kcal mol-1 is larger than previous theoretical works and well above the usually accepted value of 2.0 ± 0.4 kcal mol-1, measured at room temperature. Inclusion of temperature and entropy at 298 K does not seem to be able to solve this discrepancy. The present theoretical barrier height is therefore the recommended value. The exo-ergicity of the HCO (DCO) dissociation reaction, predicted to be -13.36 ± 0.57 (-14.72 ± 0.57) kcal mol-1, is slightly below the experimental value. Finally, all tested density functionals fail to reproduce accurately both the formation and dissociation barriers.
Efficient yet accurate approximations for ab initio calculations of alcohol cluster thermochemistry.
Umer, Muhammad; Kopp, Wassja A; Leonhard, Kai
2015-12-01
We have calculated the binding enthalpies and entropies of gas phase alcohol clusters from ethanol to 1-decanol. In addition to the monomers, we have investigated dimers, tetramers, and pentamers. Geometries have been obtained at the B3LYP/TZVP level and single point energy calculations have been performed with the Resolution of the Identity-MP2 (RIMP2) method and basis set limit extrapolation using aug-cc-pVTZ and aug-cc-pVQZ basis sets. Thermochemistry is calculated with decoupled hindered rotor treatment for large amplitude motions. The results show three points: First, it is more accurate to transfer the rigid-rotor harmonic oscillator entropies from propanol to longer alcohols than to compute them with an ultra-fine grid and tight geometry convergence criteria. Second, the computational effort can be reduced considerably by using dimerization energies of longer alcohols at density functional theory (B3LYP) level plus a RIMP2 correction obtained from 1-propanol. This approximation yields results almost with the same accuracy as RIMP2 - both methods differ for 1-decanol only 0.4 kJ/mol. Third, the entropy of dimerization including the hindered rotation contribution is converged at 1-propanol with respect to chain length. This allows for a transfer of hindered rotation contributions from smaller alcohols to longer ones which reduces the required computational and man power considerably. PMID:26646881
NASA Astrophysics Data System (ADS)
Makode, Chandrabhan; Sanyal, Sankar P.
2011-09-01
We have investigated the structural and electronic properties of monophospides of thorium, uranium and neptunium. The total energy as a function of volume is obtained by means of the self-consistent tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). From the present study with the help of total energy calculations it is found that ThP, UP and NpP are stable in NaCl-type structure at ambient pressure. The structural stability of ThP, UP and NpP changes under the application of pressure. We predict a structural phase transition from NaCl-type (B 1-phase) structure to CsCl-type (B 2-phase) structure for these phospides in the pressure range of 37.0-24.0 GPa (ThP-NpP). We also calculate lattice parameter ( a0), bulk modulus ( B0), band structure and density of states. From energy band diagram it is observed that ThP, UP and NpP exhibit metallic behavior. The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.
Efficient yet accurate approximations for ab initio calculations of alcohol cluster thermochemistry
NASA Astrophysics Data System (ADS)
Umer, Muhammad; Kopp, Wassja A.; Leonhard, Kai
2015-12-01
We have calculated the binding enthalpies and entropies of gas phase alcohol clusters from ethanol to 1-decanol. In addition to the monomers, we have investigated dimers, tetramers, and pentamers. Geometries have been obtained at the B3LYP/TZVP level and single point energy calculations have been performed with the Resolution of the Identity-MP2 (RIMP2) method and basis set limit extrapolation using aug-cc-pVTZ and aug-cc-pVQZ basis sets. Thermochemistry is calculated with decoupled hindered rotor treatment for large amplitude motions. The results show three points: First, it is more accurate to transfer the rigid-rotor harmonic oscillator entropies from propanol to longer alcohols than to compute them with an ultra-fine grid and tight geometry convergence criteria. Second, the computational effort can be reduced considerably by using dimerization energies of longer alcohols at density functional theory (B3LYP) level plus a RIMP2 correction obtained from 1-propanol. This approximation yields results almost with the same accuracy as RIMP2 — both methods differ for 1-decanol only 0.4 kJ/mol. Third, the entropy of dimerization including the hindered rotation contribution is converged at 1-propanol with respect to chain length. This allows for a transfer of hindered rotation contributions from smaller alcohols to longer ones which reduces the required computational and man power considerably.
NASA Astrophysics Data System (ADS)
Kong, Bo; Zhang, Yachao
2016-07-01
The electronic structures of the cubic GdH3 are extensively investigated using the ab initio many-body GW calculations treating the Gd 4f electrons either in the core (4f-core) or in the valence states (4f-val). Different degrees of quasiparticle (QP) self-consistent calculations with the different starting points are used to correct the failures of the GGA/GGA + U/HSE03 calculations. In the 4f-core case, GGA + G0W0 calculations give a fundamental band gap of 1.72 eV, while GGA+ GW0 or GGA + GW calculations present a larger band gap. In the 4f-val case, the nonlocal exchange-correlation (xc) functional HSE03 can account much better for the strong localization of the 4f states than the semilocal or Hubbard U corrected xc functional in the Kohn-Sham equation. We show that the fundamental gap of the antiferromagnetic (AFM) or ferromagnetic (FM) GdH3 can be opened up by solving the QP equation with improved starting point of eigenvalues and wave functions given by HSE03. The HSE03 + G0W0 calculations present a fundamental band gap of 2.73 eV in the AFM configuration, and the results of the corresponding GW0 and GW calculations are 2.89 and 3.03 eV, respectively. In general, for the cubic structure, the fundamental gap from G0W0 calculations in the 4f-core case is the closest to the real result. By G0W0 calculations in the 4f-core case, we find that H or Gd defects can strongly affect the band structure, especially the H defects. We explain the mechanism in terms of the possible electron correlation on the hydrogen site. Under compression, the insulator-to-metal transition in the cubic GdH3 occurs around 40 GPa, which might be a satisfied prediction.
NASA Astrophysics Data System (ADS)
Lemke, Kono H.; Seward, Terry M.
2008-07-01
Reports of the high ion content of steam and low-density supercritical fluids date back to the work of Carlon [Carlon H. R. (1980) Ion content of air humidified by boiling water. J. Appl.Phys.51, 171-173], who invoked ion and neutral-water clustering as mechanism to explain why ions partition into the low-density aqueous phase. Mass spectrometric, vibrational spectroscopic measurements and quantum chemical calculations have refined this concept by proposing strongly bound ion-solvent aggregates and water clusters such as Eigen- and Zundel-type proton clusters H 3O +·(H 2O) m and the more weakly bound water oligomers (H 2O) m. The extent to which these clusters affect fluid chemistry is determined by their abundance, however, little is known regarding the stability of such moieties in natural low-density high-temperature fluids. Here we report results from quantum chemical calculations using chemical-accuracy multi-level G3 (Curtiss-Pople) and CBS-Q theory (Peterson) to address this question. In particular, we have investigated the cluster structures and clustering equilibria for the ions HO·(HO)m(HS)n,NH4+·(HO)m(HS)n and H 3S +·(H 2O) m(H 2S) n, where m ⩽ 6 and n ⩽ 4, at 300-1000 K and 1 bar as well as under vapor-liquid equilibrium conditions between 300 and 646 K. We find that incremental hydration enthalpies and entropies derived from van't Hoff analyses for the attachment of H 2O and H 2S onto H 3O +, NH4+ and H 3S + are in excellent agreement with experimental values and that the addition of water to all three ions is energetically more favorable than solvation by H 2S. As clusters grow in size, the energetic trends of cluster hydration begin to reflect those for bulk H 2O liquids, i.e. calculated hydration enthalpies and entropies approach values characteristic of the condensation of bulk water (Δ Ho = -44.0 kJ mol -1, Δ So = -118.8 J K mol -1). Water and hydrogen sulfide cluster calculations at higher temperatures indicate that a significant
Tripathi, A.N.; Smith, V.H. Jr. K7L3N6); Kaijser, P.; Siemens, A.G. ); Diercksen, G.H.F. )
1990-03-01
Isotropic scattering functions and Compton profiles together with their directional components for several directions relevant to the molecular structure of C{sub 2}H{sub 2} and C{sub 2}H{sub 4} have been evaluated for {ital ab} {ital initio} self-consistent field and configuration-interaction wave functions. The internally folded density (reciprocal form factor) {ital B}({ital r}) is calculated and discussed as are various momentum expectation values. Comparison is made with available experimental and other theoretical results.
Ab initio calculation of the $$np \\to d ³$$ radiative capture process
Beane, Silas R.; Chang, Emmanuel; Detmold, William; Orginos, Kostas; Parreño, Assumpta; Savage, Martin J.; Tiburzi, Brian C.
2015-09-24
In this study, lattice QCD calculations of two-nucleon systems are used to isolate the short-distance two-body electromagnetic contributions to the radiative capture processmore » $$np \\to d\\gamma$$, and the photo-disintegration processes $$\\gamma^{(\\ast)} d \\to np$$. In nuclear potential models, such contributions are described by phenomenological meson-exchange currents, while in the present work, they are determined directly from the quark and gluon interactions of QCD. Calculations of neutron-proton energy levels in multiple background magnetic fields are performed at two values of the quark masses, corresponding to pion masses of $$m_\\pi \\sim 450$$ and 806 MeV, and are combined with pionless nuclear effective field theory to determine these low-energy inelastic processes. Extrapolating to the physical pion mass, a cross section of $$\\sigma^{lqcd}(np\\to d\\gamma)=332.4({\\tiny \\begin{array}{l}+5.4 \\\\ - 4.7\\end{array}})\\ mb$$ is obtained at an incident neutron speed of $$v=2,200\\ m/s$$, consistent with the experimental value of $$\\sigma^{expt}(np \\to d\\gamma) = 334.2(0.5)\\ mb$$.« less
Traveling Wave Ion Mobility Mass Spectrometry and Ab Initio Calculations of Phosphoric Acid Clusters
NASA Astrophysics Data System (ADS)
Lavanant, Hélène; Tognetti, Vincent; Afonso, Carlos
2014-04-01
Positive and negative ion electrospray mass spectra obtained from 50 mM phosphoric acid solutions presented a large number of phosphoric acid clusters: [(H3PO4)n + zH] z+ or [(H3PO4)n - zH] z- , with n up to 200 and z up to 4 for positively charged clusters, and n up to 270 and z up to 7 for negatively charged cluster ions. Ion mobility experiments allowed very explicit separation of the different charge states. Because of the increased pressures involved in ion mobility experiments, dissociation to smaller clusters was observed both in the trap and transfer areas. Voltages along the ion path could be optimized so as to minimize this effect, which can be directly associated with the cleavage of hydrogen bonds. Having excluded the ion mobility times that resulted from dissociated ions, each cluster ion appeared at a single drift time. These drift times showed a linear progression with the number of phosphoric atoms for cluster ions of the same charge state. Cross section calculations were carried out with MOBCAL on DFT optimized geometries with different hydrogen locations and with three types of atomic charges. DFT geometry optimizations yielded roughly spherical structures. Our results for nitrogen gas interaction cross sections showed that values were dependent on the atomic charges definition used in the MOBCAL calculation. This pinpointed the necessity to define a clear theoretical framework before any comparative interpretations can be attempted with uncharacterized compounds.
Ab initio Calculation of the np→dγ Radiative Capture Process.
Beane, Silas R; Chang, Emmanuel; Detmold, William; Orginos, Kostas; Parreño, Assumpta; Savage, Martin J; Tiburzi, Brian C
2015-09-25
Lattice QCD calculations of two-nucleon systems are used to isolate the short-distance two-body electromagnetic contributions to the radiative capture process np→dγ, and the photo-disintegration processes γ^{(*)}d→np. In nuclear potential models, such contributions are described by phenomenological meson-exchange currents, while in the present work, they are determined directly from the quark and gluon interactions of QCD. Calculations of neutron-proton energy levels in multiple background magnetic fields are performed at two values of the quark masses, corresponding to pion masses of m_{π}~450 and 806 MeV, and are combined with pionless nuclear effective field theory to determine the amplitudes for these low-energy inelastic processes. At m_{π}~806 MeV, using only lattice QCD inputs, a cross section σ^{806 MeV}~17 mb is found at an incident neutron speed of v=2,200 m/s. Extrapolating the short-distance contribution to the physical pion mass and combining the result with phenomenological scattering information and one-body couplings, a cross section of σ^{lqcd}(np→dγ)=334.9(+5.2-5.4) mb is obtained at the same incident neutron speed, consistent with the experimental value of σ^{expt}(np→dγ)=334.2(0.5) mb. PMID:26451545
Ab initio calculation of the $np \\to d ³$ radiative capture process
Beane, Silas R.; Chang, Emmanuel; Detmold, William; Orginos, Kostas; Parreño, Assumpta; Savage, Martin J.; Tiburzi, Brian C.
2015-09-24
In this study, lattice QCD calculations of two-nucleon systems are used to isolate the short-distance two-body electromagnetic contributions to the radiative capture process $np \\to d\\gamma$, and the photo-disintegration processes $\\gamma^{(\\ast)} d \\to np$. In nuclear potential models, such contributions are described by phenomenological meson-exchange currents, while in the present work, they are determined directly from the quark and gluon interactions of QCD. Calculations of neutron-proton energy levels in multiple background magnetic fields are performed at two values of the quark masses, corresponding to pion masses of $m_\\pi \\sim 450$ and 806 MeV, and are combined with pionless nuclear effective field theory to determine these low-energy inelastic processes. Extrapolating to the physical pion mass, a cross section of $\\sigma^{lqcd}(np\\to d\\gamma)=332.4({\\tiny \\begin{array}{l}+5.4 \\\\ - 4.7\\end{array}})\\ mb$ is obtained at an incident neutron speed of $v=2,200\\ m/s$, consistent with the experimental value of $\\sigma^{expt}(np \\to d\\gamma) = 334.2(0.5)\\ mb$.
Lavanant, Hélène; Tognetti, Vincent; Afonso, Carlos
2014-04-01
Positive and negative ion electrospray mass spectra obtained from 50 mM phosphoric acid solutions presented a large number of phosphoric acid clusters: [(H3PO4)n + zH](z+) or [(H3PO4)n - zH](z-), with n up to 200 and z up to 4 for positively charged clusters, and n up to 270 and z up to 7 for negatively charged cluster ions. Ion mobility experiments allowed very explicit separation of the different charge states. Because of the increased pressures involved in ion mobility experiments, dissociation to smaller clusters was observed both in the trap and transfer areas. Voltages along the ion path could be optimized so as to minimize this effect, which can be directly associated with the cleavage of hydrogen bonds. Having excluded the ion mobility times that resulted from dissociated ions, each cluster ion appeared at a single drift time. These drift times showed a linear progression with the number of phosphoric atoms for cluster ions of the same charge state. Cross section calculations were carried out with MOBCAL on DFT optimized geometries with different hydrogen locations and with three types of atomic charges. DFT geometry optimizations yielded roughly spherical structures. Our results for nitrogen gas interaction cross sections showed that values were dependent on the atomic charges definition used in the MOBCAL calculation. This pinpointed the necessity to define a clear theoretical framework before any comparative interpretations can be attempted with uncharacterized compounds.
Bond dissociation energy of the phenol O sbnd H bond from ab initio calculations
NASA Astrophysics Data System (ADS)
da Silva, Gabriel; Chen, Chiung-Chu; Bozzelli, Joseph W.
2006-06-01
The phenol O sbnd H bond dissociation energy (BDE) is currently disputed, despite its importance in combustion chemistry and in the reactions of antioxidants such as vitamin E. We have studied this BDE using the computational methods G3, G3B3 and CBS-APNO, with bond-isodesmic work reactions. These calculations yield a BDE of 89.0 ± 1.0 kcal mol -1, which supports other recent calculations [B.J. Costa Cabral, S. Canuto, Chem. Phys. Lett. 406 (2005) 300]. From our BDE we determine the enthalpy of formation of the phenoxy radical to be 13.9 ± 1.0 kcal mol -1. Comparison of the phenol C sbnd H BDE with the vinyl alcohol H sbnd CH dbnd CHOH BDE reveals the bond in phenol to be around 3 kcal mol -1 stronger than that in vinyl alcohol. Replacement of a H atom with the OH group on benzene or ethylene strengthens the C sbnd H bonds on adjacent carbons by 3 to 4 kcal mol -1.
Laser Spectroscopy and AB Initio Calculations on the TaF Molecule
NASA Astrophysics Data System (ADS)
Ng, Kiu Fung; Zou, Wenli; Liu, Wenjian; Cheung, Allan S. C.
2016-06-01
Electronic transition spectrum of the tantalum monoflouride (TaF) molecule in the spectral region between 448 and 520 nm has been studied using the technique of laser-ablation/reaction free jet expansion and laser induced fluorescence spectroscopy. TaF molecule was produced by reacting laser-ablated tantalum atoms with sulfur hexafluoride gas seeded in argon. Sixteen vibrational bands with resolved rotational structure have been recorded and analyzed, which were organized into six electronic transition systems and the ground state has been identified to be the X3Σ-(0+) state with bond length, ro, and equilibrium vibrational frequency, ωe, determined to be 1.8209 Å and 700.1 wn respectively. In addition, four vibrational bands belong to another transition system involving lower state with Ω = 2 component has also been analyzed. All observed transitions are with ΔΩ = 0. Least-squares fit of the measured line positions yielded molecular constants for the electronic states involved. The Λ-S and Ω states of TaF were calculated at the state-averaged complete active space self-consistent field (SA-CASSCF) and the subsequent internally contracted multi-reference configuration interaction with singles and doubles and Davidson's cluster correction (MRCISD+Q) levels of theory with the active space of 4 electrons in 6 orbitals, that is, the molecular orbitals corresponding to Ta 5d6s are active. The spin-orbit coupling (SOC) is calculated by the state-interaction approach at the SA-CASSCF level via the relativistic effective core potentials (RECPs) spin-orbit operator, where the diagonal elements of the spin-orbit matrix are replaced by the above MRCISD+Q energies. The spectroscopic properties of the ground and many low-lying electronic states of the TaF molecule will be reported. With respect to the observed electronic states in this work, the calculated results are in good agreement with our experimental determinations. This work represents the first experimental
Determining the phase diagram of lithium via ab initio calculation and ramp compression
NASA Astrophysics Data System (ADS)
Shulenburger, Luke; Seagle, Chris; Haill, Thomas; Harding, Eric
2015-06-01
Diamond anvil cell experiments have shown elemental lithium to have an extraordinarily complex phase diagram under pressure exhibiting numerous solid phases at pressures below 1 Mbar, as well as a complicated melting behavior. We explore this phase diagram utilizing a combination of quantum mechanical calculations and ramp compression experiments performed on Sandia National Laboratories' Z-machine. We aim to extend our knowledge of the high pressure behavior to moderate temperatures at pressures above 50 GPa with a specific focus on the melt line above 70 GPa. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the US Dept of Energy's Natl. Nuclear Security Administration under Contract DE-AC04-94AL85000.
Thermo-physical properties of LiH at high pressures by ab initio calculations
NASA Astrophysics Data System (ADS)
Mukherjee, D.; Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.; Sikka, S. K.
2011-05-01
First principles calculations have been carried out to analyze structural stability and to determine the equation of state and elastic constants of LiH as a function of pressure. The comparison of total energies of B1 and B2 structures determined as a function of compression suggests the B1 → B2 transition at ˜ 327 GPa. Various physical quantities including zero pressure equilibrium volume, bulk modulus, pressure derivative of bulk modulus, Debye temperature, bulk sound speed, Hugoniot parameter `s' and Gruneisen parameter have been derived. All these physical quantities compare well with the available experimental data. The single crystal elastic constants have been evaluated up to the B1→B2 transition pressure.
On dynamic and elastic stability of group IIIB metal carbides: Ab-initio calculations
NASA Astrophysics Data System (ADS)
Sahoo, B. D.; Mukherjee, D.; Joshi, K. D.; Gupta, Satish C.
2012-06-01
The elastic and dynamic stabilities of IIIB metal carbides in NaCl (B1 phase) type fcc structure have been investigated at zero pressure by first-principles calculations using the plane-wave pseudopotential method. The analysis of elastic moduli and phonon dispersion relations in ScC and YC suggest that the B1 phase is stable both elastically as well as dynamically at zero pressure. In contrast, for LaC, we find that the shear elastic modulus C' (=(C11-C12)/2) is negative at zero pressure and also the phonon frequencies in various directions of the Brillouin zone are imaginary, indicative of elastic as well as dynamic instability of B1 phase at zero pressure in this compound.
Ab initio calculations of zinc-blende CrAs/GaAs superlattices
NASA Astrophysics Data System (ADS)
Nagao, Kazutaka; Shirai, Masafumi; Miura, Yoshio
2004-06-01
We investigate the properties of interfaces between highly spin-polarized zinc-blende (ZB) CrAs and GaAs using first-principles density functional calculations. It is found that the local spin polarization at the Fermi level is very high even at ZB-CrAs/GaAs interface and then exhibits gradual decay from the interface to the inside of GaAs, reflecting their ideal structural matching. We further study the properties of ZB-CrAs/GaAs multilayer where two-monolayer ZB-CrAs and two-monolayer GaAs stack alternately. The multilayer shows high spin polarization throughout its entire region. Since the spin polarization is found insensitive to Cr-Ga substitutional disorder, both ZB-CrAs and ZB-CrAs/GaAs multilayer may work as efficient filter for spin-polarized current injection into GaAs.
Ab initio calculations of quasiparticle band structure in correlated systems: LDA++ approach
NASA Astrophysics Data System (ADS)
Lichtenstein, A. I.; Katsnelson, M. I.
