Ab initio Bogoliubov coupled cluster theory
NASA Astrophysics Data System (ADS)
Signoracci, Angelo; Hagen, Gaute; Duguet, Thomas
2014-09-01
Coupled cluster (CC) theory has become a standard method in nuclear theory for realistic ab initio calculations of medium mass nuclei, but remains limited by its requirement of a Slater determinant reference state which reasonably approximates the nuclear system of interest. Extensions of the method, such as equation-of-motion CC, permit the calculation of nuclei with one or two nucleons added or removed from a doubly magic core, yet still only a few dozen nuclei are accessible with modern computational restrictions. In order to extend the applicability of ab initio methods to open-shell systems, the superfluid nature of nuclei must be taken into account. By utilizing Bogoliubov algebra and employing spontaneous symmetry breaking with respect to particle number conservation, superfluid systems can be treated by a single reference state. An ab initio theory to include correlations on top of a Bogoliubov reference state has been developed in the guise of standard CC theory. The formalism and first results of this Bogoliubov coupled cluster theory will be presented to demonstrate the applicability of the method.
Ab initio quantum dynamics using coupled-cluster.
Kvaal, Simen
2012-05-21
The curse of dimensionality (COD) limits the current state-of-the-art ab initio propagation methods for non-relativistic quantum mechanics to relatively few particles. For stationary structure calculations, the coupled-cluster (CC) method overcomes the COD in the sense that the method scales polynomially with the number of particles while still being size-consistent and extensive. We generalize the CC method to the time domain while allowing the single-particle functions to vary in an adaptive fashion as well, thereby creating a highly flexible, polynomially scaling approximation to the time-dependent Schrödinger equation. The method inherits size-consistency and extensivity from the CC method. The method is dubbed orbital-adaptive time-dependent coupled-cluster, and is a hierarchy of approximations to the now standard multi-configurational time-dependent Hartree method for fermions. A numerical experiment is also given. PMID:22612082
Point defect modeling in materials: Coupling ab initio and elasticity approaches
NASA Astrophysics Data System (ADS)
Varvenne, Céline; Bruneval, Fabien; Marinica, Mihai-Cosmin; Clouet, Emmanuel
2013-10-01
Modeling point defects at an atomic scale requires careful treatment of the long-range atomic relaxations. This elastic field can strongly affect point defect properties calculated in atomistic simulations because of the finite size of the system under study. This is an important restriction for ab initio methods which are limited to a few hundred atoms. We propose an original approach coupling ab initio calculations and linear elasticity theory to obtain the properties of an isolated point defect for reduced supercell sizes. The reliability and benefit of our approach are demonstrated for three problematic cases: the self-interstitial in zirconium, clusters of self-interstitials in iron, and the neutral vacancy in silicon.
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. ).
Ab initio determination of effective electron-phonon coupling factor in copper
NASA Astrophysics Data System (ADS)
Ji, Pengfei; Zhang, Yuwen
2016-04-01
The electron temperature Te dependent electron density of states g (ε), Fermi-Dirac distribution f (ε), and electron-phonon spectral function α2 F (Ω) are computed as prerequisites before achieving effective electron-phonon coupling factor Ge-ph. The obtained Ge-ph is implemented into a molecular dynamics (MD) and two-temperature model (TTM) coupled simulation of femtosecond laser heating. By monitoring temperature evolutions of electron and lattice subsystems, the result utilizing Ge-ph from ab initio calculation shows a faster decrease of Te and increase of Tl than those using Ge-ph from phenomenological treatment. The approach of calculating Ge-ph and its implementation into MD-TTM simulation is applicable to other metals.
A coupled channel study of HN2 unimolecular decay based on a global ab initio potential surface
NASA Technical Reports Server (NTRS)
Koizumi, Hiroyasu; Schatz, George C.; Walch, Stephen P.
1991-01-01
The unimolecular decay lifetimes of several vibrational states of HN2 are determined on the basis of an accurate coupled channel dynamics study using a global analytical potential surface. The surface reproduces the ab initio points with an rms error of 0.08 kcal/mol for energies below 20 kcal/mol. Modifications to the potential that describe the effect of improving the basis set in the ab initio calculations are provided. Converged coupled channel calculations are performed for the ground rotational state of HN2 to determine the lifetimes of the lowest ten vibrational states. Only the ground vibrational state (000) and first excited bend (001) are found to have lifetimes longer than 1 ps. The lifetimes of these states are estimated at 3 x 10 to the -9th and 2 x 10 to the -10th s, respectively. Variation of these results with quality of the ab initio calculations is not more than a factor of 5.