1998-03-01
We discuss a general approach to a realistic theory of the electronic structure in materials containing correlated d or f electrons. The main feature of this approach is the taking into account of the energy dependence of the electron self-energy with the momentum dependence being neglected (local approximation). It allows us to consider such correlation effects as the non-Fermi-step form of the distribution function, the enhancement of the effective mass including Kondo resonances,'' the appearance of the satellites in the electron spectra, etc. To specify the form of the self-energy, it is useful to distinguish (according to the ratio of the on-site Coulomb energy U to the bandwidth W) three regimes-strong, moderate, and weak correlations. In the case of strong interactions (U/W>1-rare-earth system) the Hubbard-I approach is the most suitable. Starting from an exact atomic Green function with the constrained density matrix nmm' the band-structure problem is formulated as the functional problem on nmm' for f electrons and the standard local-denisty-approximation functional for delocalized electrons. In the case of moderate correlations (U/W~1-metal-insulator regime, Kondo systems) we start from the d=∞ dynamical mean-field iterative perturbation scheme of Kotliar and co-workers and also make use of our multiband atomic Green function for constrained nmm'. Finally for the weak interactions (U/W<1-transition metals) the self-consistent diagrammatic fluctuation-exchange approach of Bickers and Scalapino is generalized to the realistic multiband case. We present two-band, two-dimensional model calculations for all three regimes. A realistic calculation in the Hubbard-I scheme with the exact solution of the on-site multielectron problem for f(d) shells was performed for mixed-valence 4f compound TmSe, and for the classical Mott insulator NiO.
Accurate ab initio calculations which demonstrate a 3 Pi u ground state for Al2
NASA Technical Reports Server (NTRS)
Bauschlicher, Charles W., Jr.; Partridge, Harry; Langhoff, Stephen R.; Taylor, Peter R.; Walch, Stephen P.
1986-01-01
The spectroscopic parameters and separations between the three low-lying X 3 Pi u, A 3 Sigma g -, and a 1 Sigma g + states of Al2 are studied as a function of both the one-particle and n-particle basis set. Approximate correlation treatments are calibrated against full Cl calculations correlating the six valence electrons in a double-zeta plus two d-function basis set. Since the CASSCF/MRCI 3 Pi u to 3 Sigma g - separation is in excellent agreement wtih the FCI value, the MRCI calculations were carried out in an extended (20s13p6d4f)/(6s5p3d2f) gaussian basis. Including a small correction for relativistic effects, the best estimate is that 3 Sigma g - state lies 174/cm above the 3 Pi u ground state. The 1 Sigma g + state lies at least 2000/cm higher in energy. At the CPF level, inclusion of 2s and 2p correlation has little effect on D sub e, reduces T sub e by only 26/cm, and shortens the bond lengths by about 0.02 a sub o. Further strong support for a 3 Pi u ground state comes from the experimental absorption spectra, since both observed transitions can be convincingly assigned as 3 Pi u yields 3 Pi g. The (2) 3 Pi g state is observed to be sensitive to the level of correlation treatment, and to have its minimum shifted to shorter rho values, such that the strongest experimental absorption peak probably corresponds to the 0 yields 2 transition.
Tsuzuki, Seiji; Hayamizu, Kikuko; Seki, Shiro; Ohno, Yasutaka; Kobayashi, Yo; Miyashiro, Hajime
2008-08-14
Interactions of the lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) complex with N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium (DEME), 1-ethyl-3-methylimidazolium (EMIM) cations, neutral diethylether (DEE), and the DEMETFSA complex were studied by ab initio molecular orbital calculations. An interaction energy potential calculated for the DEME cation with the LiTFSA complex has a minimum when the Li atom has contact with the oxygen atom of DEME cation, while potentials for the EMIM cation with the LiTFSA complex are always repulsive. The MP2/6-311G**//HF/6-311G** level interaction energy calculated for the DEME cation with the LiTFSA complex was -18.4 kcal/mol. The interaction energy for the neutral DEE with the LiTFSA complex was larger (-21.1 kcal/mol). The interaction energy for the DEMETFSA complex with LiTFSA complex is greater (-23.2 kcal/mol). The electrostatic and induction interactions are the major source of the attraction in the two systems. The substantial attraction between the DEME cation and the LiTFSA complex suggests that the interaction between the Li cation and the oxygen atom of DEME cation plays important roles in determining the mobility of the Li cation in DEME-based room temperature ionic liquids.
Zhang, Wen-Jing; Hou, Gao-Lei; Wang, Peng; Xu, Hong-Guang; Feng, Gang; Xu, Xi-Ling; Zheng, Wei-Jun
2015-08-07
To understand the microsolvation of sodium acetate (CH{sub 3}COONa, NaOAc) in water, we studied NaOAc(H{sub 2}O){sub n}{sup −} (n = 0–3) clusters by photoelectron spectroscopy. We also investigated the structures of NaOAc(H{sub 2}O){sub n}{sup −} (n = 0–5) anions and NaOAc(H{sub 2}O){sub n} (n = 0–7) neutrals by quantum chemistry calculations. By comparing the theoretical results with the photoelectron experiment, the most probable structures of NaOAc(H{sub 2}O){sub n}{sup −/0} (n = 0–3) were determined. The study also shows that, with increasing n, the solvent-separated ion pair (SSIP) structures of NaOAc(H{sub 2}O){sub n}{sup −} anions become nearly energetically degenerate with the contact ion pair (CIP) structures at n = 5, while the SSIP structures of the neutral NaOAc(H{sub 2}O){sub n} clusters appear at n = 6 and become dominant at n = 7.
Ab-Initio Calculations of the Electronic Properties of Boron Nitride
NASA Astrophysics Data System (ADS)
Stewart, Anthony; Khamala, Bethuel; Hart, Daniel; Bagayoko, Diola
2014-03-01
The potential of Boron Nitride (BN) in nanotechnology is tremendous. BN in its bulk form has a wide band gap with excellent thermal and chemical stability. BN structures can be tailored using various techniques in order to obtain desired materials properties. The State-of-the-art Proton Exchange Membrane Fuel Cell (PEMFCs) technology exploits graphitized carbon as a support for platinum-type catalysts. However, some forms of carbon are susceptible to long-term durability issues such as corrosion which is a detriment to fuel cell performance and viability. Novel non-carbon supports such as BN may provide a pathway for addressing the durability and performance issues associated with carbon support materials. We present preliminary theoretical studies, using an linear combination of atomic orbital (LCAO) quantum chemistry package from Ames Laboratory, of the electronic properties of this potentially important material. Our calculated band gap of 6.48 eV for the cubic structure, obtained with an LDA potential and the BZW-EF method, is in agreement with experiment. LASIGMA/ NNSA_MSIP.
NASA Astrophysics Data System (ADS)
Oliveira, Micael; Komarova-Vladimirova, Ksenia; Remacle, Francoise; Vertraete, Matthieu
The London-van der Waals dispersion forces arising from instantaneously induced dipoles in molecules are a key ingredient in a wide range of phenomena in physics, chemistry, and biology. Therefore, the ability to control and manipulate dispersion forces between atoms and molecules is of great importance. Because those dispersion interactions depend crucially on the electronic properties of the molecular systems, a simple route to achieve this would consist in manipulating their electronic states. The recent development of ultra-short optical pulses has given researchers unprecedented control over the electronic degrees of freedom. These pulses, tailored in their frequency and envelope, allow the generation of a strongly out of equilibrium population of electronic states. In this talk we show how the Hamacker constants characterizing the London-van der Waals interaction between two molecules subject to an optical pulse can be calculated using time-dependent density functional theory (TD-DFT) or standard quantum chemistry methods and present several test cases of molecules subjected to IR and UV attosecond pulses.
Anisotropic elasticity of IVB transition-metal mononitrides determined by ab initio calculations
Nagao, Shijo; Nordlund, K.; Nowak, R.
2006-04-01
Elastic parameters of IVB transition-metal mononitrides, TiN, ZrN, and HfN in the cubic NaCl crystal structure have been calculated by means of density-functional theory with the generalized gradient approximation. The elastic constants c{sub 11}, c{sub 12}, and c{sub 44} were shown to be sufficiently converged with the density of the k-point mesh in the deformed Brillouin zone to discuss the elastic anisotropy of the systems. It was found that the anisotropy coefficient {kappa}{identical_to}(c{sub 11}-c{sub 12})/2c{sub 44} increases with the atomic number of the metal element, i.e., HfN exhibits as strong anisotropy as {kappa}=2.02. The Young's modulus of HfN along <100> is approximately two times higher than that along <111>. Moreover, analysis of the deformation energy by the applied strain modes shows that this elastic anisotropy originates from the strong covalent bonding between metal and nitrogen atoms along <100>.
NASA Astrophysics Data System (ADS)
Vieira, R. B. L.; Vilão, R. C.; Marinopoulos, A. G.; Gordo, P. M.; Paixão, J. A.; Alberto, H. V.; Gil, J. M.; Weidinger, A.; Lichti, R. L.; Baker, B.; Mengyan, P. W.; Lord, J. S.
2016-09-01
We present a systematic study of isolated hydrogen in diverse forms of ZrO2 (zirconia), both undoped and stabilized in the cubic phase by additions of transition-metal oxides (Y2O3,Sc2O3 , MgO, CaO). Hydrogen is modeled by using muonium as a pseudoisotope in muon-spin spectroscopy experiments. The muon study is also supplemented with first-principles calculations of the hydrogen states in scandia-stabilized zirconia by conventional density-functional theory (DFT) as well as a hybrid-functional approach which admixes a portion of exact exchange to the semilocal DFT exchange. The experimentally observable metastable states accessible by means of the muon implantation allowed us to probe two distinct hydrogen configurations predicted theoretically: an oxygen-bound configuration and a quasiatomic interstitial one with a large isotropic hyperfine constant. The neutral-oxygen-bound configuration is characterized by an electron spreading over the neighboring zirconium cations, forming a polaronic state with a vanishingly small hyperfine interaction at the muon. The atom-like interstitial muonium is observed also in all samples but with different fractions. The hyperfine interaction is isotropic in calcia-doped zirconia [Aiso=3.02 (8 ) GHz], but slightly anisotropic in the nanograin yttria-doped zirconia [Aiso=2.1 (1 ) GHz, D =0.13 (2 ) GHz] probably due to muons stopping close to the interface regions between the nanograins in the latter case.
Ab initio calculations of optical properties of B2C graphene sheet
NASA Astrophysics Data System (ADS)
Shahrokhi, Masoud; Naderi, Sirvan; Fathalian, Ali
2012-06-01
The electronic and the linear optical properties of a monolayer B2C graphene sheet are investigated through the density functional theory. The dielectric tensor is derived within the random phase approximation (RPA). Specifically, the dielectric function, absorption coefficient, optical conductivity, extinction index, loss function, reflectivity, and the refraction index of the monolayer B2C graphene sheet are calculated for both parallel and perpendicular electric field polarizations. The results show that the optical spectra are anisotropic along these two polarizations. For the electric field parallel to monolayer B2C graphene E∥x, adding the intraband transitions contribution, will change the optical spectra of a monolayer B2C graphene sheet significantly, while in the electric field perpendicular to monolayer B2C graphene E∥z did not change. Adding the intraband transitions contribution shows that the dielectric function has singularity at zero frequency because of the metallic behavior of a monolayer B2C graphene sheet. Also by considering the intraband transitions contribution for energy loss function in the electric field parallel to monolayer B2C graphene, the first plasmon frequency peak has been shifted from 2.76 eV to 4 eV.
Ab initio calculation of pentacene-PbSe hybrid interface for photovoltaic applications.
Roy, P; Nguyen, Thao P
2016-07-21
We perform density functional theory (DFT) quantum chemical calculations for the pentacene-PbSe hybrid interface at both molecular and crystal levels. At the interface, the parallel orientation of pentacene on the PbSe surface is found to be the most favorable, analogous to a pentacene-gold interface. The molecule-surface distance and the value of charge transfer from one pentacene molecule to the PbSe surface are estimated at around 4.15 Å and 0.12 e(-) respectively. We found that, standard-LDA/GGA-PBE/hybrid/meta-GGA xc-functionals incorrectly determine the band gaps of both pentacene and PbSe and leads to a failed prediction of the energy alignment in this system. So, we use a relativistic G0W0 functional and accurately model the electronic properties of pentacene and PbSe in both bulk material and near the interface. An energy shift of 0.23 eV, due to the difference in work function at the interface was supplemented after a detailed analysis of the electrostatic potential. The highest occupied molecular orbital level of pentacene is 0.01 eV above PbSe while the lowest unoccupied molecular orbital of pentacene lies 1.70 eV above PbSe, allowing both electrons and holes to transfer along the donor-acceptor junction. Our results provide additional insights into the electronic structure properties of the pentacene-PbSe heterojunction and establish it as a promising and efficient candidate for photovoltaic applications. PMID:27332630
Multiple time step integrators in ab initio molecular dynamics
Luehr, Nathan; Martínez, Todd J.; Markland, Thomas E.
2014-02-28
Multiple time-scale algorithms exploit the natural separation of time-scales in chemical systems to greatly accelerate the efficiency of molecular dynamics simulations. Although the utility of these methods in systems where the interactions are described by empirical potentials is now well established, their application to ab initio molecular dynamics calculations has been limited by difficulties associated with splitting the ab initio potential into fast and slowly varying components. Here we present two schemes that enable efficient time-scale separation in ab initio calculations: one based on fragment decomposition and the other on range separation of the Coulomb operator in the electronic Hamiltonian. We demonstrate for both water clusters and a solvated hydroxide ion that multiple time-scale molecular dynamics allows for outer time steps of 2.5 fs, which are as large as those obtained when such schemes are applied to empirical potentials, while still allowing for bonds to be broken and reformed throughout the dynamics. This permits computational speedups of up to 4.4x, compared to standard Born-Oppenheimer ab initio molecular dynamics with a 0.5 fs time step, while maintaining the same energy conservation and accuracy.
Interaction between single vacancies in graphene sheet: An ab initio calculation
NASA Astrophysics Data System (ADS)
Scopel, W. L.; Paz, Wendel S.; Freitas, Jair C. C.
2016-08-01
In order to investigate the interaction between single vacancies in a graphene sheet, we have used spin-polarized density functional theory (DFT). Two distinct configurations were considered, either with the two vacancies located in the same sublattice or in different sublattices, and the effect of changing the separation between the vacancies was also studied. Our results show that the ground state of the system is indeed magnetic, but the presence of the vacancies in the same sublattice or in different sublattices and the possible topological configurations can lead to different contributions from the π and σ orbitals to magnetism. On the other hand, our findings reveal that the net magnetic moment of the system with the two vacancies in the same sublattice move towards the value of the magnetic moment per isolated vacancy with the increase of the distance between the vacancies, which is ascribed to the different contributions due to π electrons. Moreover, it is also found that the local magnetic moments for vacancies in the same sublattice are in parallel configuration, while they have different orientations when the vacancies are created in different sublattices. So, our findings have clearly evidenced how difficult it would be to observe experimentally the emergence of magnetic order in graphene-based systems containing randomly created atomic vacancies, since the energy difference between cases of antiferromagnetic and ferromagnetic order decreases quickly with the increase in the distance separating each vacancy pair.
Durig, James R; Panikar, Savitha S; Obenchain, Daniel A; Bills, Brandon J; Lohan, Patrick M; Peebles, Rebecca A; Peebles, Sean A; Groner, Peter; Guirgis, Gamil A; Johnston, Michael D
2012-01-28
The microwave spectrum (6500-18 ,500 MHz) of 1-fluoro-1-silacyclopentane, c-C(4)H(8)SiHF has been recorded and 87 transitions for the (28)Si, (29)Si, (30)Si, and (13)C isotopomers have been assigned for a single conformer. Infrared spectra (3050-350 cm(-1)) of the gas and solid and Raman spectrum (3100-40 cm(-1)) of the liquid have also been recorded. The vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twist form. Ab initio calculations with a variety of basis sets up to MP2(full)/aug-cc-pVTZ predict the envelope-axial and envelope-equatorial conformers to be saddle points with nearly the same energies but much lower energy than the planar conformer. By utilizing the microwave rotational constants for seven isotopomers ((28)Si, (29)Si, (30)Si, and four (13)C) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters have been obtained for the twist conformer. The heavy atom distances in Å are: r(0)(SiC(2)) = 1.875(3); r(0)(SiC(3)) = 1.872(3); r(0)(C(2)C(4)) = 1.549(3); r(0)(C(3)C(5)) = 1.547(3); r(0)(C(4)C(5)) = 1.542(3); r(0)(SiF) = 1.598(3) and the angles in degrees are: [angle]CSiC = 96.7(5); [angle]SiC(2)C(4) = 103.6(5); [angle]SiC(3)C(5) = 102.9(5); [angle]C(2)C(4)C(5) = 108.4(5); [angle]C(3)C(5)C(4) = 108.1(5); [angle]F(6)Si(1)C(2) = 110.7(5); [angle]F(6)Si(1)C(3) = 111.6(5). The heavy atom ring parameters are compared to the corresponding r(s) parameters. Normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values, and infrared band contours. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.
Kroes, Geert-Jan; Pavanello, Michele; Blanco-Rey, María; Alducin, Maite; Auerbach, Daniel J
2014-08-01
Energy loss from the translational motion of an atom or molecule impinging on a metal surface to the surface may determine whether the incident particle can trap on the surface, and whether it has enough energy left to react with another molecule present at the surface. Although this is relevant to heterogeneous catalysis, the relative extent to which energy loss of hot atoms takes place to phonons or electron-hole pair (ehp) excitation, and its dependence on the system's parameters, remain largely unknown. We address these questions for two systems that present an extreme case of the mass ratio of the incident atom to the surface atom, i.e., H + Cu(111) and H + Au(111), by presenting adiabatic ab initio molecular dynamics (AIMD) predictions of the energy loss and angular distributions for an incidence energy of 5 eV. The results are compared to the results of AIMDEFp calculations modeling energy loss to ehp excitation using an electronic friction ("EF") model applied to the AIMD trajectories, so that the energy loss to the electrons is calculated "post" ("p") the computation of the AIMD trajectory. The AIMD calculations predict average energy losses of 0.38 eV for Cu(111) and 0.13-0.14 eV for Au(111) for H-atoms that scatter from these surfaces without penetrating the surface. These energies closely correspond with energy losses predicted with Baule models, which is suggestive of structure scattering. The predicted adiabatic integral energy loss spectra (integrated over all final scattering angles) all display a lowest energy peak at an energy corresponding to approximately 80% of the average adiabatic energy loss for non-penetrative scattering. In the adiabatic limit, this suggests a way of determining the approximate average energy loss of non-penetratively scattered H-atoms from the integral energy loss spectrum of all scattered H-atoms. The AIMDEFp calculations predict that in each case the lowest energy loss peak should show additional energy loss in the
Kroes, Geert-Jan Pavanello, Michele; Blanco-Rey, María; Alducin, Maite
2014-08-07
Energy loss from the translational motion of an atom or molecule impinging on a metal surface to the surface may determine whether the incident particle can trap on the surface, and whether it has enough energy left to react with another molecule present at the surface. Although this is relevant to heterogeneous catalysis, the relative extent to which energy loss of hot atoms takes place to phonons or electron-hole pair (ehp) excitation, and its dependence on the system's parameters, remain largely unknown. We address these questions for two systems that present an extreme case of the mass ratio of the incident atom to the surface atom, i.e., H + Cu(111) and H + Au(111), by presenting adiabatic ab initio molecular dynamics (AIMD) predictions of the energy loss and angular distributions for an incidence energy of 5 eV. The results are compared to the results of AIMDEFp calculations modeling energy loss to ehp excitation using an electronic friction (“EF”) model applied to the AIMD trajectories, so that the energy loss to the electrons is calculated “post” (“p”) the computation of the AIMD trajectory. The AIMD calculations predict average energy losses of 0.38 eV for Cu(111) and 0.13-0.14 eV for Au(111) for H-atoms that scatter from these surfaces without penetrating the surface. These energies closely correspond with energy losses predicted with Baule models, which is suggestive of structure scattering. The predicted adiabatic integral energy loss spectra (integrated over all final scattering angles) all display a lowest energy peak at an energy corresponding to approximately 80% of the average adiabatic energy loss for non-penetrative scattering. In the adiabatic limit, this suggests a way of determining the approximate average energy loss of non-penetratively scattered H-atoms from the integral energy loss spectrum of all scattered H-atoms. The AIMDEFp calculations predict that in each case the lowest energy loss peak should show additional energy loss
NASA Astrophysics Data System (ADS)
Kroes, Geert-Jan; Pavanello, Michele; Blanco-Rey, María; Alducin, Maite; Auerbach, Daniel J.
2014-08-01
Energy loss from the translational motion of an atom or molecule impinging on a metal surface to the surface may determine whether the incident particle can trap on the surface, and whether it has enough energy left to react with another molecule present at the surface. Although this is relevant to heterogeneous catalysis, the relative extent to which energy loss of hot atoms takes place to phonons or electron-hole pair (ehp) excitation, and its dependence on the system's parameters, remain largely unknown. We address these questions for two systems that present an extreme case of the mass ratio of the incident atom to the surface atom, i.e., H + Cu(111) and H + Au(111), by presenting adiabatic ab initio molecular dynamics (AIMD) predictions of the energy loss and angular distributions for an incidence energy of 5 eV. The results are compared to the results of AIMDEFp calculations modeling energy loss to ehp excitation using an electronic friction ("EF") model applied to the AIMD trajectories, so that the energy loss to the electrons is calculated "post" ("p") the computation of the AIMD trajectory. The AIMD calculations predict average energy losses of 0.38 eV for Cu(111) and 0.13-0.14 eV for Au(111) for H-atoms that scatter from these surfaces without penetrating the surface. These energies closely correspond with energy losses predicted with Baule models, which is suggestive of structure scattering. The predicted adiabatic integral energy loss spectra (integrated over all final scattering angles) all display a lowest energy peak at an energy corresponding to approximately 80% of the average adiabatic energy loss for non-penetrative scattering. In the adiabatic limit, this suggests a way of determining the approximate average energy loss of non-penetratively scattered H-atoms from the integral energy loss spectrum of all scattered H-atoms. The AIMDEFp calculations predict that in each case the lowest energy loss peak should show additional energy loss in the
NASA Astrophysics Data System (ADS)
Kozin, Igor N.; Jensen, Per; Li, Yan; Buenker, Robert J.; Hirsch, Gerhard; Klee, Stefan
1997-01-01
The present work reports an ab initioMRD-CI calculation of the dipole moment surfaces for the electronic ground state of the H 2Te molecule. Using the ab initioresults, we calculate the vibrational transition moments, and we simulate the far-infrared spectrum of H 2Te by means of the MORBID program system. We obtain the equilibrium value of the dipole moment from the ab initiocalculation as 0.377 Debye based on our initial theoretical treatment which was employed over a wide range of molecular geometries. However, the use of an improved AO basis at the equilibrium geometry of H 2Te lowers this result to 0.298 Debye. The comparison of our simulated far-infrared spectrum with the experimental spectrum suggests that this value is too large, and that the correct value is certainly larger than 0.19 Debye and very probably smaller than 0.26 Debye. From the ab initiodata, we predict many vibrational transition moments for H 2Te, D 2Te, and HDTe. We hope that these results will be of assistance in the interpretation of the rotation-vibration spectrum of these molecules.
Chen, Lina; Woon, David E; Dunning, Thom H
2013-05-23
Based on detailed, high level ab initio calculations on a number of halogenated compounds of second row, late p-block elements, the SF(n), ClF(n), PFn, SCl(n), and SF(n)Cl families, we found that a new type of bond--the recoupled pair bond--accounts for the ability of these elements to form hypervalent, or hypercoordinated, compounds. Hypervalent molecules are formed when it is energetically favorable for the electrons in a lone pair orbital to be recoupled, allowing each of the electrons to form chemical bonds with ligands. In this paper, we characterize the structures and energies of the ground and low-lying excited states of the ClF(n)(+) (n = 1-6) ions, using high level ab initio methods [MRCI, CCSD(T)/RCCSD(T)] with large correlation consistent basis sets. We computed a number of quantities, including ClF(n)(+) structures, bond dissociation energies, and ClF(n) ionization energies and compared our results with the available experimental data. Both the bond dissociation energies and the ionization energies oscillate, variations that are readily explained using the recoupled pair bonding model. Comparisons are drawn between the ClF(n)(+) cations and their counterparts in the isoelectronic SF(n) series, which possess many similarities. We found two significant differences between the ClF(n)(+) and the SF(n) series: (i) the bond dissociation energies of ClF(n)(+) are much weaker than those of the corresponding SF(n) species, and (ii) there is no stable (3)A2 state in ClF2(+) corresponding to the stable state found in SF2. An examination of the Mulliken populations at the HF/AVTZ level for ClF(n)(+) and SF(n) species predicts that the F atom in the axial (recoupled pair bonding) position is more highly charged than the F atom in the equatorial (covalent bonding) position; there is also less charge transfer to the F atoms in ClF(n)(+) than in SF(n). The positive charge on Cl(+) makes it more difficult for an F atom to attract electrons from Cl(+) than from S and
Surin, L. A.; Potapov, A.; Schlemmer, S.; Dolgov, A. A.; Tarabukin, I. V.; Panfilov, V. A.; Kalugina, Y. N.; Faure, A.; Avoird, A. van der
2015-03-21
The rotational spectrum of the van der Waals complex NH{sub 3}–CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 112–139 GHz. Newly observed and assigned transitions belong to the K = 0–0, K = 1–1, K = 1–0, and K = 2–1 subbands correlating with the rotationless (j{sub k}){sub NH3} = 0{sub 0} ground state of free ortho-NH{sub 3} and the K = 0–1 and K = 2–1 subbands correlating with the (j{sub k}){sub NH3} = 1{sub 1} ground state of free para-NH{sub 3}. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. Some of these transitions are continuations to higher J values of transition series observed previously [C. Xia et al., Mol. Phys. 99, 643 (2001)], the other transitions constitute newly detected subbands. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the ortho-NH{sub 3}–CO and para-NH{sub 3}–CO complexes. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of NH{sub 3}–CO has been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations and an augmented correlation-consistent triple zeta basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the N atom closest to the CO subunit and binding energy D{sub e} = 359.21 cm{sup −1}. The bound rovibrational levels of the NH{sub 3}–CO complex were calculated for total angular momentum J = 0–6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D{sub 0} are 210.43 and 218.66 cm{sup −1} for ortho-NH{sub 3}–CO and para-NH{sub 3}–CO, respectively.
Chen, K X; Wu, J A; Ji, R Y
1987-09-01
We investigated the cis- and trans-isomers of Pt(NH3)2Cl2 and [Pt(NH3)2]2+ using a quantum chemical non-empirical calculation method, the pseudopotential valence electron-only ab initio method. The electronic structure and electrostatic potential counter maps were in turn determined through the wave functions so obtained. There was a sharp difference between the dipole moments of the cis- and trans-isomers. The electrostatic counter maps of the isomers also had remarkably different features. Based on the interaction between the platinum (II) coordination compound and the base pairs of nucleic acid, the difference in antitumour activity of the isomeric compounds was discussed. It is pointed out that the key factor for antitumour activity is that the platinum (II) coordination compound must be mutually complementary with the target acceptor in both configuration and bonding activity. This mutual-complementary requirement includes a bonding ability of the platinum complex with two negative centers in DNA, so as to form an intrastrand crosslink with two neighbouring guanines.
NASA Astrophysics Data System (ADS)
Wolff, Stephen K.; Jayatilaka, Dylan; Chandler, Graham S.
1995-09-01
Spin-orbit interaction plays a significant role in determining the magnetic density in some transition metal complexes. We present a new ab initio technique, based on an extension of unrestricted Hartree-Fock theory, which includes nonperturbatively these spin-orbit effects, and simultaneously also the effects of a finite magnetic field. We also present a new and efficient method for calculating magnetic structure factors, based on the current density rather than magnetic dipole moment density, for a crystal composed of noninteracting molecular fragments. These structure factors are directly comparable to polarized neutron diffraction experiments. Results for the Cs3CoCl5 crystal are compared with experiment and previous studies. Without one-electron spin-orbit coupling terms, the magnitudes of the predicted structure factors are on average 10-15 % too low, whereas, with the spin-orbit terms, the magnitudes are 25-30% too high. Using an effective nuclear charge for Co in the spin-orbit term brings the results into much better agreement, and suggests that the two-electron spin-orbit shielding terms omitted in the present work are important. For over one quarter of the reflections studied, the magnetic contribution to the structure factors is more than 20% of the nuclear contribution.
Ab initio calculation of proton dynamics in topaz-OH (Al2SiO4(OH)2)
NASA Astrophysics Data System (ADS)
Churakov, S. V.; Wunder, B.; Parrinello, M.
2003-04-01
The OH-endmember of topaz (topaz-OH), Al2SiO4(OH)2, which was first synthesized by [1] in multi-anvil experiments at P above 7.0 GPa, is proposed to be an important water-containing mineral in deep subduction zones. Up to now the hydrogen position has not been located in the structure of topaz-OH. However, this is strongly required for a reliable thermodynamic description of this phase. According to [2], two non-equivalent OH-positions should exist, which would lead to a reduction of symmetry from Pbnm, known for natural, normally F-rich topaz, to Pbn21, by loss of the mirror plane in (001). However, this reduction of symmetry was not resolved from their conventional single-crystal X-ray diffraction data. The purpose of this study was to obtain the most stable configuration of protons in the structure of topaz-OH, by DFT ab initio quantum mechanic calculations[3]. Three principally different structures with distinct orientation of protons were obtained. In all configurations hydrogen atoms are located on oxygen sites not bonded to silicon. Protons were found to occupy either all H1 or all H2 sites (structure I and II) or be equally distributed between H1 and H2 positions (structure III) as proposed earlier by [2]. The first two configurations maintain Pbnm symmetry, while the mirror plane is lost in the structure III. Due to the lower static lattice energy and entropy effect at finite temperature the protons in topaz-OH are presumably oriented according to the mixed configuration III. The thermal stability of this structure was confirmed by MD simulations at 50, 300 and 600K. The IR-spectra calculated from dynamic trajectories were found to be in excellent agreement with the spectroscopic measurements. At the temperatures above 300K a dynamic transfer of protons between H1 and H2 sites have been observed. This transition is recorded in high temperature Far-IR spectra of topaz. [1] Wunder B., Rubie D.C., Ross II C.R., Medenbach O., Seifert F., Schreyer W. (1993) Am
Dashnau, Jennifer L; Zelent, Bogumil; Vanderkooi, Jane M
2005-04-01
In order to correlate how the solvent affects emission properties of tryptophan, the fluorescence and phosphorescence emission spectra of tryptophan and indole model compounds were compared for solid sugar glass (trehalose/sucrose) matrix and glycerol/water solution and under the same conditions, these matrices were examined by infrared spectroscopy. Temperature was varied from 290 to 12 K. In sugar glass, the fluorescence and phosphorescence emission spectra are constant over this temperature range and the fluorescence remains red shifted; these results are consistent with the static interaction of OH groups with tryptophan in the sugar glass. In sugar glass containing water, the water retains mobility over the entire temperature range as indicated by the HOH infrared bending frequency. The fluorescence of tryptophan in glycerol/water shifts to the blue as temperature decreases and the frequency change of the absorption of the HOH bend mode is larger than in the sugar glass. These results suggest rearrangement of glycerol and water molecules over the entire temperature change. Shifts in the fluorescence emission maximum of indole and tryptophan were relatively larger than shifts for the phosphorescence emission-as expected for the relatively smaller excited triplet state dipole for tryptophan. The fluorescence emission of tryptophan in glycerol/water at low temperature has maxima at 312, 313, and 316 nm at pH 1.4, 7.0, and 10.6, respectively. The spectral shifts are interpreted to be an indication of a charge, or Stark phenomena, effect on the excited state molecule, as supported by ab initio calculations. To check whether the amino acid remains charged over the temperature range, the infrared spectrum of alanine was monitored over the entire range of temperature. The ratio of infrared absorption characteristic of carboxylate/carbonyl was constant in glycerol/water and sugar glass, which indicates that the charge was retained. Tryptophan buried in proteins, namely
NASA Astrophysics Data System (ADS)
Liang, T.; Raston, P.; Douberly, G. E.
2013-06-01
The HOOO hydridotrioxygen radical and its deuterated analog (DOOO) have been isolated in helium nanodroplets following the in-situ association reaction between OH and O_2. The infrared spectrum in the 3500-3700 cm^{-1} region reveals bands that are assigned to the ν_1 (OH stretch) fundamental and ν_1+ν_6 (OH stretch plus torsion) combination band of the trans-HOOO isomer. The helium droplet spectrum is assigned on the basis of a detailed comparison to the infrared spectrum of HOOO produced in the gas phase [E. L. Derro, T. D. Sechler, C. Murray, and M. I. Lester, J. Chem. Phys. 128, 244313 (2008)]. Despite the characteristic low temperature and rapid cooling of helium nanodroplets, there is no evidence for the formation of a weakly bound OH-O_2 van der Waals complex, which implies the absence of a kinetically significant barrier in the entrance channel of the reaction. There is also no spectroscopic evidence for the formation of cis-HOOO, which is predicted by theory to be nearly isoenergetic to the trans isomer. Stark spectroscopy of the trans-HOOO species provides vibrationally averaged dipole moment components that qualitatively disagree with predictions obtained from CCSD(T) computations at the equilibrium, planar geometry, indicating a floppy complex undergoing large-amplitude motion about the torsional coordinate. Under conditions that favor the introduction of multiple O_2 molecules to the droplets, bands associated with larger H/DOOO-(O_2)_n clusters are observed shifted 1-10 cm^{-1} to the red of the trans-H/DOOO ν_1 bands. Detailed ab initio calculations are carried out for multiple isomers of cis- and trans-HO_3-O_2, corresponding to either hydrogen or oxygen bonded van der Waals complexes. Comparisons to theory suggest that the structure of the HO_3-O_2 complex formed in helium droplets is a hydrogen-bonded ^4A' species consisting of a trans-HO_3 core. The computed binding energy of the complex is approximately 240 cm^{-1}. Despite the weak
Ribeiro, M.
2015-06-21
Ab initio calculations of hydrogen-passivated Si nanowires were performed using density functional theory within LDA-1/2, to account for the excited states properties. A range of diameters was calculated to draw conclusions about the ability of the method to correctly describe the main trends of bandgap, quantum confinement, and self-energy corrections versus the diameter of the nanowire. Bandgaps are predicted with excellent accuracy if compared with other theoretical results like GW, and with the experiment as well, but with a low computational cost.
Reeh, S; Kasprzak, M; Klusmann, C D; Stalf, F; Music, D; Ekholm, M; Abrikosov, I A; Schneider, J M
2013-06-19
The elastic properties of fcc Fe-Mn-X (X = Cr, Co, Ni, Cu) alloys with additions of up to 8 at.% X were studied by combinatorial thin film growth and characterization and by ab initio calculations using the disordered local moments (DLM) approach. The lattice parameter and Young's modulus values change only marginally with X. The calculations and experiments are in good agreement. We demonstrate that the elastic properties of transition metal alloyed Fe-Mn can be predicted by the DLM model.
Papasavva, S.; Tai, S.; Esslinger, A.; Illinger, K.H.; Kenny, J.E.
1995-03-16
We have investigated the feasibility of using ab initio molecular orbital methods for predicting the global warming potential of the proposed chlorofluorocarbon (CFC) substitute CF{sub 3}CH{sub 2}F, HFC-134a. Various levels of theory and basis sets were used to optimize geometry and calculate harmonic vibrational frequencies and infrared intensities for the molecule using the GAUSSIAN 92 software package. In attempting to assess the quality of the computations, we found it necessary to reconsider the vibrational assignments available in the literature. On the basis of the current assignment, we find that for the highest level calculation, MP2/6-31G{sup **}, the calculated harmonic frequencies agree extremely well with the experimentally observed ones at frequencies below 800 cm{sup {minus}1}, with a systematic error toward higher calculated frequencies becoming apparent above 800 cm{sup {minus}1}. At lower levels of theory, the systematic error is apparent at all frequencies. The regularity of the deviation between calculated and observed frequencies makes ab initio calculations of vibrational frequencies much more useful than semiempirical calculations, which tend to show random deviations, as demonstrated with a PM3-UHF calculation in this work. The calculated absolute intensities are in good agreement with the limited experimental measurements previously reported. 23 refs., 3 figs., 5 tabs.
Ab-Initio Based Computation of Rate Constants for Spin Forbidden Metalloprotein-Substrate Reactions
NASA Astrophysics Data System (ADS)
Ozkanlar, Abdullah; Rodriguez, Jorge H.
2007-03-01
Some chemical and biochemical reactions are non-adiabatic processes whereby the total spin angular momentum, before and after the reaction, is not conserved. These are named spin- forbidden reactions. The application of ab-initio methods, such as spin density functional theory (SDFT), to the prediction of rate constants is a challenging task of fundamental and practical importance. We apply non-adiabatic transition state theory (NA-TST) in conjuntion with SDFT to predict the rate constant of the spin- forbidden recombination of carbon monoxide with iron tetracarbonyl. To model the surface hopping probability between singlet and triplet states, the Landau-Zener formalism is used. The lowest energy point for singlet-triplet crossing, known as minimum energy crossing point (MECP), was located and used to compute, in a semi-quantum approach, reaction rate constants at 300 K. The predicted rates are in very good agreement with experiment. In addition, we present results for the spin- forbidden ligand binding reactions of iron-containing heme proteins such as myoglobin.
Optimized energy landscape exploration using the ab initio based activation-relaxation technique
NASA Astrophysics Data System (ADS)
Machado-Charry, Eduardo; Béland, Laurent Karim; Caliste, Damien; Genovese, Luigi; Deutsch, Thierry; Mousseau, Normand; Pochet, Pascal
2011-07-01
Unbiased open-ended methods for finding transition states are powerful tools to understand diffusion and relaxation mechanisms associated with defect diffusion, growth processes, and catalysis. They have been little used, however, in conjunction with ab initio packages as these algorithms demanded large computational effort to generate even a single event. Here, we revisit the activation-relaxation technique (ART nouveau) and introduce a two-step convergence to the saddle point, combining the previously used Lanczós algorithm with the direct inversion in interactive subspace scheme. This combination makes it possible to generate events (from an initial minimum through a saddle point up to a final minimum) in a systematic fashion with a net 300-700 force evaluations per successful event. ART nouveau is coupled with BigDFT, a Kohn-Sham density functional theory (DFT) electronic structure code using a wavelet basis set with excellent efficiency on parallel computation, and applied to study the potential energy surface of C20 clusters, vacancy diffusion in bulk silicon, and reconstruction of the 4H-SiC surface.
NASA Astrophysics Data System (ADS)
Dash, S.; Joshi, N.; Drera, G.; Ghosh, P.; Magnano, E.; Bondino, F.; Galinetto, P.; Mozzati, M. C.; Salvinelli, G.; Aguekian, V.; Sangaletti, L.
2016-03-01
The electronic properties of the Mn:GaSe interface, produced by evaporating Mn at room temperature on a ɛ -GaSe(0001) single-crystal surface, have been studied by soft x-ray spectroscopies, and the experimental results are discussed at the light of ab initio DFT+U calculations of a model Ga1 -xMnxSe (x =0.055 ) surface alloy. Consistently with these calculations that also predict a high magnetic moment for the Mn ions (4.73 -4.83 μB), XAS measurements at the Mn L edge indicate that Mn diffuses into the lattice as a Mn2 + cation with negligible crystal-field effects. Ab initio calculations also show that the most energetically favorable lattice sites for Mn diffusion are those where Mn substitutes Ga cations in the Ga layers of the topmost Se-Ga-Ga-Se sandwich. Mn s and p states are found to strongly hybridize with Se and Ga p states, while weaker hybridization is predicted for Mn d states with Se s and p orbitals. Furthermore, unlike other Mn-doped semiconductors, there is strong interaction between the Ga -s and Mn -dz2 states. The effects of hybridization of Mn 3 d electrons with neighboring atoms are still clearly detectable from the characteristic charge-transfer satellites observed in the photoemission spectra. The Mn 3 d spectral weight in the valence band is probed by resonant photoemission spectroscopy at the Mn L edge, which also allowed an estimation of the charge transfer (Δ =2.95 eV) and Mott-Hubbard (U =6.4 eV) energies on the basis of impurity-cluster configuration-interaction model of the photoemission process. The Mott-Hubbard correlation energy U is consistent with the Ueff on-site Coulomb repulsion parameter (5.84 eV) determined for the ab initio calculations.
NASA Astrophysics Data System (ADS)
Aarset, Kirsten; Hagen, Kolbjørn; Stølevik, Reidar
2001-06-01
The structure and conformational composition of 2-chlorobutane and 2-bromobutane have been studied by gas-phase electron diffraction (GED) at 25°C, together with ab initio molecular orbital calculations (HF/6-311+G(d,p)). These molecules may exist as three distinguishable conformers (G+, A, and G-). The symbols refer to anti (A) with a torsion angle Φ2(X 8-C 2-C 3-C 4) of about 180° and gauche (G+ and G-) with torsion angles Φ2(X 8-C 2-C 3-C 4) of about +60° and 300°(-60°), respectively. It was not possible; from our GED-data alone, to accurately determine the conformational composition because the distance distributions for two of the conformers (G+ and G-) are very similar. The conformational composition for 2-chlorobutane obtained from the ab initio calculations (G+ 62%, A 25% G- 13%) was found to fit the experimental data quite well. For 2-bromobutane the ab initio calculated conformational composition (G+ 58%, A 28% G- 14%) did not, however, fit the experimental data. Here a much better fit was obtained by using only 10% of the A conformer and using the relative energy for the two gauche conformers, as obtained in the ab initio molecular orbital calculations, to calculate the relative amounts of the two gauche forms (G+ 73%, A 10% G- 17%). The results for the principal distances ( rg) and angles ∠ α for the G+ conformer of 2-chlorobutane, with estimated 2 σ uncertainties, obtained from the combined GED/ab initio study are: r( C1- C2)=1.524(3) Å, r( C2- C3)=1.528(3) Å, r( C3- C4)=1.539(3) Å, r( C- Cl)=1.812(3) Å, r( C- H) ave=1.098(4) Å, ∠C 1C 2C 3=111.5(16)°, ∠C 2C 3C 4=113.3(5)°, ∠C 1C 2C1=110.4(9)°. The results for the G+ conformer of 2-bromobutane are: r( C1- C2)=1.526(4) Å, r( C2- C3)=1.530(4) Å, r( C3- C4)=1.540(4) Å, r( C- Br)=1.982(5) Å, r( C- H) ave=1.111(8) Å, ∠C 1C 2C 3=112.5(16)°, ∠C 2C 3C 4=114.6(15)°, ∠C 1C 2Br=110.1(16)°. Only average values for r(C-C), r(C-H), ∠CCC, and ∠CCH could be determined in the
Laasonen, Kari
2013-01-01
In this chapter, an introduction to ab initio molecular dynamics (AIMD) has been given. Many of the basic concepts, like the Hellman-Feynman forces, the difference between the Car-Parrinello molecular dynamics and AIMD, have been explained. Also a very versatile AIMD code, the CP2K, has been introduced. On the application, the emphasis was on the aqueous systems and chemical reactions. The biochemical applications have not been discussed in depth.
NASA Astrophysics Data System (ADS)
Abe, Akihiro; Furuya, Hidemine; Ichimura, Noriko; Kawauchi, Susumu
1997-02-01
The gas-phase NMR analysis of 5-methoxy-1,3-dioxanes was carried out. The conformational energies estimated from the observed coupling constant data were compared with the results of ab initio MO calculations using d95 + (2df,p) basis set at the MP2 level. While the energy difference between the axial-out and equatorial-out forms was in a reasonable agreement, the 1,5 interaction energy between the methoxy methyl and the ring oxygens was not in accord.
Hua -Gen Yu; Han, Huixian; Guo, Hua
2016-03-29
Vibrational energy levels of the ammonium cation (NH4+) and its deuterated isotopomers are calculated using a numerically exact kinetic energy operator on a recently developed nine-dimensional permutation invariant semiglobal potential energy surface fitted to a large number of high-level ab initio points. Like CH4, the vibrational levels of NH4+ and ND4+ exhibit a polyad structure, characterized by a collective quantum number P = 2(v1 + v3) + v2 + v4. As a result, the low-lying vibrational levels of all isotopomers are assigned and the agreement with available experimental data is better than 1 cm–1.