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
Ab initio Mapping of Interlayer Coupling in Transition Metal Dichalcogenides and Graphene
NASA Astrophysics Data System (ADS)
Fang, Shiang; Kaxiras, Efthimios
Two-dimensional layered materials cover a wide variety of physics phenomena, such as topological phases, superconductivity, magnetism and charge density waves. Owing to the layered geometry and the van der Waals interactions in between, stacks of these van der Waals layered materials provide a venue to create a heterostructure with various physics properties. The interaction between different physics properties is particular interesting to engineer the material with the desired properties. One of the crucial ingredient in understanding the heterostructure is the interlayer coupling in between. In the literature, such kind of coupling has been proposed in various empirical forms. However, a true ab initio coupling model is still lacking. For the first time, here we have derived such interlayer coupling model from the first principle calculations based on the Wannier transformation of graphene stacks. We further investigate the Fermi velocity renormalization, van Hove singularities and the moire pattern for electron localization. Such microscopic understanding of the interlayer coupling would shed light on orbital hybridization and transport in multilayer stacks. This work was supported by the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319, and by ARO MURI Award No. W911NF-14-0247.
Harris, Travis V.; Morokuma, Keiji; Kurashige, Yuki; Yanai, Takeshi
2014-02-07
The applicability of ab initio multireference wavefunction-based methods to the study of magnetic complexes has been restricted by the quickly rising active-space requirements of oligonuclear systems and dinuclear complexes with S > 1 spin centers. Ab initio density matrix renormalization group (DMRG) methods built upon an efficient parameterization of the correlation network enable the use of much larger active spaces, and therefore may offer a way forward. Here, we apply DMRG-CASSCF to the dinuclear complexes [Fe{sub 2}OCl{sub 6}]{sup 2−} and [Cr{sub 2}O(NH{sub 3}){sub 10}]{sup 4+}. After developing the methodology through systematic basis set and DMRG M testing, we explore the effects of extended active spaces that are beyond the limit of conventional methods. We find that DMRG-CASSCF with active spaces including the metal d orbitals, occupied bridging-ligand orbitals, and their virtual double shells already capture a major portion of the dynamic correlation effects, accurately reproducing the experimental magnetic coupling constant (J) of [Fe{sub 2}OCl{sub 6}]{sup 2−} with (16e,26o), and considerably improving the smaller active space results for [Cr{sub 2}O(NH{sub 3}){sub 10}]{sup 4+} with (12e,32o). For comparison, we perform conventional MRCI+Q calculations and find the J values to be consistent with those from DMRG-CASSCF. In contrast to previous studies, the higher spin states of the two systems show similar deviations from the Heisenberg spectrum, regardless of the computational method.
An ab-initio coupled mode theory for near field radiative thermal transfer.
Chalabi, Hamidreza; Hasman, Erez; Brongersma, Mark L
2014-12-01
We investigate the thermal transfer between finite-thickness planar slabs which support surface phonon polariton modes (SPhPs). The thickness-dependent dispersion of SPhPs in such layered materials provides a unique opportunity to manipulate and enhance the near field thermal transfer. The key accomplishment of this paper is the development of an ab-initio coupled mode theory that accurately describes all of its thermal transfer properties. We illustrate how the coupled mode parameters can be obtained in a direct fashion from the dispersion relation of the relevant modes of the system. This is illustrated for the specific case of a semi-infinite SiC substrate placed in close proximity to a thin slab of SiC. This is a system that exhibits rich physics in terms of its thermal transfer properties, despite the seemingly simple geometry. This includes a universal scaling behavior of the thermal conductance with the slab thickness and spacing. The work highlights and further increases the value of coupled mode theories in rapidly calculating and intuitively understanding near-field transfer. PMID:25606933
Ab initio phonon coupling and optical response of hot electrons in plasmonic metals
NASA Astrophysics Data System (ADS)
Brown, Ana M.; Sundararaman, Ravishankar; Narang, Prineha; Goddard, William A.; Atwater, Harry A.
2016-08-01
Ultrafast laser measurements probe the nonequilibrium dynamics of excited electrons in metals with increasing temporal resolution. Electronic structure calculations can provide a detailed microscopic understanding of hot electron dynamics, but a parameter-free description of pump-probe measurements has not yet been possible, despite intensive research, because of the phenomenological treatment of electron-phonon interactions. We present ab initio predictions of the electron-temperature dependent heat capacities and electron-phonon coupling coefficients of plasmonic metals. We find substantial differences from free-electron and semiempirical estimates, especially in noble metals above transient electron temperatures of 2000 K, because of the previously neglected strong dependence of electron-phonon matrix elements on electron energy. We also present first-principles calculations of the electron-temperature dependent dielectric response of hot electrons in plasmonic metals, including direct interband and phonon-assisted intraband transitions, facilitating complete theoretical predictions of the time-resolved optical probe signatures in ultrafast laser experiments.
Ab initio Bogoliubov coupled cluster theory for open-shell nuclei
Signoracci, Angelo J.; Duguet, Thomas; Hagen, Gaute; Jansen, G. R.