Yu, Hua-Gen; Han, Huixian; Guo, Hua
2016-04-14
Vibrational energy levels of the ammonium cation (NH4(+)) and its deuterated isotopomers are calculated using a numerically exact kinetic energy operator on a recently developed nine-dimensional permutation invariant semiglobal potential energy surface fitted to a large number of high-level ab initio points. Like CH4, the vibrational levels of NH4(+) and ND4(+) exhibit a polyad structure, characterized by a collective quantum number P = 2(v1 + v3) + v2 + v4. The low-lying vibrational levels of all isotopomers are assigned and the agreement with available experimental data is better than 1 cm(-1).
Ab initio calculations of the 2p3/2-2p1/2 fine-structure splitting in boronlike ions
NASA Astrophysics Data System (ADS)
Artemyev, A. N.; Shabaev, V. M.; Tupitsyn, I. I.; Plunien, G.; Surzhykov, A.; Fritzsche, S.
2013-09-01
We have performed ab initio QED calculations of the (1s)2(2s)22p3/2-(1s)2(2s)22p1/2 fine-structure splitting along the boron isoelectronic sequence for all ions with 17≤Z≤100. This level splitting was evaluated within the extended Furry picture and by making use of four different screening potentials in order to estimate the effects of interelectronic correlations. The accuracy of the predicted transition energies has been improved significantly when compared with previous computations.
Selvam, Parthiban; Peguin, Robson P S; Chokshi, Udayan; da Rocha, Sandro R P
2006-10-10
In situ high-pressure tensiometry and ab initio calculations were used to rationally design surfactants for the 1,1,1,2-tetrafluoroethane-water (HFA134a|W) interface. Nonbonded pair interaction (binding) energies (E(b)) of the complexes between HFA134a and candidate surfactant tails were used to quantify the HFA-philicity of selected moieties. The interaction between HFA134a and an ether-based tail was shown to be predominantly electrostatic in nature and much more favorable than that between HFA134a and a methyl-based fragment. The interfacial activity of (i) amphiphiles typically found in FDA-approved pressurized metered-dose inhaler (pMDI) formulations, (ii) a series of nonionic surfactants with methylene-based tails, and (iii) a series of nonionic surfactants with ether-based tails was investigated at the HFA134a|W interface using in situ tensiometry. This is the first time that the tension of the surfactant-modified HFA134a|W interface has been reported in the literature. The ether-based surfactants were shown to be very interfacially active, with tension decreasing by as much as 27 mN.m(-)(1). However, the methyl-based surfactants, including those from FDA-approved formulations, did not exhibit high activity at the HFA134a|W interface. These results are in direct agreement with the E(b) calculations. Significant differences in interfacial activity are noted for surfactants at the 2H,3H-perfluoropentane (HPFP)|water and HFA134a|W interfaces. Care should be taken, therefore, when results from the mimicking solvent (HPFP) are extrapolated to HFA134a-based systems. The results shown here are of relevance in the selection of surfactants capable of forming and stabilizing reverse aqueous aggregates in HFA-based pMDIs, which are promising formulations for the systemic delivery of biomolecules to and through the lungs.
Towards Accurate Ab Initio Predictions of the Spectrum of Methane
NASA Technical Reports Server (NTRS)
Schwenke, David W.; Kwak, Dochan (Technical Monitor)
2001-01-01
We have carried out extensive ab initio calculations of the electronic structure of methane, and these results are used to compute vibrational energy levels. We include basis set extrapolations, core-valence correlation, relativistic effects, and Born- Oppenheimer breakdown terms in our calculations. Our ab initio predictions of the lowest lying levels are superb.
NASA Astrophysics Data System (ADS)
Abadias, G.; Kanoun, M. B.; Goumri-Said, S.; Koutsokeras, L.; Dub, S. N.; Djemia, Ph.
2014-10-01
The structure, phase stability, and mechanical properties of ternary alloys of the Zr-Ta-N system are investigated by combining thin-film growth and ab initio calculations. Zr1-xTaxN films with 0≤x≤1 were deposited by reactive magnetron cosputtering in Ar +N2 plasma discharge and their structural properties characterized by x-ray diffraction. We considered both ordered and disordered alloys, using supercells and special quasirandom structure approaches, to account for different possible metal atom distributions on the cation sublattice. Density functional theory within the generalized gradient approximation was employed to calculate the electronic structure as well as predict the evolution of the lattice parameter and key mechanical properties, including single-crystal elastic constants and polycrystalline elastic moduli, of ternary Zr1-xTaxN compounds with cubic rocksalt structure. These calculated values are compared with experimental data from thin-film measurements using Brillouin light scattering and nanoindentation tests. We also study the validity of Vegard's empirical rule and the effect of growth-dependent stresses on the lattice parameter. The thermal stability of these Zr1-xTaxN films is also studied, based on their structural and mechanical response upon vacuum annealing at 850 °C for 3 h. Our findings demonstrate that Zr1-xTaxN alloys with Ta fraction 0.51⩽x⩽0.78 exhibit enhanced toughness, while retaining high hardness ˜30 GPa, as a result of increased valence electron concentration and phase stability tuning. Calculations performed for disordered or ordered structures both lead to the same conclusion regarding the mechanical behavior of these nitride alloys, in agreement with recent literature findings [H. Kindlund, D. G. Sangiovanni, L. Martinez-de-Olcoz, J. Lu, J. Jensen, J. Birch, I. Petrov, J. E. Greene, V. Chirita, and L. Hultman, APL Materials 1, 042104 (2013), 10.1063/1.4822440].
NASA Astrophysics Data System (ADS)
Zhu, He; Jha, Kshitij C.; Bhatta, Ram S.; Tsige, Mesfin; Dhinojwala, Ali
2015-03-01
The chemical composition and molecular structure of polymeric surfaces are important in understanding wetting, adhesion, and friction. Here, we combine interface-sensitive sum frequency generation spectroscopy (SFG), all-atom molecular dynamics (MD) simulations, and ab initio calculations to understand the composition and the orientation of chemical groups on poly(methyl methacrylate) (PMMA) surface as a function of tacticity and temperature. The SFG spectral features for isotactic and syndiotactic PMMA surfaces are similar and the dominant peak in the spectra corresponds to the ester-methyl groups. The SFG spectra for solid and melt states are very similar for both syndiotactic and isotactic PMMA. In comparison, the MD simulation results show that both the ester-methyl and the α-methyl groups of syndiotactic-PMMA are ordered and tilted towards the surface normal. For the isotactic-PMMA, the α-methyl groups are less ordered compared to their ester-methyl groups. The backbone methylene groups have a broad angular distribution and on average tilt along the surface plane, independent of tacticity and temperature. We have compared the SFG results with theoretical spectra calculated using MD simulations and ab initio calculations. National Science Foundation.
Wang, Xue B.; Woo, Hin-koon; Wang, Lai S.; Minofar, Babak; Jungwirth, Pavel
2006-04-20
The electronic structure and electron affinity of the acetyloxyl radical (CH3COO) were investigated by low-temperature anion photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of the acetate anion (CH3COO-) were obtained at two photon energies (355 and 266 nm) and under three different temperatures (300, 70, and 20 K) using a new low temperature ion-trap photoelectron spectroscopy apparatus. In contrast to a featureless spectrum at 300 K, a well-resolved vibrational progression corresponding to the OCO bending mode was observed at low temperatures in the 355 nm spectrum, yielding an accurate electron affinity for the acetyloxyl radical as 3.250 + 0.010 eV. This experimental result is supported by ab initio calculations, which also indicate three low-lying electronic states observed in the 266 nm spectrum. The calculations suggest a 19° decrease of the OCO angle upon detaching an electron from acetate, consistent with the vibrational progression observed experimentally.
Ruiz, Camilo; Plaja, Luis; Roso, Luis; Becker, Andreas
2006-02-10
We present ab initio computations of the ionization of two-electron atoms by short pulses of intense linearly polarized Ti:sapphire laser radiation beyond the one-dimensional approximation. In the model the electron correlation is included in its full dimensionality, while the center-of-mass motion is restricted along the polarization axis. Our results exhibit a rich double ionization quantum dynamics in the direction transversal to the field polarization, which is neglected in the previous models based on the one-dimensional approximation.
Vadali, Ramkumar V; Shi, Yan; Kumar, Sameer; Kale, Laxmikant V; Tuckerman, Mark E; Martyna, Glenn J
2004-12-01
Many systems of great importance in material science, chemistry, solid-state physics, and biophysics require forces generated from an electronic structure calculation, as opposed to an empirically derived force law to describe their properties adequately. The use of such forces as input to Newton's equations of motion forms the basis of the ab initio molecular dynamics method, which is able to treat the dynamics of chemical bond-breaking and -forming events. However, a very large number of electronic structure calculations must be performed to compute an ab initio molecular dynamics trajectory, making the efficiency as well as the accuracy of the electronic structure representation critical issues. One efficient and accurate electronic structure method is the generalized gradient approximation to the Kohn-Sham density functional theory implemented using a plane-wave basis set and atomic pseudopotentials. The marriage of the gradient-corrected density functional approach with molecular dynamics, as pioneered by Car and Parrinello (R. Car and M. Parrinello, Phys Rev Lett 1985, 55, 2471), has been demonstrated to be capable of elucidating the atomic scale structure and dynamics underlying many complex systems at finite temperature. However, despite the relative efficiency of this approach, it has not been possible to obtain parallel scaling of the technique beyond several hundred processors on moderately sized systems using standard approaches. Consequently, the time scales that can be accessed and the degree of phase space sampling are severely limited. To take advantage of next generation computer platforms with thousands of processors such as IBM's BlueGene, a novel scalable parallelization strategy for Car-Parrinello molecular dynamics is developed using the concept of processor virtualization as embodied by the Charm++ parallel programming system. Charm++ allows the diverse elements of a Car-Parrinello molecular dynamics calculation to be interleaved with low
NASA Astrophysics Data System (ADS)
Dahlqvist, Martti; Hotokka, Matti; Räsänen, Markku
1998-04-01
The infrared spectra of monomeric pinacol molecules (2,3-dimethyl-2,3-butanediol; (CH 3) 2C(OH)C(OH)(CH 3) 2) have been recorded in the gas phase and dilute nonpolar solutions, and in an argon matrix. The vibrational data are consistent with the intramolecularly hydrogen-bonded G-type (gauche with respect to the central C-C bond) conformers and there is no evidence for the T-type (trans with respect to the central C-C bond) conformers, which have been observed in the condensed phases. This was confirmed by studying the infrared region 835-815 cm -1, which was found to be the most indicative to show spectral changes within the type of the conformers. In this region the band of the T-type conformers (assigned to the hybridized asymmetric vibration of the central CC and CO stretching modes) disappears when going from the condensed phases to phases, where pinacol molecules are monomeric. Ab initio HF/6-311G** (MP2/6-311G**) calculations support the experimental findings; the calculated relative energies for the tGg', gGg', g'Gg', tTt, and gTg' conformers are 0.0 (0.0), 3.4 (3.4), 5.1 (5.9), 7.9 (11.3), and 12.0 (14.0) kJ mol -1, respectively. Consequently, only the G-type conformers are sufficiently populated to give rise to observable spectral lines. Both experimental findings and theoretical calculations demonstrated that the bands in the argon matrix spectrum of pinacol are due to the most stable tGg' conformer. Although the ab initio calculations predict that also the gGg' and g'Gg' conformers are present in the gas phase and in dilute nonpolar solutions their existence could not be confirmed experimentally. Hence, we conclude that the conformation sensitive bands may coincide in the spectra. The HF/6-311G** ab initio calculations for vibrational frequencies of pinacol are consistent with this conclusion, suggesting only small differences between the wavenumbers of the G-type conformers. Pinacol does not show infrared-induced photorotamerization in the low
NASA Astrophysics Data System (ADS)
Bittner, Dror M.; Zaleski, Daniel P.; Stephens, Susanna L.; Tew, David P.; Walker, Nicholas R.; Legon, Anthony C.
2015-04-01
The H3N⋯CuCl monomer has been generated and isolated in the gas phase through laser vaporisation of a copper sample in the presence of low concentrations of NH3 and CCl4 in argon. The resulting complex cools to a rotational temperature approaching 2 K during supersonic expansion of the gas sample and is characterised by broadband rotational spectroscopy between 7 and 18.5 GHz. The spectra of six isotopologues are measured and analysed to determine rotational, B0; centrifugal distortion, DJ, DJK; and nuclear quadrupole coupling constants of Cu, Cl, and 14N nuclei, χaa (X). The geometry of the complex is C3v with the N, Cu, and Cl atoms located on the a inertial axis. Bond distances and the ∠(H —N⋯Cu) bond angle within the complex are precisely evaluated through fitting of geometrical parameters to the experimentally determined moments of inertia and through ab initio calculations at the CCSD(T)(F12*)/AVQZ level. The r(Cu —Cl), r(Cu —N), and ∠(H —N⋯Cu) parameters are, respectively, evaluated to be 2.0614(7) Å, 1.9182(13) Å, and 111.40(6)° in the r0 geometry, in good agreement with the ab initio calculations. Geometrical parameters evaluated for the isolated complex are compared with those established crystallographically for a solid-state sample of [Cu(NH3)Cl].
Mattioli, Giuseppe; Zaharieva, Ivelina; Dau, Holger; Guidoni, Leonardo
2015-08-19
Amorphous transition-metal (hydr)oxides are considered as the most promising catalysts that promote the oxidation of water to molecular oxygen, protons, and "energized" electrons, and, in turn, as fundamental parts of "artificial leaves" that can be exploited for large scale generation of chemical fuels (e.g., hydrogen) directly from sunlight. We present here a joint theoretical-experimental investigation of electrodeposited amorphous manganese oxides with different catalytic activities toward water oxidation (MnCats). Combining the information content of X-ray absorption fine structure (XAFS) measurements with the predictive power of ab initio calculations based on density functional theory, we have been able to identify the essential structural and electronic properties of MnCats. We have elucidated (i) the localization and structural connection of Mn(II), Mn(III), and Mn(IV) ions in such amorphous oxides and (ii) the distribution of protons at the MnCat/water interface. Our calculations result in realistic 3D models of the MnCat atomistic texture, formed by the interconnection of small planar Mn-oxo sheets cross-linked through different kinds of defective Mn atoms, isolated or arranged in closed cubane-like units. Essential for the catalytic activity is the presence of undercoordinated Mn(III)O5 units located at the boundary of the amorphous network, where they are ready to act as hole traps that trigger the oxidation of neighboring water molecules when the catalyst is exposed to an external positive potential. The present validation of a sound 3D model of MnCat improves the accuracy of XAFS fits and opens the way for the development of mechanistic schemes of its functioning beyond a speculative level.
Papatheodorou, G.N.; Curtiss, L.A.; Maroni, V.A.
1983-03-15
Raman spectra of gaseous NH/sub 3/:AlCl/sub 3/ and NH/sub 3/:AlBr/sub 3/ were recorded at 400 /sup 0/C. The observed Raman frequencies in combination with reported infrared frequencies for NH/sub 3/:AlCl/sub 3/ and ab initio molecular orbital calculations on NH/sub 3/:AlCl/sub 3/ and NH/sub 3/:AlF/sub 3/ were used to derive force constants for all three NH/sub 3/:AlX/sub 3/ complexes (X = F, Cl, Br) based on an adjusted valence force field (AVFF) concept. The resulting force constant calculations produced complete sets of A/sub 1/ and E mode frequencies for each complex. Statistical mechanical analyses were then performed using the A/sub 1/ and E mode frequencies together with estimated values for the torsional mode of each complex and published enthalpy data. From these analyses, the relative thermodynamic stability of each complex was determined. At 700 and 1000 K, NH/sub 3/:AlCl/sub 3/ was found to be more stable than NH/sub 3/:AlBr/sub 3/. It was further predicted that the reaction of gaseous NH/sub 3/ with solid AlF/sub 3/ to form NH/sub 3/:AlF/sub 3/ is not favored in this temperature range, which provides an explanation for the lack of success in prior efforts to produce NH/sub 3/:AlF/sub 3/.
Mattioli, Giuseppe; Zaharieva, Ivelina; Dau, Holger; Guidoni, Leonardo
2015-08-19
Amorphous transition-metal (hydr)oxides are considered as the most promising catalysts that promote the oxidation of water to molecular oxygen, protons, and "energized" electrons, and, in turn, as fundamental parts of "artificial leaves" that can be exploited for large scale generation of chemical fuels (e.g., hydrogen) directly from sunlight. We present here a joint theoretical-experimental investigation of electrodeposited amorphous manganese oxides with different catalytic activities toward water oxidation (MnCats). Combining the information content of X-ray absorption fine structure (XAFS) measurements with the predictive power of ab initio calculations based on density functional theory, we have been able to identify the essential structural and electronic properties of MnCats. We have elucidated (i) the localization and structural connection of Mn(II), Mn(III), and Mn(IV) ions in such amorphous oxides and (ii) the distribution of protons at the MnCat/water interface. Our calculations result in realistic 3D models of the MnCat atomistic texture, formed by the interconnection of small planar Mn-oxo sheets cross-linked through different kinds of defective Mn atoms, isolated or arranged in closed cubane-like units. Essential for the catalytic activity is the presence of undercoordinated Mn(III)O5 units located at the boundary of the amorphous network, where they are ready to act as hole traps that trigger the oxidation of neighboring water molecules when the catalyst is exposed to an external positive potential. The present validation of a sound 3D model of MnCat improves the accuracy of XAFS fits and opens the way for the development of mechanistic schemes of its functioning beyond a speculative level. PMID:26226190
NASA Astrophysics Data System (ADS)
Zarhri, Z.; Ziat, Y.; El Rhazouani, O.; Benyoussef, A.; Elkenz, A.
2016-07-01
The ab-initio calculations based on the Korringa Kohn Rostoker approximation approach combined with coherent potential approximation (KKR-CPA), were used to study the magnetic properties of the titanium anti-site (TiO) and chromium (Cr) doped TiO2. In the considered systems, we used different concentrations for TiO defect and Cr doping. In TiO2(0.98)(TiO)0.02, the obtained results indicate that TiO is a donor having half-metal behavior. TiO[3d] band is located at the Fermi level, although isn't 100% polarized, the ferromagnetic (FM) state is verified as being more stable than disordered local moment (DLM) state. For Ti0.98Cr0.02O2 the Cr doping introduced new states which give the material half-metallic feature. The majority spin of Cr impurities are located at the Fermi level and the conduction electrons around the Fermi level are 100% spin polarized. This indicates the stability of (FM) state. Moreover, in Ti0.98Cr0.02O2(0.98)(TiO)0.02, the top of the valence band is shifted to lower energy compared to pure TiO2, and the n-type of TiO2 is verified. The majority spin of Cr[3d] are located at 0.025 Ry close to the Fermi level. The predicted Curie temperatures (Tc) were calculated using the mean field approximation (MFA) and we predicted that TiO defect in Cr doped TiO2 makes Tc higher. This kind of defect makes the material useful for spinotronics's applications and devices.
NASA Astrophysics Data System (ADS)
Muñoz, Emiliano L.; Mercurio, Marcio E.; Cordeiro, Moacir R.; Pereira, Luciano F. D.; Carbonari, Artur W.; Rentería, Mario
2012-08-01
In this work, we present results of Time-Differential γ-γ Perturbed-Angular-Correlations (PAC) experiments performed in 111Cd-doped ZnO semiconductor. The PAC technique has been applied in order to characterize the electric-field-gradient (EFG) tensor at (111In (EC)→) 111Cd nuclei located, as was later demonstrated, at defect-free cation sites of the ZnO host structure. The PAC experiments were performed in the temperature range of 77-1075 K. At first glance, the unexpected presence of low-intensity dynamic hyperfine interactions was observed, which were analyzed with a perturbation factor based on the Bäverstam and Othaz model. The experimental EFG results were compared with ab initio calculations performed with the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) method, in the framework of the Density Functional Theory (DFT), using the Wien2K code. The presence of the dynamic hyperfine interactions has been analyzed enlightened by the FP-APW+lo calculations of the EFG performed as a function of the charge state of the cell. We could correlate the large strength of the dynamic hyperfine interaction with the strong variation of the EFG due to changes in the electronic charge distribution in the Cd vicinity during the time-window of the PAC measurement. It was also revealed that the Cd impurity decays to a final stable neutral charge state (Cd2+) fast enough (in few ns) to produce the nearly undamped observed PAC spectra.
Durig, James R; Zheng, Chao; Williams, Michael J; Stidham, Howard D; Guirgis, Gamil A
2004-06-01
The infrared spectra (3200-400 cm(-1)) of krypton solutions of 1,3-difluoropropane, FCH2CH2CH2F, at variable temperatures (-105 to -150 degrees C) have been recorded. Additionally, the infrared spectra (3200-50 cm(-1)) of the gas and solid have been recorded as well as the Raman spectrum of the liquid. From a comparison of the spectra of the fluid phases with that in the solid, all of the fundamental vibrations of the C2 conformer (gauche-gauche) where the first gauche indicates the form for one of the CH2F groups and the second gauche the other CH2F, and many of those for the C1 form (trans-gauche) have been identified. Tentative assignments have been made for a few of the fundamentals of the other two conformers, i.e. C2v (trans-trans) and Cs (gauche-gauche'). By utilizing six pairs of fundamentals for these two conformers in the krypton solutions, an enthalpy difference of 277 +/- 28 cm(-1) (3.31 +/- 0.33 kJ mol(-1)) has been obtained for the C2 versus C1 conformer with the C2 conformer the more stable form. For the C2v conformer, the enthalpy difference has been determined to be 716 +/- 72 cm(-1) (8.57 +/- 0.86 kJ mol(-1)) and for the Cs form 971 +/- 115 cm(-1) (11.6 +/- 1.4 kJ mol(-1)). It is estimated that there is 64 +/- 3% of the C2 form, 34 +/-3% of the C1 form, 1% of the C2v form and 0.6% of the Cs conformer present at ambient temperature. Equilibrium geometries and total energies of the four stable conformers have been determined from ab initio calculations with full electron correlation by the perturbation method to second order as well as by hybrid density functional theory calculations with the B3LYP method using a number of basis sets. The MP2 calculations predict the C1 conformer stability to be slightly higher than the experimentally determined value whereas for the C2v and Cs conformers the predicted energy difference is much larger than the experimental value. The B3LYP calculations predict a better energy difference for both the C1 and C2v as
Sládek, Vladimír; Lukeš, Vladimír; Ilčin, Michal; Biskupič, Stanislav
2012-03-15
The ground state ab initio CCSD(T) potential curves using various basis sets (aug-cc-pVXZ-PP (X = D, T, Q, 5)) is obtained for the dimers of helium with IIb group metals. The effect of the position of the (mid) bond-functions on the interaction energy is discussed. A Symmetry Adapted Perturbation Theory decomposition of the interaction energy is provided and the trends in the dimer stabilizing and destabilizing contributions are depicted. The spline fitted potential curves are applied together with rigorous statistical formulae in order to obtain the transport coefficients (viscosity coefficients, diffusion coefficients) and the second virial coefficient both for pure constituents and mixtures. The obtained theoretical results are compared with available experimental data. Molecular dynamics is used to obtain reliable values of the diffusion coefficients for all the systems under study.
Half-metallic behaviour in doped TiO2 (rutile) with double impurities: ab initio calculation
NASA Astrophysics Data System (ADS)
Fakhim Lamrani, A.; Belaiche, M.; Benyoussef, A.; El Kenz, A.
2013-12-01
Dilute magnetic oxides are without doubt among the most interesting classes of magnetic materials. However, the nature of their electronic structure and magnetic exchange is far from understood. Here, we apply the ab initio augmented spherical wave (ASW) method, with corrected generalised gradient approximation to study the electronic structure and magnetic properties of doped TiO2 rutile with double impurities. The study reveals a half-metallic ferromagnetic behaviour for Ti1-2x Cr x Mo x O2, and the local magnetic moments of the impurities and their oxidation states agree with the charge transfer between Cr and Mo, which would lead to the ferromagnetic state through the double-exchange mechanism in transition metal oxides.
NASA Astrophysics Data System (ADS)
Li, Yaping; Liu, Zhimin; Jentoft, Friederike; Wang, Sanwu
2015-03-01
Biomass is an important renewable energy resource. Cresol is one of components in crude bio-oil generated from biomass, and hydrogenation of cresol is often involved in the upgrading process. We studied catalytic hydrogenation of cresol on the Pt(111) surface with and without the presence of water. In particular, we used first-principles density-functional theory and ab initio molecular dynamics simulations to obtain adsorption geometries, binding energies, reaction energies, activation energies, and reaction pathways for hydrogenation of cresol with possible products of 2-methylcyclohexanone and 2-methylcyclohexanol. Our theoretical results are used to explain the available experimental measurements, which show a strong influence of water. Supported by DOE (DE-SC0004600). This research used the supercomputer resources at NERSC, of XSEDE, at TACC and at the Tandy Supercomputing Center.
Fit Point-Wise AB Initio Calculation Potential Energies to a Multi-Dimension Long-Range Model
NASA Astrophysics Data System (ADS)
Zhai, Yu; Li, Hui; Le Roy, Robert J.
2016-06-01
A potential energy surface (PES) is a fundamental tool and source of understanding for theoretical spectroscopy and for dynamical simulations. Making correct assignments for high-resolution rovibrational spectra of floppy polyatomic and van der Waals molecules often relies heavily on predictions generated from a high quality ab initio potential energy surface. Moreover, having an effective analytic model to represent such surfaces can be as important as the ab initio results themselves. For the one-dimensional potentials of diatomic molecules, the most successful such model to date is arguably the ``Morse/Long-Range'' (MLR) function developed by R. J. Le Roy and coworkers. It is very flexible, is everywhere differentiable to all orders. It incorporates correct predicted long-range behaviour, extrapolates sensibly at both large and small distances, and two of its defining parameters are always the physically meaningful well depth {D}_e and equilibrium distance r_e. Extensions of this model, called the Multi-Dimension Morse/Long-Range (MD-MLR) function, linear molecule-linear molecule systems and atom-non-linear molecule system. have been applied successfully to atom-plus-linear molecule, linear molecule-linear molecule and atom-non-linear molecule systems. However, there are several technical challenges faced in modelling the interactions of general molecule-molecule systems, such as the absence of radial minima for some relative alignments, difficulties in fitting short-range potential energies, and challenges in determining relative-orientation dependent long-range coefficients. This talk will illustrate some of these challenges and describe our ongoing work in addressing them. Mol. Phys. 105, 663 (2007); J. Chem. Phys. 131, 204309 (2009); Mol. Phys. 109, 435 (2011). Phys. Chem. Chem. Phys. 10, 4128 (2008); J. Chem. Phys. 130, 144305 (2009) J. Chem. Phys. 132, 214309 (2010) J. Chem. Phys. 140, 214309 (2010)
Kolar, J. E-mail: sny@iceht.forth.gr; Strizik, L.; Kohoutek, T.; Wagner, T.; Voyiatzis, G. A.; Chrissanthopoulos, A.; Yannopoulos, S. N. E-mail: sny@iceht.forth.gr
2013-11-28
Photostructural changes—the hallmark of non-crystalline chalcogenides—are in essence the basis of a number of photoinduced effects, i.e., changes of their physical properties, which are exploited in a variety of applications, especially in photonics and optoelectronics. Despite the vast number of investigations of photostructural changes, there is currently lack of systematic studies on how the thermal history, which affects glass structure, modifies the extent of photostructural changes. In this article, we study the role of thermal history on photostructural changes in glassy As{sub 15}S{sub 85}. This particular sulfur-rich composition has been chosen based on the colossal photostructural response it exhibits under near-band gap light irradiation, which inherently originates from its nanoscale phase-separated nature. To control the thermal history, the glass was quenched to various temperatures and each of these quenched products was annealed under four different conditions. Off-resonant Raman scattering was used to study the equilibrium study of each product. Structural changes of interest involve changes of the sulfur atoms participating into S{sub 8} rings and S{sub n} chains. Their ratio was found to depend on quenching/annealing conditions. Near-band gap light was used to perturb the rings-to-chain ratio and at the same time to record these changes through Raman scattering, revealing an intricate behavior of photostructural changes. Ab initio calculations were employed to determine the stability of various sulfur clusters/molecules thus aiding the correlation of the particular photo-response of glassy As{sub 15}S{sub 85} with its structural constituents.
Petrenko, Y M
2015-01-01
Ab initio quantum mechanics studies for the detection of structure and dipole structure peculiarities of Hoogsteen base pairs relative to Watson-Crick base pairs, were performed during our work. These base pairs are formed as a result of complementary interactions. It was revealed, that adenine-thymine Hoogsteen base pair and adenine-thymine Watson-Crick base pairs can be formed depending on initial configuration. Cytosine-guanine Hoogsteen pairs are formed only when cytosine was originally protonated. Both types of Hoogsteen pairs have noticeable difference in the bond distances and angles. These differences appeared in purine as well as in pyrimidine parts of the pairs. Hoogsteen pairs have mostly shorter hydrogen bond lengths and significantly larger angles of hydrogen bonds and larger angles between the hydrogen bonds than Watson-Crick base pairs. Notable differences are also observed with respect to charge distribution and dipole moment. Quantitative data on these differences are shown in our work. It is also reported that the values of local parameters (according to Cambridge classification of the parameters which determine DNA properties) in Hoogsteen base pairs, are greatly different from Watson-Crick ones.
NASA Astrophysics Data System (ADS)
Scanlon, Kerin; Eades, Robert A.; Dixon, David A.
Certain first and second derivatives of the dipole moment function for SF 6 have been calculated from ab initio molecular orbital theory in the finite difference approximation. All calculations were carried out in normal coordinates with an experimental force field and geometry. A double-zeta basis with d-functions on the sulfur was employed. Calculated harmonic frequencies in cm -1 are 1084 (ω 3) and (648(ω 4) and calculated intensities in km/mole are 1612 (ν 3) and 209 (ν 4). Second derivatives of the dipole moment function for ν 1 + ν 3 x and ν 2 a+ν 3 x in units of e-amu -1 Å are 0.0552 and -0.0119, respectively. A comparison with the values determined from an STO-3G basis is presented together with a comparison of the derived quantities from experiment.
Nikolov, S; Fabritius, H; Petrov, M; Friák, M; Lymperakis, L; Sachs, C; Raabe, D; Neugebauer, J
2011-02-01
Recently, we proposed a hierarchical model for the elastic properties of mineralized lobster cuticle using (i) ab initio calculations for the chitin properties and (ii) hierarchical homogenization performed in a bottom-up order through all length scales. It has been found that the cuticle possesses nearly extremal, excellent mechanical properties in terms of stiffness that strongly depend on the overall mineral content and the specific microstructure of the mineral-protein matrix. In this study, we investigated how the overall cuticle properties changed when there are significant variations in the properties of the constituents (chitin, amorphous calcium carbonate (ACC), proteins), and the volume fractions of key structural elements such as chitin-protein fibers. It was found that the cuticle performance is very robust with respect to variations in the elastic properties of chitin and fiber proteins at a lower hierarchy level. At higher structural levels, variations of design parameters such as the volume fraction of the chitin-protein fibers have a significant influence on the cuticle performance. Furthermore, we observed that among the possible variations in the cuticle ingredients and volume fractions, the experimental data reflect an optimal use of the structural variations regarding the best possible performance for a given composition due to the smart hierarchical organization of the cuticle design.
Durig, James R; Zheng, Chao
2007-11-01
Variable temperature (-105 to -150 degrees C) studies of the infrared spectra (3500-400 cm(-1)) of ethylisothiocyanate, CH(3)CH(2)NCS, dissolved in liquid krypton have been recorded. Additionally the infrared spectra of the gas and solid have been re-investigated. These spectroscopic data indicate a single conformer in all physical states with a large number of molecules in the gas phase at ambient temperature in excited states of the CN torsional mode which has a very low barrier to conformational interchange. To aid in the analyses of the vibrational and rotational spectra, ab initio calculations have been carried out by the perturbation method to the second order (MP2) with valence and core electron correlation using a variety of basis sets up to 6-311+G(2df,2pd). With the smaller basis sets up to 6-311+G(d,p) and cc-PVDZ, the cis conformer is indicated as a transition state with all larger basis sets the cis conformer is the only stable form. The predicted energy difference from these calculations between the cis form and the higher energy trans conformer is about 125 cm(-1) which represents essentially the barrier to internal rotation of the NCS group (rotation around NC axis). Density functional theory calculation by the B3LYP method with the same basis sets predicts this barrier to be about 25 cm(-1). By utilizing the previously reported microwave rotational constants with the structural parameters predicted by the ab initio MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters have been obtained for the cis form. The determined heavy atom parameters are: r(NC)=1.196(5), r(CS)=1.579(5), r(CN)=1.439(5), r(CC)=1.519(5)A for the distances and angles of angleCCN=112.1(5), angleCNC=146.2(5), angleNCS=174.0(5) degrees . The centrifugal distortion constants, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and compared to experimental
NASA Astrophysics Data System (ADS)
Téllez, Claudio A.; Hollauer, Eduardo; Felcman, Judith; Lopes, Damiana C. N.; Cattapan, Renata A.
2002-07-01
Cadmium- n-di-isopropylphosphorylguanidine-di-chloride (CdDPGCl 2) was synthesized in the solid phase and characterized previously. The Fourier transform infrared and Raman spectra of (CdDPGCl 2) in the solid state were recorded and analyzed. Emphasis was placed on the vibrational assignment of the [(O 2P=O{CdCl 2}HN=C)] fragment of the complete molecular structure. With the aim of assisting the vibrational assignment of the experimental spectra, a comparison with the spectra of N-di-isopropylphosphorylguanidine ligand was carried out and ab initio calculations have been performed with several effective core potentials and valence basis sets (Hay-Wadt (HW) and Stevens-Basch-Krauss (SBK)). Due to our limited computational resources, hydrogen atoms replaced the isopropyl groups. The calculated geometrical parameters showed excellent agreement with the experimental, as well as the RHF/MP2 calculated infrared wave numbers, when compared to the IR/Raman experimental wave numbers.
Ab initio many-body calculations of the (3)H(d,n)(4)He and (3)He(d,p)(4)He fusion reactions.
Navrátil, Petr; Quaglioni, Sofia
2012-01-27
We apply the ab initio no-core shell model combined with the resonating-group method approach to calculate the cross sections of the (3)H(d,n)(4)He and (3)He(d,p)(4)He fusion reactions. These are important reactions for the big bang nucleosynthesis and the future of energy generation on Earth. Starting from a selected similarity-transformed chiral nucleon-nucleon interaction that accurately describes two-nucleon data, we performed many-body calculations that predict the S factor of both reactions. Virtual three-body breakup effects are obtained by including excited pseudostates of the deuteron in the calculation. Our results are in satisfactory agreement with experimental data and pave the way for microscopic investigations of polarization and electron-screening effects, of the (3)H(d,γn)(4)He bremsstrahlung and other reactions relevant to fusion research. PMID:22400830
NASA Astrophysics Data System (ADS)
Śmialek, Malgorzata A.; Łabuda, Marta; Guthmuller, Julien; Hubin-Franskin, Marie-Jeanne; Delwiche, Jacques; Hoffmann, Søren Vrønning; Jones, Nykola C.; Mason, Nigel J.; Limão-Vieira, Paulo
2016-06-01
The high-resolution vacuum ultraviolet photoabsorption spectrum of ethyl acetate, C4H8O2, is presented over the energy range 4.5-10.7 eV (275.5-116.0 nm). Valence and Rydberg transitions and their associated vibronic series observed in the photoabsorption spectrum, have been assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. Also, the photoabsorption cross sections have been used to calculate the photolysis lifetime of this ester in the upper stratosphere (20-50 km). Calculations have also been carried out to determine the ionisation energies and fine structure of the lowest ionic state of ethyl acetate and are compared with a newly recorded photoelectron spectrum (from 9.5 to 16.7 eV). Vibrational structure is observed in the first photoelectron band of this molecule for the first time.
NASA Technical Reports Server (NTRS)
Schwenke, David W.; Walch, Stephen P.; Taylor, Peter R.
1991-01-01
Extensive ab initio calculations on the ground state potential energy surface of H2 + H2O were performed using a large contracted Gaussian basis set and a high level of correlation treatment. An analytical representation of the potential energy surface was then obtained which reproduces the calculated energies with an overall root-mean-square error of only 0.64 mEh. The analytic representation explicitly includes all nine internal degrees of freedom and is also well behaved as the H2 dissociates; it thus can be used to study collision-induced dissociation or recombination of H2. The strategy used to minimize the number of energy calculations is discussed, as well as other advantages of the present method for determining the analytical representation.
Marchetti, Barbara; Karsili, Tolga N V; Ashfold, Michael N R; Domcke, Wolfgang
2016-07-27
The availability of non-radiative decay mechanisms by which photoexcited molecules can revert to their ground electronic state, without experiencing potentially deleterious chemical transformation, is fundamental to molecular photostability. This Perspective Article combines results of new ab initio electronic structure calculations and prior experimental data in an effort to systematise trends in the non-radiative decay following UV excitation of selected families of heterocyclic molecules. We start with the prototypical uni- and bicyclic molecules phenol and indole, and explore the structural and photophysical consequences of incorporating progressively more nitrogen atoms within the respective ring structures en route to the DNA bases thymine, cytosine, adenine and guanine. For each of the latter, we identify low energy non-radiative decay pathways via conical intersections with the ground state potential energy surface accessed by out-of-plane ring deformations. This is followed by summary descriptions and illustrations of selected rival (electron driven H atom transfer) non-radiative excited state decay processes that demand consideration once the nucleobases are merely components in larger biomolecular systems like nucleosides, and both individual and stacked base-pairs. PMID:26980149
Marchetti, Barbara; Karsili, Tolga N V; Ashfold, Michael N R; Domcke, Wolfgang
2016-07-27
The availability of non-radiative decay mechanisms by which photoexcited molecules can revert to their ground electronic state, without experiencing potentially deleterious chemical transformation, is fundamental to molecular photostability. This Perspective Article combines results of new ab initio electronic structure calculations and prior experimental data in an effort to systematise trends in the non-radiative decay following UV excitation of selected families of heterocyclic molecules. We start with the prototypical uni- and bicyclic molecules phenol and indole, and explore the structural and photophysical consequences of incorporating progressively more nitrogen atoms within the respective ring structures en route to the DNA bases thymine, cytosine, adenine and guanine. For each of the latter, we identify low energy non-radiative decay pathways via conical intersections with the ground state potential energy surface accessed by out-of-plane ring deformations. This is followed by summary descriptions and illustrations of selected rival (electron driven H atom transfer) non-radiative excited state decay processes that demand consideration once the nucleobases are merely components in larger biomolecular systems like nucleosides, and both individual and stacked base-pairs.
Acceleration of the Convergence in ab initio Atomic Relaxations
NASA Astrophysics Data System (ADS)
Zhao, Zhengji; Wang, Lin-Wang; Meza, Juan
2006-03-01
Atomic relaxations is often required to accurately describe the properties of nanosystems. In ab initio calculations, a common practice is to use a standard search algorithm, such as BFGS (Broyden-Fletcher-Goldfarb-Shanno) or CG (conjugate gradient) method, which starts the atomic relaxations without any knowledge of the Hessian matrix of the system. For example, the initial Hessian in BFGS method is often set to identity, and there is no preconditioning to CG method. One way to accelerate the convergence of the atomic relaxations is to estimate an approximate Hessian matrix of the system and then use it as the initial Hessian in BFGS method or a preconditioner in CG method. Previous attempts to obtain the approximated Hessian were focused on the use of classical force field models which rely on the existence of good parameters. Here, we present an alternative method to estimate the Hessian matrix of a nanosystem. First, we decompose the system into motifs which consist of a few atoms, then calculate the Hessian matrix elements on different motif types from ab initio calculations for small prototype systems. Then we generate the Hessian Matrix of the whole system by putting together these motif Hessians. We have applied our motif-based Hessian matrix in ab initio atomic relaxations in several bulk (with/without impurity) and quantum dot systems, and have found a speed up factor of 2 to 4 depending on the system size.
Scifoni, E; Bodo, E; Gianturco, F A
2005-06-01
Ab initio calculations at the multiconfiguration self-consistent field level followed by a multireference configuration interaction were carried out along the two possible collinear approaches of the [LiHHe]+ system, while a three-dimensional calculation of the structures of that complex with LiH+ kept at its equilibrium geometry was also completed at the same level of accuracy. The interaction forces of the lowest two electronic states indicate possible reactive behavior, with the first excited potential-energy surface clearly showing a barrierless path to HeH+ product formation. The details of the reactive pathways and their possible bearing on reaction processes, which could occur at the low temperature of a He nanodroplet holding LiH+ as an impurity, are analyzed and discussed. PMID:15974673
NASA Astrophysics Data System (ADS)
Nikolopoulos, L. A. A.
2003-02-01
A package is presented for the fully ab-initio calculation of one- and two-photon ionization cross sections for two-electron atomic systems (H -, He, Mg, Ca, …) under strong laser fields, within lowest-order perturbation theory (LOPT) and in the dipole approximation. The atomic structure is obtained through configuration interaction (CI) of antisymmetrized two-electron states expanded in a B-spline finite basis. The formulation of the theory and the relevant codes presented here represent the accumulation of work over the last ten years [1-11,13-15]. Extensions to more than two-photon ionization is straightforward. Calculation is possible for both the length and velocity form of the laser-atom interaction operator. The package is mainly, written in standard FORTRAN language and uses the publicly available libraries SLATEC, LAPACK and BLAS.
Gerber, Iann C; Jolibois, Franck
2015-05-14
Chemical shift requires the knowledge of both the sample and a reference magnetic shielding. In few cases as nitrogen (15N), the standard experimental reference corresponds to its liquid phase. Theoretical estimate of NMR magnetic shielding parameters of compounds in their liquid phase is then mandatory but usually replaced by an easily-get gas phase value, forbidding direct comparisons with experiments. We propose here to combine ab initio molecular dynamic simulations with the calculations of magnetic shielding using GIAO approach on extracted cluster's structures from MD. Using several computational strategies, we manage to accurately calculate 15N magnetic shielding of nitromethane in its liquid phase. Theoretical comparison between liquid and gas phase allows us to extrapolate an experimental value for the 15N magnetic shielding of nitromethane in gas phase between -121.8 and -120.8 ppm.
Guo, Q.X.; Zhao, Y.S.; Jiang, C.; Mao, W.L.; Wang, Z.W.; /Cornell U., CHESS
2009-06-09
Sm{sub 2}O{sub 3} was compressed at room temperature up to 44.0 GPa and then decompressed back to ambient pressure. In situ X-ray diffraction was used to monitor the structural changes in the sample. A cubic to hexagonal phase transformation was observed in Sm{sub 2}O{sub 3} for the first time. After decompression back to ambient pressure, the hexagonal phase was not quenchable and transformed to a monoclinic phase. Ab initio Density-Functional-Theory (DFT) calculations were performed to obtain theoretical data for comparison with the experimental results and elucidation of the transformation mechanism. A possible phase transformation mechanism that is consistent with the experimental results and theoretical calculations is proposed.
NASA Astrophysics Data System (ADS)
Joshi, Bhawani Datt; Srivastava, Anubha; Tandon, Poonam; Jain, Sudha
2011-11-01
Yohimbine hydrochloride (YHCl) is an aphrodisiac and promoted for erectile dysfunction, weight loss and depression. The optimized geometry, total energy, potential energy surface and vibrational wavenumbers of yohimbine hydrochloride have been determined using ab initio, Hartree-Fock (HF) and density functional theory (DFT/B3LYP) method with 6-311++G(d,p) basis set. A complete vibrational assignment is provided for the observed Raman and IR spectra of YHCl. The UV absorption spectrum was examined in ethanol solvent and compared with the calculated one in gas phase as well as in solvent environment (polarizable continuum model, PCM) using TD-DFT/6-31G basis set. These methods are proposed as a tool to be applied in the structural characterization of YHCl. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with frontier orbital gap are presented.
Ohyama, Tatsuya; Hayakawa, Masato; Nishikawa, Shin; Kurita, Noriyuki
2011-06-01
Transcription mechanisms of gene information from DNA to mRNA are essentially controlled by regulatory proteins such as a lactose repressor (LacR) protein and ligand molecules. Biochemical experiments elucidated that a ligand binding to LacR drastically changes the mechanism controlled by LacR, although the effect of ligand binding has not been clarified at atomic and electronic levels. We here investigated the effect of ligand binding on the specific interactions between LacR and operator DNA by the molecular simulations combined with classical molecular mechanics and ab initio fragment molecular orbital methods. The results indicate that the binding of anti-inducer ligand strengthens the interaction between LacR and DNA, which is consistent with the fact that the binding of anti-inducer enhances the repression of gene transcription by LacR. It was also elucidated that hydrating water molecules existing between LacR and DNA contribute to the specific interactions between LacR and DNA. PMID:21328406
Rayón, Víctor M; Valdés, Haydee; Díaz, Natalia; Suárez, Dimas
2008-02-01
A systematic theoretical study on several models of Zn(II) complexes has been carried out employing both ab initio correlated wave function and density functional methods. The performance of five different functionals namely PW91, PBE, B3LYP, MPWLYP1M, and TPSS in the prediction of metal-ligand bond distances, binding energies, and proton affinities has been assessed comparing the results to those obtained with the MP2 and CCSD(T) wave function methodologies. Several basis sets ranging from double-ζ up to quintuple-ζ quality have been used, including the recently developed all-electron correlation consistent basis sets for zinc. It is shown that all the tested functionals overestimate both the metal-ligand bond distances and the binding energies, being that the B3LYP and TPSS functionals are the ones that perform the best. An analysis of the metal-ligand interaction energy shows that induction and charge-transfer effects play a prominent role in the bonding of these systems, even for those complexes with the less polarizable ligands. This finding highlights the importance of a correct description of the polarization of the monomers' charge densities by any theoretical method which aims to be applied to the study of Zn(II) complexes.
Kraisler, Eli; Makov, Guy; Kelson, Itzhak
2010-10-15
The total energies and the spin states for atoms and their first ions with Z=1-86 are calculated within the the local spin-density approximation (LSDA) and the generalized-gradient approximation (GGA) to the exchange-correlation (xc) energy in density-functional theory. Atoms and ions for which the ground-state density is not pure-state v-representable are treated as ensemble v-representable with fractional occupations of the Kohn-Sham system. A recently developed algorithm which searches over ensemble v-representable densities [E. Kraisler et al., Phys. Rev. A 80, 032115 (2009)] is employed in calculations. It is found that for many atoms, the ionization energies obtained with the GGA are only modestly improved with respect to experimental data, as compared to the LSDA. However, even in those groups of atoms where the improvement is systematic, there remains a non-negligible difference with respect to the experiment. The ab initio electronic configuration in the Kohn-Sham reference system does not always equal the configuration obtained from the spectroscopic term within the independent-electron approximation. It was shown that use of the latter configuration can prevent the energy-minimization process from converging to the global minimum, e.g., in lanthanides. The spin values calculated ab initio fit the experiment for most atoms and are almost unaffected by the choice of the xc functional. Among the systems with incorrectly obtained spin, there exist some cases (e.g., V, Pt) for which the result is found to be stable with respect to small variations in the xc approximation. These findings suggest a necessity for a significant modification of the exchange-correlation functional, probably of a nonlocal nature, to accurately describe such systems.
Vondrásek, Jirí; Bendová, Lada; Klusák, Vojtech; Hobza, Pavel
2005-03-01
The formation of a hydrophobic core of globular proteins is believed to be the consequence of exterior hydrophobic forces of entropic nature. This, together with the low occurrence of hydrogen bonds in the protein core, leads to the opinion that the energy contribution of core formation to protein folding and stability is negligible. We show that stabilization inside the hydrophobic core of a small protein, rubredoxin, determined by means of high-level correlated ab initio calculations (complete basis set limit of MP2 stabilization energy + CCSD(T) correction term), amounted to approximately 50 kcal/mol. These results clearly demonstrate strong attraction inside a hydrophobic core. This finding may lead to substantial changes in the current view of protein folding. We also point out the inability of the DFT/B3LYP method to describe a strong attraction between studied amino acids.
He, P; Ma, X; Zhang, J W; Zhao, H B; Lüpke, G; Shi, Z; Zhou, S M
2013-02-15
The dependence of the intrinsic Gilbert damping parameter α(0) on the spin-orbital coupling strength ξ is investigated in L1(0) ordered FePd(1-x) Pt(x) films by time-resolved magneto-optical Kerr effect measurements and spin-dependent ab initio calculations. Continuous tuning of α(0) over more than one order of magnitude is realized by changing the Pt/Pd concentration ratio showing that α(0) is proportional to ξ(2) as changes of other leading parameters are found to be negligible. The perpendicular magnetic anisotropy is shown to have a similar variation trend with x. The present results may facilitate the design and fabrication of new magnetic alloys with large perpendicular magnetic anisotropy and tailored damping properties. PMID:25166400
Ferreira da Silva, F; Lange, E; Limão-Vieira, P; Jones, N C; Hoffmann, S V; Hubin-Franskin, M-J; Delwiche, J; Brunger, M J; Neves, R F C; Lopes, M C A; de Oliveira, E M; da Costa, R F; Varella, M T do N; Bettega, M H F; Blanco, F; García, G; Lima, M A P; Jones, D B
2015-10-14
The electronic spectroscopy of isolated furfural (2-furaldehyde) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 3.5-10.8 eV energy-range, with absolute cross section measurements derived. Electron energy loss spectra are also measured over a range of kinematical conditions. Those energy loss spectra are used to derive differential cross sections and in turn generalised oscillator strengths. These experiments are supported by ab initio calculations in order to assign the excited states of the neutral molecule. The good agreement between the theoretical results and the measurements allows us to provide the first quantitative assignment of the electronic state spectroscopy of furfural over an extended energy range.
NASA Astrophysics Data System (ADS)
Ferreira da Silva, F.; Lange, E.; Limão-Vieira, P.; Jones, N. C.; Hoffmann, S. V.; Hubin-Franskin, M.-J.; Delwiche, J.; Brunger, M. J.; Neves, R. F. C.; Lopes, M. C. A.; de Oliveira, E. M.; da Costa, R. F.; Varella, M. T. do N.; Bettega, M. H. F.; Blanco, F.; García, G.; Lima, M. A. P.; Jones, D. B.
2015-10-01
The electronic spectroscopy of isolated furfural (2-furaldehyde) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 3.5-10.8 eV energy-range, with absolute cross section measurements derived. Electron energy loss spectra are also measured over a range of kinematical conditions. Those energy loss spectra are used to derive differential cross sections and in turn generalised oscillator strengths. These experiments are supported by ab initio calculations in order to assign the excited states of the neutral molecule. The good agreement between the theoretical results and the measurements allows us to provide the first quantitative assignment of the electronic state spectroscopy of furfural over an extended energy range.
Tamarany, Rizcky; Lee, Seung-Cheol; Kim, Hyung-Jun; Choi, Jung-Hae
2013-02-07
Ab initio calculations were performed to investigate the initial deposition of epitaxial MgO on GaAs(001)-{beta}2(2 Multiplication-Sign 4). The differences between the chemical bonding of Mg-As and O-As were characterized by the adsorption energies of atomic O and Mg at several symmetrically distinct sites, and O bonding was substantially stronger than that of Mg. Thermodynamics were analyzed through the introduction of environmental chemical potentials simulating in situ growth conditions by the sputtering of a stoichiometric MgO target. A surface phase diagram was generated under Mg and O environments with constrained equilibrium, and the results explained the initial formation of an epitaxial MgO phase on GaAs(001)-{beta}2(2 Multiplication-Sign 4).
He, P; Ma, X; Zhang, J W; Zhao, H B; Lüpke, G; Shi, Z; Zhou, S M
2013-02-15
The dependence of the intrinsic Gilbert damping parameter α(0) on the spin-orbital coupling strength ξ is investigated in L1(0) ordered FePd(1-x) Pt(x) films by time-resolved magneto-optical Kerr effect measurements and spin-dependent ab initio calculations. Continuous tuning of α(0) over more than one order of magnitude is realized by changing the Pt/Pd concentration ratio showing that α(0) is proportional to ξ(2) as changes of other leading parameters are found to be negligible. The perpendicular magnetic anisotropy is shown to have a similar variation trend with x. The present results may facilitate the design and fabrication of new magnetic alloys with large perpendicular magnetic anisotropy and tailored damping properties.
NASA Technical Reports Server (NTRS)
Bowman, Joel M.; Gazdy, Bela; Bentley, Joseph A.; Lee, Timothy J.; Dateo, Christopher E.
1993-01-01
A potential energy surface for the HCN/HNC system which is a fit to extensive, high-quality ab initio, coupled-cluster calculations is presented. All HCN and HNC states with energies below the energy of the first delocalized state are reported and characterized. Vibrational transition energies are compared with all available experimental data on HCN and HNC, including high CH-overtone states up to 23,063/cm. A simulation of the (A-tilde)-(X-tilde) stimulated emission pumping (SEP) spectrum is also reported, and the results are compared to experiment. Franck-Condon factors are reported for odd bending states of HCN, with one quantum of vibrational angular momentum, in order to compare with the recent assignment by Jonas et al. (1992), on the basis of axis-switching arguments of a number of previously unassigned states in the SEP spectrum.
Ferreira da Silva, F.; Lange, E.; Limão-Vieira, P. E-mail: michael.brunger@flinders.edu.au; Jones, N. C.; Hoffmann, S. V.; Hubin-Franskin, M.-J.; Delwiche, J.; Brunger, M. J. E-mail: michael.brunger@flinders.edu.au; and others
2015-10-14
The electronic spectroscopy of isolated furfural (2-furaldehyde) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 3.5–10.8 eV energy-range, with absolute cross section measurements derived. Electron energy loss spectra are also measured over a range of kinematical conditions. Those energy loss spectra are used to derive differential cross sections and in turn generalised oscillator strengths. These experiments are supported by ab initio calculations in order to assign the excited states of the neutral molecule. The good agreement between the theoretical results and the measurements allows us to provide the first quantitative assignment of the electronic state spectroscopy of furfural over an extended energy range.
Foyevtsova, Kateryna; Krogel, Jaron T; Kim, Jeongnim; Kent, Paul R; Dagotto, Elbio R; Reboredo, Fernando A
2014-01-01
In view of the continuous theoretical efforts aimed at an accurate microscopic description of the strongly correlated transition metal oxides and related materials, we show that with continuum quantum Monte Carlo (QMC) calculations it is possible to obtain the value of the spin superexchange coupling constant of a copper oxide in a quantitatively excellent agreement with experiment. The variational nature of the QMC total energy allows us to identify the best trial wave function out of the available pool of wave functions, which makes the approach essentially free from adjustable parameters and thus truly ab initio. The present results on magnetic interactions suggest that QMC is capable of accurately describing ground state properties of strongly correlated materials.
NASA Astrophysics Data System (ADS)
Ventura, Oscar N.; Segovia, Marc
2005-02-01
The experimental enthalpy of formation of perfluoropropane (C 3F 8), reported originally as -1729 kJ/mol and latter corrected to -1784.7 kJ/mol, is reexamined at the light of density functional and model chemistry (G3, CBS-4, CBS-Q) calculations of several isodesmic reactions relating C 3F 8 to smaller fluoroalkanes. The average enthalpy of formation of C 3F 8 obtained from all reactions studied was -1739 ± 12 kJ/mol at the DFT level and -1748 ± 12 kJ/mol at the ab initio level, thus ruling out the larger experimental value. A value of -1732 ± 5 kJ/mol is recommended from careful analysis of the theoretical results.
Ferreira da Silva, F; Lange, E; Limão-Vieira, P; Jones, N C; Hoffmann, S V; Hubin-Franskin, M-J; Delwiche, J; Brunger, M J; Neves, R F C; Lopes, M C A; de Oliveira, E M; da Costa, R F; Varella, M T do N; Bettega, M H F; Blanco, F; García, G; Lima, M A P; Jones, D B
2015-10-14
The electronic spectroscopy of isolated furfural (2-furaldehyde) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 3.5-10.8 eV energy-range, with absolute cross section measurements derived. Electron energy loss spectra are also measured over a range of kinematical conditions. Those energy loss spectra are used to derive differential cross sections and in turn generalised oscillator strengths. These experiments are supported by ab initio calculations in order to assign the excited states of the neutral molecule. The good agreement between the theoretical results and the measurements allows us to provide the first quantitative assignment of the electronic state spectroscopy of furfural over an extended energy range. PMID:26472380
Bittner, Dror M.; Zaleski, Daniel P.; Stephens, Susanna L.; Walker, Nicholas R. E-mail: a.c.legon@bristol.ac.uk; Tew, David P.; Legon, Anthony C. E-mail: a.c.legon@bristol.ac.uk
2015-04-14
The H{sub 3}N⋯CuCl monomer has been generated and isolated in the gas phase through laser vaporisation of a copper sample in the presence of low concentrations of NH{sub 3} and CCl{sub 4} in argon. The resulting complex cools to a rotational temperature approaching 2 K during supersonic expansion of the gas sample and is characterised by broadband rotational spectroscopy between 7 and 18.5 GHz. The spectra of six isotopologues are measured and analysed to determine rotational, B{sub 0}; centrifugal distortion, D{sub J}, D{sub JK}; and nuclear quadrupole coupling constants of Cu, Cl, and {sup 14}N nuclei, χ{sub aa} (X). The geometry of the complex is C{sub 3v} with the N, Cu, and Cl atoms located on the a inertial axis. Bond distances and the ∠(H —N⋯Cu) bond angle within the complex are precisely evaluated through fitting of geometrical parameters to the experimentally determined moments of inertia and through ab initio calculations at the CCSD(T)(F12*)/AVQZ level. The r(Cu —Cl), r(Cu —N), and ∠(H —N⋯Cu) parameters are, respectively, evaluated to be 2.0614(7) Å, 1.9182(13) Å, and 111.40(6)° in the r{sub 0} geometry, in good agreement with the ab initio calculations. Geometrical parameters evaluated for the isolated complex are compared with those established crystallographically for a solid-state sample of [Cu(NH{sub 3})Cl].
NASA Astrophysics Data System (ADS)
Schatz, George C.; Walch, Stephen P.; Wagner, Albert F.
1980-11-01
We present ab initio (GVB-POL-CI) calculations for enough of the region about the abstraction and exchange saddle points for H(T)+CH4(CD4) to perform a full normal mode analysis of the transition states. The resulting normal mode frequencies are compared to four other published surfaces: an ab initio UHF-SCF calculation by Carsky and Zahradnik, a semiempirical surface by Raff, and two semiempirical surfaces by Kurylo, Hollinden, and Timmons. Significant quantitative and qualitative differences exist between the POL-CI results and those of the other surfaces. Transition state theory rate constants and vibrationally adiabatic reaction threshold energies were computed for all surfaces and compared to available experimental values. For abstraction, the POL-CI rates are in good agreement with experimental rates and in better agreement than are the rates of any of the other surfaces. For exchange, uncertainties in the experimental values and in the importance of vibrationally nonadiabatic effects cloud the comparison of theory to experiment. Tentative conclusions are that the POL-CI barrier is too low by several kcal. Unless vibrationaly nonadiabatic effects are severe, the POL-CI surface is still in better agreement with experiment than are the other surfaces. The rates for a simple 3-atom transition state theory model (where CH3 is treated as an atom) are compared to the rates for the full 6-atom model. The kinetic energy coupling of reaction coordinate modes to methyl group modes is identified as being of primary importance in determining the accuracy of the 3-atom model for this system. Substantial coupling in abstraction, but not exchange, causes the model to fail for abstraction but succeed for exchange.
NASA Astrophysics Data System (ADS)
Qureshi, N.; Zbiri, M.; Rodríguez-Carvajal, J.; Stunault, A.; Ressouche, E.; Hansen, T. C.; Fernández-Díaz, M. T.; Johnson, M. R.; Fuess, H.; Ehrenberg, H.; Sakurai, Y.; Itou, M.; Gillon, B.; Wolf, Th.; Rodríguez-Velamazan, J. A.; Sánchez-Montero, J.
2009-03-01
We present a combination of ab initio calculations, magnetic Compton scattering, and polarized neutron experiments, which elucidate the density distribution of unpaired electrons in the kagome staircase system Co3V2O8 . Ab initio wave functions were used to calculate the spin densities in real and momentum spaces, which show good agreement with the respective experiments. It has been found that the spin polarized orbitals are equally distributed between the t2g and the eg levels for the spine (s) Co ions while the eg orbitals of the cross-tie (c) Co ions only represent 30% of the atomic spin density. Furthermore, the results reveal that the magnetic moments of the cross-tie Co ions, which are significantly smaller than those of the spine Co ions in the zero-field ferromagnetic structure, do not saturate by applying an external magnetic field of 2 T along the easy axis a . In turn, the increasing bulk magnetization, which can be observed by field dependent macroscopic measurements, originates from induced magnetic moments on the O and V sites. The refined individual magnetic moments are μ(Coc)=1.54(4)μB , μ(Cos)=2.87(3)μB , μ(V)=0.41(4)μB , μ(O1)=0.05(5)μB , μ(O2)=0.35(5)μB , and μ(O3)=0.36(5)μB combining to the same macroscopic magnetization value, which was previously only attributed to the Co ions.
NASA Astrophysics Data System (ADS)
Tanuwijaya, V. V.; Hidayat, N. N.; Agusta, M. K.; Dipojono, H. K.
2015-09-01
One of the biggest challenge in material technology for hydrogen storage application is to increase hydrogen uptake in room temperature and pressure. As a class of highly porous material, Metal-Organic Frameworks (MOF) holds great potential with its tunable structure. However, little is known about the effect of metal cluster to its hydrogen storage capability. Investigation on this matter has been carried out carefully on small cluster of Zn and Cu-based MOF using first principles method. The calculation of two distinct building units of MOFs, namely octahedral and paddle-wheel models, have been done with B3LYP density functional method using 6-31G(d,p) and LANL2DZ basis sets. From geometry optimization of Zn-based MOF linked by benzene-dicarboxylate (MOF-5), it is found that hydrogen tends to keep distance from metal cluster group and stays above benzene ring. In the other hand, hydrogen molecule prefers to stay atop of the exposed Cu atom in Cu-based MOF system linked by the same linker group (Cu-bdc). Calculated hydrogen binding enthalpies for Zn and Cu octahedral cages at ZnO3 sites are 1.64kJ/mol and 2.73kJ/mol respectively, while hydrogen binding enthalpies for Zn and Cu paddle-wheel cages calculated on top of metal atoms are found to be at 6.05kJ/mol and 6.10kJ/mol respectively. Major difference between Zn-MOF-5 and Cu-bdc hydrogen uptake performance might be caused by unsaturated metal sites present in Cu-bdc system and the influence of their geometric structures, although a small difference on binding energy in the type of transition metal used is also observed. The comparison between Zn and Cu-based MOF may contribute to a comprehensive understanding of metal clusters and the importance of selecting best transition metal for design and synthesis of metal-organic frameworks.
Tanuwijaya, V. V.; Hidayat, N. N. Agusta, M. K. Dipojono, H. K.
2015-09-30
One of the biggest challenge in material technology for hydrogen storage application is to increase hydrogen uptake in room temperature and pressure. As a class of highly porous material, Metal-Organic Frameworks (MOF) holds great potential with its tunable structure. However, little is known about the effect of metal cluster to its hydrogen storage capability. Investigation on this matter has been carried out carefully on small cluster of Zn and Cu-based MOF using first principles method. The calculation of two distinct building units of MOFs, namely octahedral and paddle-wheel models, have been done with B3LYP density functional method using 6-31G(d,p) and LANL2DZ basis sets. From geometry optimization of Zn-based MOF linked by benzene-dicarboxylate (MOF-5), it is found that hydrogen tends to keep distance from metal cluster group and stays above benzene ring. In the other hand, hydrogen molecule prefers to stay atop of the exposed Cu atom in Cu-based MOF system linked by the same linker group (Cu-bdc). Calculated hydrogen binding enthalpies for Zn and Cu octahedral cages at ZnO{sub 3} sites are 1.64kJ/mol and 2.73kJ/mol respectively, while hydrogen binding enthalpies for Zn and Cu paddle-wheel cages calculated on top of metal atoms are found to be at 6.05kJ/mol and 6.10kJ/mol respectively. Major difference between Zn-MOF-5 and Cu-bdc hydrogen uptake performance might be caused by unsaturated metal sites present in Cu-bdc system and the influence of their geometric structures, although a small difference on binding energy in the type of transition metal used is also observed. The comparison between Zn and Cu-based MOF may contribute to a comprehensive understanding of metal clusters and the importance of selecting best transition metal for design and synthesis of metal-organic frameworks.
A highly accurate ab initio potential energy surface for methane
NASA Astrophysics Data System (ADS)
Owens, Alec; Yurchenko, Sergei N.; Yachmenev, Andrey; Tennyson, Jonathan; Thiel, Walter
2016-09-01
A new nine-dimensional potential energy surface (PES) for methane has been generated using state-of-the-art ab initio theory. The PES is based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit and incorporates a range of higher-level additive energy corrections. These include core-valence electron correlation, higher-order coupled cluster terms beyond perturbative triples, scalar relativistic effects, and the diagonal Born-Oppenheimer correction. Sub-wavenumber accuracy is achieved for the majority of experimentally known vibrational energy levels with the four fundamentals of 12CH4 reproduced with a root-mean-square error of 0.70 cm-1. The computed ab initio equilibrium C-H bond length is in excellent agreement with previous values despite pure rotational energies displaying minor systematic errors as J (rotational excitation) increases. It is shown that these errors can be significantly reduced by adjusting the equilibrium geometry. The PES represents the most accurate ab initio surface to date and will serve as a good starting point for empirical refinement.
A highly accurate ab initio potential energy surface for methane.
Owens, Alec; Yurchenko, Sergei N; Yachmenev, Andrey; Tennyson, Jonathan; Thiel, Walter
2016-09-14
A new nine-dimensional potential energy surface (PES) for methane has been generated using state-of-the-art ab initio theory. The PES is based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit and incorporates a range of higher-level additive energy corrections. These include core-valence electron correlation, higher-order coupled cluster terms beyond perturbative triples, scalar relativistic effects, and the diagonal Born-Oppenheimer correction. Sub-wavenumber accuracy is achieved for the majority of experimentally known vibrational energy levels with the four fundamentals of (12)CH4 reproduced with a root-mean-square error of 0.70 cm(-1). The computed ab initio equilibrium C-H bond length is in excellent agreement with previous values despite pure rotational energies displaying minor systematic errors as J (rotational excitation) increases. It is shown that these errors can be significantly reduced by adjusting the equilibrium geometry. The PES represents the most accurate ab initio surface to date and will serve as a good starting point for empirical refinement. PMID:27634258
A highly accurate ab initio potential energy surface for methane.
Owens, Alec; Yurchenko, Sergei N; Yachmenev, Andrey; Tennyson, Jonathan; Thiel, Walter
2016-09-14
A new nine-dimensional potential energy surface (PES) for methane has been generated using state-of-the-art ab initio theory. The PES is based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit and incorporates a range of higher-level additive energy corrections. These include core-valence electron correlation, higher-order coupled cluster terms beyond perturbative triples, scalar relativistic effects, and the diagonal Born-Oppenheimer correction. Sub-wavenumber accuracy is achieved for the majority of experimentally known vibrational energy levels with the four fundamentals of (12)CH4 reproduced with a root-mean-square error of 0.70 cm(-1). The computed ab initio equilibrium C-H bond length is in excellent agreement with previous values despite pure rotational energies displaying minor systematic errors as J (rotational excitation) increases. It is shown that these errors can be significantly reduced by adjusting the equilibrium geometry. The PES represents the most accurate ab initio surface to date and will serve as a good starting point for empirical refinement.
Grid-based methods for biochemical ab initio quantum chemical applications
Colvin, M.E.; Nelson, J.S.; Mori, E.
1997-01-01
A initio quantum chemical methods are seeing increased application in a large variety of real-world problems including biomedical applications ranging from drug design to the understanding of environmental mutagens. The vast majority of these quantum chemical methods are {open_quotes}spectral{close_quotes}, that is they describe the charge distribution around the nuclear framework in terms of a fixed analytic basis set. Despite the additional complexity they bring, methods involving grid representations of the electron or solvent charge can provide more efficient schemes for evaluating spectral operators, inexpensive methods for calculating electron correlation, and methods for treating the electrostatic energy of salvation in polar solvents. The advantage of mixed or {open_quotes}pseudospectral{close_quotes} methods is that they allow individual non-linear operators in the partial differential equations, such as coulomb operators, to be calculated in the most appropriate regime. Moreover, these molecular grids can be used to integrate empirical functionals of the electron density. These so-called density functional methods (DFT) are an extremely promising alternative to conventional post-Hartree Fock quantum chemical methods. The introduction of a grid at the molecular solvent-accessible surface allows a very sophisticated treatment of a polarizable continuum solvent model (PCM). Where most PCM approaches use a truncated expansion of the solute`s electric multipole expansion, e.g. net charge (Born model) or dipole moment (Onsager model), such a grid-based boundary-element method (BEM) yields a nearly exact treatment of the solute`s electric field. This report describes the use of both DFT and BEM methods in several biomedical chemical applications.
Ab initio derivation of model energy density functionals
NASA Astrophysics Data System (ADS)
Dobaczewski, Jacek
2016-08-01
I propose a simple and manageable method that allows for deriving coupling constants of model energy density functionals (EDFs) directly from ab initio calculations performed for finite fermion systems. A proof-of-principle application allows for linking properties of finite nuclei, determined by using the nuclear nonlocal Gogny functional, to the coupling constants of the quasilocal Skyrme functional. The method does not rely on properties of infinite fermion systems but on the ab initio calculations in finite systems. It also allows for quantifying merits of different model EDFs in describing the ab initio results.
Venturinelli Jannuzzi, Sergio Augusto; Phung, Quan Manh; Domingo, Alex; Formiga, André Luiz Barboza; Pierloot, Kristine
2016-06-01
Important electromeric states in manganese-oxo porphyrins MnO(P)(+) and MnO(PF4)(+) (porphyrinato or meso-tetrafluoroporphyrinato) have been investigated with correlated ab initio methods (CASPT2, RASPT2), focusing on their possible role in multistate reactivity patterns in oxygen transfer (OAT) reactions. Due to the lack of oxyl character, the Mn(V) singlet ground state is kinetically inert. OAT reactions should therefore rather proceed through thermally accessible triplet and quintet states that have a more pronounced oxyl character. Two states have been identified as possible candidates: a Mn(V) triplet state and a Mn(IV)O(L(•)a2u)(+) quintet state. The latter state is high-lying in MnO(P)(+) but is stabilized by the substitutions of H by F at the meso carbons (where the a2u orbital has a significant amplitude). Oxyl character and Mn-O bond weakening in these two states stems from the fact that the Mn-O π* orbitals become singly (triplet) or doubly occupied (quintet). Moreover, an important role for the reactivity of the triplet state is also likely to be played by the π bond that has an empty π* orbital, because of the manifest diradical character of this π bond, revealed by the CASSCF wave function. Interestingly, the diradical character of this bond increases when the Mn-O bond is stretched, while the singly occupied π* orbital looses its oxygen radical contribution. The RASPT2 results were also used as a benchmark for the description of excited state energetics and Mn-O oxyl character with a wide range of pure and hybrid density functionals. With the latter functionals both the Mn(V) → Mn(IV) promotion energy and the diradical character of the π bond (with empty π*) are found to be extremely dependent on the contribution of exact exchange. For this reason, pure functionals are to be preferred.
Zhao, Ya-Ru; Kuang, Xiao-Yu; Zheng, Bao-Bing; Wang, Su-Juan; Li, Yan-Fang
2012-01-01
Ab initio methods based on density functional theory at BP86 level were applied to the study of the geometrical structures, relative stabilities, and electronic properties of small bimetallic Be(2)Au(n) (n = 1-9) clusters. The optimized geometries reveal that the most stable isomers have 3D structures at n = 3, 5, 7, 8, and 9. Here, the relative stabilities were investigated in terms of the averaged atomic binding energies, fragmentation energies and second-order difference of energies. The results show that the planar Be(2)Au(4) structure is the most stable structure for Be(2)Au(n) clusters. The HOMO-LUMO gap, vertical ionization potential, vertical electron affinity and chemical hardness exhibit a pronounced even-odd alternating phenomenon. In addition, charge transfer and natural electron configuration were analyzed and compared.
Howell, Sarah L; Matthewson, Benjamin J; Polson, Matthew I J; Burrell, Anthony K; Gordon, Keith C
2004-05-01
A series of bridging ligands, dipyrido[2,3-a:3',2'-c]phenazine (ppb), dipyrido[2,3-a:3',2'-c]-6,7-dichlorophenazine (ppbCl2), and dipyrido[2,3-a:3',2'-c]-6,7-dimethylphenazine (ppbMe2), and their binuclear copper(I) complexes have been synthesized, and their spectral properties were measured. The single-crystal structure of the complex, [(PPh3)2Cu(mu-ppbCl2)Cu(PPh3)2](BF4)2 in the monoclinic space group P21/c, 18.2590(1), 21.1833(3), 23.2960(3) A with Z = 4 is reported. The copper(I) complexes are deeply colored through MLCT transitions in the visible region. The vibrational spectra of the ligands have been modeled using ab initio hybrid density functional theory (DFT) methods (B3LYP/6-31G(d)) and compared to experimental FT-Raman and IR data. The DFT calculations are used to interpret the resonance Raman spectra, and thus the electronic spectra, of the complexes. The preferential enhancement of modes associated with the phenanthroline section of the ligands with blue excitation (lambda(exc) = 457.9 nm) over phenazine-based modes with redder excitation (lambda(exc) = 514.5 and 632.8 nm) suggests the 2 MLCT transitions terminated on different unoccupied MOs are present under the visible absorption envelope. The radical anion species of the ligands are prepared by the electrochemical reduction of the binuclear copper(I) complexes; no evidence of dechelation prevalent in other copper(I) complexes is observed. The resonance Raman spectra of the reduced complexes are dramatically different from those of the parent species. Across the series common bands are observed at about 1590 and 1570 cm(-1) which do not shift with reduction but are altered in intensity. The normal-mode analysis of the radical anion species suggests that these normal modes primarily involve bond length distortions that are unaffected by reduction.
NASA Astrophysics Data System (ADS)
Genova, Alessandro; Ceresoli, Davide; Pavanello, Michele
2016-06-01
In this work we achieve three milestones: (1) we present a subsystem DFT method capable of running ab-initio molecular dynamics simulations accurately and efficiently. (2) In order to rid the simulations of inter-molecular self-interaction error, we exploit the ability of semilocal frozen density embedding formulation of subsystem DFT to represent the total electron density as a sum of localized subsystem electron densities that are constrained to integrate to a preset, constant number of electrons; the success of the method relies on the fact that employed semilocal nonadditive kinetic energy functionals effectively cancel out errors in semilocal exchange-correlation potentials that are linked to static correlation effects and self-interaction. (3) We demonstrate this concept by simulating liquid water and solvated OH• radical. While the bulk of our simulations have been performed on a periodic box containing 64 independent water molecules for 52 ps, we also simulated a box containing 256 water molecules for 22 ps. The results show that, provided one employs an accurate nonadditive kinetic energy functional, the dynamics of liquid water and OH• radical are in semiquantitative agreement with experimental results or higher-level electronic structure calculations. Our assessments are based upon comparisons of radial and angular distribution functions as well as the diffusion coefficient of the liquid.
An ab initio-based approach to the stability of GaN(0 0 0 1) surfaces under Ga-rich conditions
NASA Astrophysics Data System (ADS)
Ito, Tomonori; Akiyama, Toru; Nakamura, Kohji
2009-05-01
Structural stability of GaN(0 0 0 1) under Ga-rich conditions is systematically investigated by using our ab initio-based approach. The surface phase diagram for GaN(0 0 0 1) including (2×2) and pseudo-(1×1) is obtained as functions of temperature and Ga beam equivalent pressure by comparing chemical potentials of Ga atom in the gas phase with that on the surface. The calculated results reveal that the pseudo-(1×1) appearing below 684-973 K changes its structure to the (2×2) with Ga adatom at higher temperatures beyond 767-1078 K via the newly found (1×1) with two adlayers of Ga. These results are consistent with the stable temperature range of both the pseudo-(1×1) and (2×2) with Ga adatom obtained experimentally. Furthermore, it should be noted that the structure with another coverage of Ga adatoms between the (1×1) and (2×2)-Ga does not appear as a stable structure of GaN(0 0 0 1). Furthermore, ghost island formation observed by scanning tunneling microscopy is discussed on the basis of the phase diagram.
Ab initio Wannier-function-based many-body approach to Born charges of crystalline insulators
NASA Astrophysics Data System (ADS)
Sony, Priya; Shukla, Alok
2004-12-01
In this paper we present an approach aimed at performing many-body calculations of Born-effective charges of crystalline insulators by including the electron-correlation effects. The scheme is implemented entirely in the real space, using Wannier functions as single-particle orbitals. Correlation effects are computed by including virtual excitations from the Hartree-Fock mean field, and the excitations are organized as per a Bethe Goldstone-like many-body hierarchy. The results of our calculations suggest that the approach presented here is promising.
Ng, Maggie; Mok, Daniel K W; Lee, Edmond P F; Dyke, John M
2013-03-15
The minimum energy path (MEP) of the reaction, CF(3)CHFCF(3) + H → transition state (TS) → CF(3)CFCF(3) + H(2), has been computed at different ab initio levels and with density functional theory (DFT) using different functionals. The computed B3LYP/6-31++G**, BH&HLYP/cc-pVDZ, BMK/6-31++G**, M05/6-31+G**, M05-2X/6-31+G**, UMP2/6-31++G**, PUMP2/6-31++G**//UMP2/6-31++G**, RCCSD(T)/aug-cc-pVDZ//UMP2/6-31++G**, RCCSD(T)/aug-cc-pVTZ(spd,sp)//UMP2//6-31++G**, RCCSD(T)/CBS//M05/6-31+G**, and RCCSD(T)/CBS//UMP2/6-31++G** MEPs, and associated gradients and Hessians, were used in reaction rate coefficient calculations based on the transition state theory (TST). Reaction rate coefficients were computed between 300 and 1500 K at various levels of TST, which include conventional TST, canonical variational TST (CVT) and improved CVT (ICVT), and with different tunneling corrections, namely, Wigner, zero-curvature, and small-curvature (SCT). The computed rate coefficients obtained at different ab initio, DFT and TST levels are compared with experimental values available in the 1000-1200 K temperature range. Based on the rate coefficients computed at the ICVT/SCT level, the highest TST level used in this study, the BH&HLYP functional performs best among all the functionals used, while the RCCSD(T)/CBS//MP2/6-31++G** level is the best among all the ab initio levels used. Comparing computed reaction rate coefficients obtained at different levels of theory shows that, the computed barrier height has the strongest effect on the computed reaction rate coefficients as expected. Variational effects on the computed rate coefficients are found to be negligibly small. Although tunneling effects are relatively small at high temperatures (~1500 K), SCT corrections are significant at low temperatures (~300 K), and both barrier heights and the magnitudes of the imaginary frequencies affect SCT corrections.
Size-Expanded yDNA bases: An Ab Initio Study
Fuentes-Cabrera, Miguel A; Sumpter, Bobby G; Lipkowski, Pawel; Wells, Jack C
2006-01-01
xDNA and yDNA are new classes of synthetic nucleic acids characterized by having base-pairs with one of the bases larger than the natural congeners. Here these larger bases are called x- and y-bases. We recently investigated and reported the structural and electronic properties of the x-bases (Fuentes-Cabrera et al. J. Phys. Chem. B 2005, 109, 21135-21139). Here we extend this study by investigating the structure and electronic properties of the y-bases. These studies are framed within our interest that xDNA and yDNA could function as nanowires, for they could have smaller HOMO-LUMO gaps than natural DNA. The limited amount of experimental structural data in these synthetic duplexes makes it necessary to first understand smaller models and, subsequently, to use that information to build larger models. In this paper, we report the results on the chemical and electronic structure of the y-bases. In particular, we predict that the y-bases have smaller HOMO-LUMO gaps than their natural congeners, which is an encouraging result for it indicates that yDNA could have a smaller HOMO-LUMO gap than natural DNA. Also, we predict that the y-bases are less planar than the natural ones. Particularly interesting are our results corresponding to yG. Our studies show that yG is unstable because it is less aromatic and has a Coulombic repulsion that involves the amino group, as compared with a more stable tautomer. However, yG has a very small HOMO-LUMO gap, the smallest of all the size-expanded bases we have considered. The results of this study provide useful information that may allow the synthesis of an yG-mimic that is stable and has a small HOMO-LUMO gap.
Ab initio search for a high permeability material based on bcc iron
NASA Astrophysics Data System (ADS)
Ostanin, S.; Staunton, J. B.; Razee, S. S.; Demangeat, C.; Ginatempo, B.; Bruno, Ezio
2004-02-01
Using the fully relativistic spin-polarized Korringa-Kohn-Rostoker method, we study the prototypical soft magnet, bcc iron. We investigate how its magnetic anisotropy (MAE) varies as a function of volume, band filling, and tetragonal distortions of the crystal lattice. We follow the trends of the linear magnetostriction and magnetic permeability. We find that a slight reduction in band filling and modest lattice expansion produces a significant magnetic softening of this model system. We explore whether this situation can be realized by doping bcc Fe with vanadium. Treating the compositional disorder with the coherent potential approximation, we calculate the magnetic anisotropy and magnetostriction trends of iron-rich Fe1-cVc disordered alloys and find the behavior to accord with the predictions from the bcc Fe model. In particular we find that for c≈0.1 the MAE is very small and the linear magnetostriction is zero. We propose Fe0.9V0.1 as a high permeability material. Fair agreement with experimental values for the MAE and magnetostriction of both Fe and FeV is found.
Multiscale Design of Advanced Materials based on Hybrid Ab Initio and Quasicontinuum Methods
Luskin, Mitchell
2014-03-12
This project united researchers from mathematics, chemistry, computer science, and engineering for the development of new multiscale methods for the design of materials. Our approach was highly interdisciplinary, but it had two unifying themes: first, we utilized modern mathematical ideas about change-of-scale and state-of-the-art numerical analysis to develop computational methods and codes to solve real multiscale problems of DOE interest; and, second, we took very seriously the need for quantum mechanics-based atomistic forces, and based our methods on fast solvers of chemically accurate methods.
De Fazio, Dario; Cavalli, Simonetta; Aquilanti, Vincenzo
2016-07-14
Quantum scattering calculations within the time-independent approach in an extended interval of energies were performed for the title reaction on four ab initio potential energy surfaces. The calculated integral cross sections, vibrational branching ratios, and rate constants are compared with scattering experiments as well as with chemical kinetics rate data available for this system for both the HF and DF channels. The calculations on the CSZ (J. Chem. Phys. 2015, 142, 024303) and LWAL (J. Chem. Phys. 2007, 127, 174302) surfaces are in close agreement between them and reproduce satisfactorily the experimental measurements. The agreement with the experiments is improved with respect to calculations on the earlier SW (J. Chem. Phys. 1996, 104, 6515) and FXZ (J. Chem. Phys. 2008, 129, 011103) surfaces. The results presented here witness the remarkable progress made by quantum chemistry calculations in describing the interatomic interactions governing the dynamics and kinetics of this reaction. They also suggest that comparison with translationally and rotationally averaged experimental observables is not sufficient to assess the relative accuracy of highly accurate potential energy surfaces. The dynamics and kinetics calculations show that temperatures lower than 50 K or molecular beam energy spread below 1 meV must be reached to discriminate the accuracy of the LWAL and the CSZ surfaces.
Can Competency-Based Training Fly?: An Overview of Key Issues for "Ab Initio" Pilot Training
ERIC Educational Resources Information Center
Franks, Peter; Hay, Stephen; Mavin, Tim
2014-01-01
Competency-based training (CBT) for pilots was formally introduced in 1999 by the Civil Aviation Safety Authority (CASA) for training leading to the issue of aeroplane private and commercial pilot licences. This initiative followed the Australian government's introduction of CBT policy for vocational and workplace training in the late 1980's.…
Development of ab initio techniques critical for future science-based explosives R&D.
Wixom, Ryan R.; Mattsson, Ann Elisabet
2013-10-01
Density Functional Theory (DFT) has emerged as an indispensable tool in materials research, since it can accurately predict properties of a wide variety of materials at both equilibrium and extreme conditions. However, for organic molecular crystal explosives, successful application of DFT has largely failed due to the inability of current exchange-correlation functionals to correctly describe intermolecular van der Waals (vdWs) forces. Despite this, we have discovered that even with no treatment of vdWs bonding, the AM05 functional and DFT based molecular dynamics (MD) could be used to study the properties of molecular crystals under compression. We have used DFT-MD to predict the unreacted Hugoniots for PETN and HNS and validated the results by comparison with crystalline and porous experimental data. Since we are also interested in applying DFT methods to study the equilibrium volume properties of explosives, we studied the nature of the vdWs bonding in pursuit of creating a new DFT functional capable of accurately describing equilibrium bonding of molecular crystals. In this report we discuss our results for computing shock Hugoniots of molecular crystals and also what was learned about the nature of bonding in these materials.
NASA Astrophysics Data System (ADS)
Pochert, Jörg; Quack, Martin; Stohner, Jürgen; Willeke, Martin
2000-08-01
We report a new mechanism for intramolecular vibrational redistribution (IVR) in CF3CHFI which couples the CH chromophore vibrations through a strong Fermi resonance to the formal CF stretching normal mode (a heavy atom frame mode) involving the trans F-atom across the CC bond. The analysis is made possible by comparing spectroscopic results with extensive ab initio calculations of the vibrational fundamental and overtone spectra in the range extending to 12 000 cm-1. Potential energy and electric dipole moment hypersurfaces are calculated ab initio by second order Møller-Plesset perturbation theory (MP2) on a grid involving the CH stretching, two CH bending modes and one high frequency CF stretching normal mode. The potentials are scaled to obtain agreement between the experimental spectrum and the theoretical spectrum calculated by a discrete variable representation technique on this grid. Both spectra are then analyzed in terms of three-dimensional (3D) and four-dimensional (4D) effective vibrational Hamiltonians including Fermi- and Darling-Dennison-type resonances between the CH stretching mode and the CH bending modes and the CF stretching mode. The interaction between the CH modes and the CF mode is clearly visible in the experimental and calculated (4D) spectra. The effective Fermi resonance coupling constants [ksff'≃(40±10) cm-1 and ksaf'≃(55±10) cm-1] coupling the CH and CF mode subspaces are of about the same magnitude as the intra-CH chromophore Fermi resonances (ksaa'≃56 cm-1 and ksbb'≃42 cm-1, coupling CH stretching mode "s" with the two CH bending modes "a" and "b"). The chiral, pseudo-Cs symmetry breaking coupling (ksab'≃11 cm-1) is complemented by an equally strong coupling through the CF mode (ksfb'≃15 cm-1). It is demonstrated that low order perturbation theoretical analysis using potential constants from a polynomial expansion to represent effective coupling constants gives inadequate results with discrepancies ranging about from
Mishra, Karuna Kara; Achary, S Nagabhusan; Chandra, Sharat; Ravindran, T R; Sinha, Anil K; Singh, Manavendra N; Tyagi, Avesh K
2016-09-01
Variable-temperature Raman spectroscopic and synchrotron X-ray diffraction studies were performed on BaTe2O6 (orthorhombic, space group: Cmcm), a mixed-valence tellurium compound with a layered structure, to understand structural stability and anharmonicity of phonons. The structural and vibrational studies indicate no phase transition in it over a wider range of temperature (20 to 853 K). The structure shows anisotropic expansion with coefficients of thermal expansion in the order αb ≫ αa > αc, which was attributed to the anisotropy in bonding and structure of BaTe2O6. Temperature evolution of Raman modes of BaTe2O6 indicated a smooth decreasing trend in mode frequencies with increasing temperature, while the full width at half-maximum (fwhm) of all modes systematically increases due to a rise in phonon scattering processes. With the use of our earlier reported isothermal mode Grüneisen parameters, thermal properties such as thermal expansion coefficient and molar specific heat are calculated. The pure anharmonic (explicit) and quasiharmonic (implicit) contribution to the total anharmonicity is delineated and compared. The temperature dependence of phonon mode frequencies and their fwhm values are analyzed by anharmonicity models, and the dominating anharmonic phonon scattering mechanism is concluded in BaTe2O6. In addition to the lattice modes, several external modes of TeOn (n = 5, 6) are found to be strongly anharmonic. The ab initio electronic structure calculations indicated BaTe2O6 is a direct band gap semiconductor with gap energy of ∼2.1 eV. Oxygen orbitals, namely, O-2p states in the valence band maximum and the sp-hybridized states in the conduction band minimum, are mainly involved in the electronic transitions. In addition a number of electronic transitions are predicted by the electronic structure calculations. Experimental photoluminescence results are adequately explained by the ab initio calculations. Further details of the structural and
Madebène, B; Asselin, P; Soulard, P; Alikhani, M E
2011-08-21
The coexistence of axial and equatorial hydrogen-bonded conformers of 1 : 1 (CH(2))(3)S-HF (and -DF) has been observed in the same adiabatic expansion of a supersonic jet seeded with argon and in a static absorption cell at room temperature. High level calculations computed the axial conformer to be the most stable one with a small energy difference with respect to the equatorial one, in full agreement with previous microwave experiments. On the grounds of band contour simulations of FTIR spectra and ab initio energetic and anharmonic vibrational calculations, two pairs of ν(s) HF donor stretching bands, observed in a series of jet-FTIR spectra at 3457.9 and 3480.5 cm(-1) have been respectively assigned to the axial and equatorial forms of the 1 : 1 complex. In the jet-FTIR spectra series with HF, the assignment of an additional broad band (about 200 cm(-1) higher in frequency with respect to ν(s)) to a 1 : 2 complex has been supported by theoretical investigations. Experimental detection of both axial and equatorial forms of a cyclic trimer has been confirmed by calculated energetic and vibrational properties. The nature of hydrogen bonding has been examined within topological frameworks. The energetic partitioning within the 1 : 1 dimers has been elucidated with SAPT techniques. Interestingly, the interconversion pathway between two 1 : 1 structures has been explored and it was seen that the formation of the 1 : 1 complex affects the interconversion barrier on the ring puckering motion. The band contour analysis of gas phase FTIR experiments provided a consistent set of vibrational frequencies and anharmonic coupling constants, in good agreement with ab initio anharmonic vibrational calculations. Finally, from a series of cell-FTIR spectra recorded at different partial pressures of (CH(2))(3)S and HF monomers, the absorption signal of the 1 : 1 complex could be isolated which enabled to estimate the equilibrium constant K(p) = 0
NASA Astrophysics Data System (ADS)
Dakkouri, Marwan; Grosser, Martin
2002-06-01
As a continuation of our systematic investigation of the effect of substituents on the ring geometry and dynamics in silacyclobutanes and in order to explore the role of the silicon atom as a mediator for electronic interactions between the attached fragments, we studied the molecular structure of 1,1-diethynylsilacyclobutane (DESCB) by means of gas-phase electron diffraction and ab initio calculations. The structural refinement of the electron diffraction data yielded the following bond lengths ( ra) and bond angles (uncertainties are 3σ): r( Si- C)=1.874(2) Å, r( Si- C)=1.817(1) Å, r(- C C-)=1.209(1) Å, r( C- C)=1.563(2) Å, ∠(C-Si-C)=79.2(6)°, ∠(C-Si-C)=106.5(6)°. The geminal Si-CC moieties were found to be bent outwards by 3.1(15)° and the puckering angle was determined to be 30.0(15)°. The evidently short Si-C bond length, which was also reproduced by the ab initio calculations, could be rationalized as being the consequence of the electronic interaction between the outer π charges of the triple bond and the 3pπ orbitals at the silicon atom. It is also likely that the conjugation of the geminal ethynyl groups leads to an enhancement of this bond contraction. Electrostatic interactions and the subsequent reduction of the covalent radius of the silicon atom may also contribute to this bond shortening. It has been found that the endocyclic Si-C bond length fits nicely within a scheme describing a monotonous decrease of the Si-C bond length with the increase of the electronegativity of the substituent in various geminally substituted silacyclobutanes. A series of related silacyclobutanes and acyclic diethynylsilanes have been studied by applying various ab initio methods and their optimized structures were compared to the structure of DESCB. Among these compounds are 1,1-dicyanosilacyclobutane (DCYSCB), which is isoelectronic to DESCB, 1,1-diethynylcyclobutane (DECB) which is isovalent to DESCB, monoethynylsilacyclobutane (MESCB
NASA Astrophysics Data System (ADS)
Antušek, Andrej; Keḑziera, Dariusz; Jackowski, Karol; Jaszuński, Michał; Makulski, Włodzimierz
2008-09-01
New values of the indirect spin-spin coupling constants in CH 4, SiH 4 and GeH 4, derived from experiment and ab initio calculations, are reported. The new experimental values of 1J(CH), 1J(SiH) and 1J(GeH) are obtained from gas-phase NMR spectra. The dependence of the measured one-bond coupling constants on the density is analysed and the results are extrapolated to zero-density point to eliminate the effects due to intermolecular forces. In the calculation of the coupling constants, at the nonrelativistic level coupled cluster singles and doubles (CCSD) perturbation theory is used and the basis set convergence of the results is discussed. The relativistic corrections are estimated from Dirac-Hartree-Fock (DHF) calculations. The final theoretical values are obtained adding available estimates of the vibrational and temperature corrections. The agreement of the calculated and experimental 1J(XH), X = C, Si, Ge, constants is very satisfying, the differences are approximately 1-3%.
NASA Astrophysics Data System (ADS)
K. Ephraim, Babu; N., Murali; K. Vijaya, Babu; B. Kishore, Babu; Veeraiah, V.
2015-01-01
Ab initio calculations are performed on the electronic, structural, elastic and optical properties of the cubic per-ovskite KCdF3. The Kohn—Sham equations are solved by applying the full potential linearized augmented plane wave (FP-LAPW) method. The exchange correlation effects are included through the local density approximation (LDA), generalized gradient approximation (GGA) and modified Becke-Johnson (mBJ) exchange potential. The calculated lattice constant is in good agreement with the experimental result. The elastic properties such as elastic constants, anisotropy factor, shear modulus, Young's modulus and Poisson's ratio are calculated. KCdF3 is ductile and elastically anisotropic. The calculations of the electronic band structure, density of states (DOS) and charge density show that this compound has an indirect energy band gap (M—Γ) with a mixed ionic and covalent bonding. The contribution of the different bands is analyzed from the total and partial density of states curves. Optical response of the dielectric functions, optical reflectivity, absorption coefficient, real part of optical conductivity, refractive index, extinction coefficient and electron energy loss, are presented for the energy range of 0-40 eV. The compound KCdF3 can be used for high-frequency optical and optoelectronic devices.
NASA Astrophysics Data System (ADS)
Sana, P.; Tetzner, H.; Dabrowski, J.; Lupina, L.; Costina, I.; Thapa, S. B.; Storck, P.; Schroeder, T.; Zoellner, M. H.
2016-10-01
Growth mechanism of ScN on Sc2O3 for integration of Ga-polar GaN on Si(111) is investigated by in-situ X-ray photoemission spectroscopy, ex-situ time-of-flight secondary ion mass spectrometry, atomic force microscopy, and ab-initio density functional theory (DFT) calculations. The ScN films are grown by molecular beam epitaxy from e-beam evaporated Sc and N plasma. The films grow in a layer-by-layer (Frank-van der Merwe, FM) fashion. Diffusion of nitrogen into Sc2O3 and segregation of oxygen onto ScN are observed. The segregated O atoms are gradually removed from the surface by N atoms from the plasma. Experiment and theory show that nitrogen cannot be efficiently incorporated into Sc2O3 by exposing it to N plasma alone, and calculations indicate that anion intermixing between ScN and Sc2O3 should be weak. On the basis of ab-initio data, the in-diffusion of N into Sc2O3 is attributed mostly to the effect of interaction between ScN ad-dimers on the Sc2O3 surface in the initial stage of growth. The segregation of O to the ScN surface is understood as driven by the tendency to compensate build-up of the electric field in the polar ScN film. This segregation is computed to be energetically favorable (by 0.4 eV per O atom) already for a monolayer of ScN; the energy gain increases to 1.0 eV and 1.6 eV per O atom for two and three ScN layers, respectively. Finally, it is verified by DFT that the ScN deposition method in which Sc metallic film is deposited first and then nitridized would lead to strong incorporation of O into the grown film, accompanied by strong reduction of the Sc2O3 substrate.
Śmiałek, M A; Łabuda, M; Guthmuller, J; Hoffmann, S V; Jones, N C; MacDonald, M A; Zuin, L; Mason, N J; Limão-Vieira, P
2015-08-13
The highest resolution vacuum ultraviolet photoabsorption spectrum of isobutyl formate, C5H10O2, yet reported is presented over the energy range 4.5-10.7 eV (275.5-118.0 nm) revealing several new spectral features. Valence and Rydberg transitions and their associated vibronic series observed in the photoabsorption spectrum have been assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. Calculations have also been carried out to determine the ionization energies and fine structure of the lowest ionic state of isobutyl formate and are compared with a newly recorded photoelectron spectrum (from 9.0 to 27.0 eV). The value of the first ionization energy was determined to be 10.508 eV (adiabatic) and 10.837 eV (vertical). New vibrational structure is observed in the first photoelectron band, predominantly resulting from C-O and C═O stretches of the molecule. The photoabsorption cross sections have been used to calculate the photolysis lifetime of isobutyl formate in the upper stratosphere (20-50 km), indicating that the hydroxyl radical processes will be the main loss process for isobutyl formate.
Śmiałek, M A; Łabuda, M; Guthmuller, J; Hoffmann, S V; Jones, N C; MacDonald, M A; Zuin, L; Mason, N J; Limão-Vieira, P
2015-08-13
The highest resolution vacuum ultraviolet photoabsorption spectrum of isobutyl formate, C5H10O2, yet reported is presented over the energy range 4.5-10.7 eV (275.5-118.0 nm) revealing several new spectral features. Valence and Rydberg transitions and their associated vibronic series observed in the photoabsorption spectrum have been assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. Calculations have also been carried out to determine the ionization energies and fine structure of the lowest ionic state of isobutyl formate and are compared with a newly recorded photoelectron spectrum (from 9.0 to 27.0 eV). The value of the first ionization energy was determined to be 10.508 eV (adiabatic) and 10.837 eV (vertical). New vibrational structure is observed in the first photoelectron band, predominantly resulting from C-O and C═O stretches of the molecule. The photoabsorption cross sections have been used to calculate the photolysis lifetime of isobutyl formate in the upper stratosphere (20-50 km), indicating that the hydroxyl radical processes will be the main loss process for isobutyl formate. PMID:26176891
NASA Astrophysics Data System (ADS)
Karoui, K.; Ben Bechir, M.; Bulou, A.; Guidara, K.; Ben Rhaiem, A.
2016-06-01
The X-ray powder diffraction pattern shows that at room temperature, [N(CH3)3H]2CuCl4is crystallized in the monoclinic system with P21/c space group. The phase transitions at T1 ≈ 292/298 K, T2 ≈ 262/264 K and T3 ≈ 241 Khave been confirmed by the differential scanning calorimetry (DSC). Besides, the different phases have been studied by Raman scattering on single crystals, in the frequency range of 60-350 cm-1. The infrared and Raman spectra are attributed on the basis of ab initio calculations of the vibrational spectra. The optimized geometries, calculated vibrational frequencies, infrared intensities and Raman activities of [CuCl4]2- and [N(CH3)3H]+were calculated using the Hartree-Fock (HF) and density functional theory (B3LYP) with 3-21G, 6-31G(d) and 6-311G(d,p)basis sets. The temperature evolution of Raman spectra reveals the existence of phase transitions at T1, T2 and T3 and confirms their nature.
Rhyman, Lydia; Armata, Nerina; Ramasami, Ponnadurai; Dyke, John M
2012-06-14
The atmospherically relevant reactions between dimethyl selenide (DMSe) and the molecular halogens (X(2) = Cl(2), Br(2), and I(2)) have been studied with ab initio calculations at the MP2/aug-cc-pVDZ level of theory. Geometry optimization calculations showed that the reactions proceed from the reagents to the products (CH(3)SeCH(2)X + HX) via three minima, a van der Waals adduct (DMSe:X(2)), a covalently bound intermediate (DMSeX(2)), and a product-like complex (CH(3)SeCH(2)X:HX). The computed potential energy surfaces are used to predict what molecular species are likely to be observed in spectroscopic experiments such as gas-phase photoelectron spectroscopy and infrared matrix isolation spectroscopy. It is concluded that, for the reactions of DMSe with Cl(2) and Br(2), the covalent intermediate should be seen in spectroscopic experiments, whereas, in the DMSe + I(2) reaction, the van der Waals adduct DMSe:I(2) should be observed. Comparison is made with previous related calculations and experiments on dimethyl sulfide (DMS) with molecular halogens. The relevance of the results to atmospheric chemistry is discussed. The DMSeX(2) and DMSe:X(2) intermediates are likely to be reservoirs of molecular halogens in the atmosphere which will lead on photolysis to ozone depletion.
Mauney, Christopher; Lazzati, Davide; Buongiorno Nardelli, Marco
2015-02-10
The binding energies of n < 100 carbon clusters are calculated using the ab initio density functional theory code Quantum Espresso. Carbon cluster geometries are determined using several levels of classical techniques and further refined using density functional theory. The resulting energies are used to compute the work of cluster formation and the nucleation rate in a saturated, hydrogen-poor carbon gas. Compared to classical calculations that adopt the capillary approximation, we find that nucleation of carbon clusters is enhanced at low temperatures and depressed at high temperatures. This difference is ascribed to the different behavior of the critical cluster size. We find that the critical cluster size is at n = 27 or n = 8 for a broad range of temperatures and saturations, instead of being a smooth function of such parameters. The results of our calculations can be used to follow carbonaceous cluster/grain formation, stability, and growth in hydrogen-poor environments, such as the inner layers of core-collapse supernovae and supernova remnants.
NASA Astrophysics Data System (ADS)
Atanelov, J.; Mohn, P.
2015-09-01
In this study we present ab initio density-functional theory calculations on stoichiometric, cation-doped, and strained GaFeO 3 . We start with a detailed discussion of the origin of the antiferromagnetic (AFM) superexchange in stoichiometric GaFeO 3 and give a molecular orbital description of the exchange mechanism derived from our calculations. In addition, we study the properties of the Fe-O-Fe bonds for different geometries to underline the angle and distance dependence of the AFM coupling as formulated in the Goodenough-Kanamori rules. We describe the AFM ground state of GaFeO 3 as a result of two intrinsic Fe-O-Fe chains that meander through the crystal along the c direction. The magnetocrystalline anisotropy energies are calculated for the stoichiometric phase with and without inner cationic site disorder, and the presence of a sublattice-dependent anisotropy is examined. Furthermore, we perform our studies of Ga2 -xFe xO3 for varying Fe concentrations x (0.0 ≤x ≤2.0 ) where at a value of x =0.0 and x =2.0 it transforms into the isomorphic ɛ -Ga 2O3 and ɛ -Fe 2O3 phases, respectively. The effect of strain was also studied. Incorporating dopants and applying strain to the simulation cell changes the intrinsic geometry and thus the magnetic properties of gallium ferrite.
Pershina, V; Borschevsky, A; Eliav, E; Kaldor, U
2008-10-14
The interaction of the inert gases Rn and element 118 with various surfaces has been studied on the basis of fully relativistic ab initio Dirac-Coulomb CCSD(T) calculations of atomic properties. The calculated polarizability of element 118, 46.3 a.u., is the largest in group 18, the ionization potential is the lowest at 8.91 eV, and the estimated atomic radius is the largest, 4.55 a.u. These extreme values reflect, in addition to the general trends in the Periodic Table, the relativistic expansion and destabilization of the outer valence 7p(3/2) orbital. Van der Waals coefficients C(3) and adsorption enthalpies DeltaH(ads) of Ne through element 118 on noble metals and inert surfaces, such as quartz, ice, Teflon, and graphite, were calculated in a physisorption model using the atomic properties obtained. The C(3) coefficients were shown to steadily increase in group 18, while the increase in DeltaH(ads) from Ne to Rn does not continue to element 118: The large atomic radius of the latter element is responsible for a decrease in the interaction energy. We therefore predict that experimental distinction between Rn and 118 by adsorption on these types of surfaces will not be feasible. A possible candidate for separating the two elements is charcoal; further study is needed to test this possibility.
Ab-initio study of transition metal hydrides
Sharma, Ramesh; Shukla, Seema Dwivedi, Shalini Sharma, Yamini
2014-04-24
We have performed ab initio self consistent calculations based on Full potential linearized augmented plane wave (FP-LAPW) method to investigate the optical and thermal properties of yttrium hydrides. From the band structure and density of states, the optical absorption spectra and specific heats have been calculated. The band structure of Yttrium metal changes dramatically due to hybridization of Y sp orbitals with H s orbitals and there is a net charge transfer from metal to hydrogen site. The electrical resistivity and specific heats of yttrium hydrides are lowered but the thermal conductivity is slightly enhanced due to increase in scattering from hydrogen sites.
Lasoroski, Aurélie; Vuilleumier, Rodolphe; Pollet, Rodolphe
2014-07-07
The electronic relaxation of gadolinium complexes used as MRI contrast agents was studied theoretically by following the short time evolution of zero-field-splitting parameters. The statistical analysis of ab initio molecular dynamics trajectories provided a clear separation between static and transient contributions to the zero-field-splitting. For the latter, the correlation time was estimated at approximately 0.1 ps. The influence of the ligand was also probed by replacing one pendant arm of our reference macrocyclic complex by a bulkier phosphonate arm. In contrast to the transient contribution, the static zero-field-splitting was significantly influenced by this substitution.
NASA Astrophysics Data System (ADS)
de la Mora, Pablo; Cosio-Castañeda, Carlos; Martinez-Anaya, Oliver; Morales, Francisco; Tavizon, Gustavo
2016-09-01
In this work, a theoretical study of the electrical properties of the Bi2-ySryIr2O7 (Bi2-ySryIr2O16O2) α-pyrochlore-type solid solution is presented. Quantum ab initio DFT(WIEN2k) calculations were performed in order to understand the electrical resistivity changes associated to the Bi substitution by Sr in this system. The main crystallographic modification associated to this substitution is the x position of the 48f oxygen (x, 1/8, 1/8) (O1); this substitution substantially modifies the Bi/Sr-O1 and Ir-O1 atomic distances, increasing the former and diminishing the latter. Experimentally, the Bi2-ySryIr2O7 samples are metallic and the electrical resistivity increases with the Sr content. Electronic structure calculations for Bi2Ir2O7 and BiSrIr2O7 show that, regardless of structural changes, there is only a small change of electrical conductivity with the Sr substitution, and the experimentally observed increase of the resistivity can be explained in terms of a larger impact on the electronic structure of both; the Sr 'impurities' as well as of the thermal Sr oscillations.
Xue, H. T.; Tang, F. L.; Lu, W. J.; Li, X. K.; Zhang, Y.; Feng, Y. D.
2014-08-07
The phase diagram of the CuInSe{sub 2}-CuGaSe{sub 2} pseudobinary system was determined using a combination of special quasirandom structure approach, ab initio density functional theory calculations, and thermodynamic modelling. It is shown that the CuIn{sub 1−x}Ga{sub x}Se{sub 2} solution phase has a tendency to phase separation at low temperature. The calculated consolute temperature is 485 K. It is found that both the binodal and spinodal curves are significantly asymmetric and on both curves there are a local maximum and a local minimum, which have not been reported in the previous studies. Our phase diagram can well explain the finding that the inhomogeneity of CuIn{sub 0.25}Ga{sub 0.75}Se{sub 2} is higher than that of CuIn{sub 0.75}Ga{sub 0.25}Se{sub 2} at the same temperature, while the previous phase diagrams cannot. Hence, our phase diagram should be more reliable and applicable.
NASA Astrophysics Data System (ADS)
Gróf, M.; Gatial, A.; Milata, V.; Prónayová, N.; Kožíšek, J.; Breza, M.; Matějka, P.
2009-12-01
The IR, Raman and NMR spectra of 3- N, N-dimethylhydrazino-2-acetyl propenenitrile (DMHAP) [(H 3C) 2N sbnd NH sbnd CH dbnd C(CN)(COCH 3)] were measured. X-ray analysis revealed that DMHAP exists in solid state as ZZa conformer. Vibrational and NMR spectra confirmed the existence of only one ZZa conformer with an intramolecular hydrogen bond in less polar solvents and next two EZa and EZs conformers of E-isomer with Z-orientation of acetyl group and anti and syn orientation of dimethylhydrazino group in more polar environments. The observed IR and Raman bands were compared with harmonic vibrational frequencies, calculated using ab initio MP2 and DFT/B3LYP methods in 6-31G∗∗ basis set, and assigned on the basis of potential energy distribution. In addition, the geometries and relative energies of the possible isomers and conformers of DMHAP were also evaluated on the same levels and compared with the X-ray data. The influence of environment polarity on this conformational equilibrium is discussed with respect to the SCRF solvent effect calculations using IEFPCM model.