2015-06-29
Background: Ab initio many-body methods have been developed over the past 10 yr to address closed-shell nuclei up to mass A≈130 on the basis of realistic two- and three-nucleon interactions. A current frontier relates to the extension of those many-body methods to the description of open-shell nuclei. Several routes to address open-shell nuclei are currently under investigation, including ideas that exploit spontaneous symmetry breaking. Purpose: Singly open-shell nuclei can be efficiently described via the sole breaking of U(1) gauge symmetry associated with particle-number conservation as a way to account for their superfluid character. While this route was recently followed within the framework of self-consistent Green's function theory, the goal of the present work is to formulate a similar extension within the framework of coupled cluster theory. Methods: We formulate and apply Bogoliubov coupled cluster (BCC) theory, which consists of representing the exact ground-state wave function of the system as the exponential of a quasiparticle excitation cluster operator acting on a Bogoliubov reference state. Equations for the ground-state energy and the cluster amplitudes are derived at the singles and doubles level (BCCSD) both algebraically and diagrammatically. The formalism includes three-nucleon forces at the normal-ordered two-body level. The first BCC code is implemented in m scheme, which will permit the treatment of doubly open-shell nuclei via the further breaking of SU(2) symmetry associated with angular momentum conservation. Results: Proof-of-principle calculations in an N_{max}=6 spherical harmonic oscillator basis for ^{16,18}O and ^{18}Ne in the BCCD approximation are in good agreement with standard coupled cluster results with the same chiral two-nucleon interaction, while ^{20}O and ^{20}Mg display underbinding relative to experiment. The breaking of U(1) symmetry, monitored by computing the variance
Ab initio Bogoliubov coupled cluster theory for open-shell nuclei
Signoracci, Angelo J.; Duguet, Thomas; Hagen, Gaute; Jansen, G. R.
2015-06-29
Background: Ab initio many-body methods have been developed over the past 10 yr to address closed-shell nuclei up to mass A≈130 on the basis of realistic two- and three-nucleon interactions. A current frontier relates to the extension of those many-body methods to the description of open-shell nuclei. Several routes to address open-shell nuclei are currently under investigation, including ideas that exploit spontaneous symmetry breaking. Purpose: Singly open-shell nuclei can be efficiently described via the sole breaking of U(1) gauge symmetry associated with particle-number conservation as a way to account for their superfluid character. While this route was recently followed withinmore » the framework of self-consistent Green's function theory, the goal of the present work is to formulate a similar extension within the framework of coupled cluster theory. Methods: We formulate and apply Bogoliubov coupled cluster (BCC) theory, which consists of representing the exact ground-state wave function of the system as the exponential of a quasiparticle excitation cluster operator acting on a Bogoliubov reference state. Equations for the ground-state energy and the cluster amplitudes are derived at the singles and doubles level (BCCSD) both algebraically and diagrammatically. The formalism includes three-nucleon forces at the normal-ordered two-body level. The first BCC code is implemented in m scheme, which will permit the treatment of doubly open-shell nuclei via the further breaking of SU(2) symmetry associated with angular momentum conservation. Results: Proof-of-principle calculations in an Nmax=6 spherical harmonic oscillator basis for 16,18O and 18Ne in the BCCD approximation are in good agreement with standard coupled cluster results with the same chiral two-nucleon interaction, while 20O and 20Mg display underbinding relative to experiment. The breaking of U(1) symmetry, monitored by computing the variance associated with the particle-number operator, is
NASA Astrophysics Data System (ADS)
Hagiwara, Yohsuke; Tateno, Masaru
2010-10-01
We review the recent research on the functional mechanisms of biological macromolecules using theoretical methodologies coupled to ab initio quantum mechanical (QM) treatments of reaction centers in proteins and nucleic acids. Since in most cases such biological molecules are large, the computational costs of performing ab initio calculations for the entire structures are prohibitive. Instead, simulations that are jointed with molecular mechanics (MM) calculations are crucial to evaluate the long-range electrostatic interactions, which significantly affect the electronic structures of biological macromolecules. Thus, we focus our attention on the methodologies/schemes and applications of jointed QM/MM calculations, and discuss the critical issues to be elucidated in biological macromolecular systems.
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
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.
Zabidi, Noriza Ahmad; Kassim, Hasan Abu; Shrivastava, Keshav N.
2008-05-20
Polonium is the only element with a simple cubic (sc) crystal structure. Atoms in solid polonium sit at the corners of a simple cubic unit cell and no where else. Polonium has a valence electron configuration 6s{sup 2}6p{sup 4} (Z = 84). The low temperature {alpha}-phase transforms into the rhombohedral (trigonal) {beta} structure at {approx}348 K. The sc {alpha}-Po unit cell constant is a = 3.345 A. The beta form of polonium ({beta}-Po) has the lattice parameters, a{sub R} = 3.359 A and a rhombohedral angle 98 deg. 13'. We have performed an ab initio electronic structure calculation by using the density functional theory. We have performed the calculation with and without spin-orbit (SO) coupling by using both the LDA and the GGA for the exchange-correlations. The k-points in a simple cubic BZ are determined by R (0.5, 0.5, 0.5), {gamma} (0, 0, 0), X (0.5, 0, 0), M (0.5, 0.5, 0) and {gamma} (0, 0, 0). Other directions of k-points are {gamma} (0, 0, 0), X (0.5, 0, 0), R (0.5, 0.5, 0.5) and {gamma} (0, 0, 0). The SO splittings of p states at the {gamma} point in the GGA+SO scheme for {alpha}-Po are 0.04 eV and 0.02 eV while for the {beta}-Po these are 0.03 eV and 0.97 eV. We have also calculated the vibrational spectra for the unit cells in both the structures. We find that exchanging of a Po atom by Pb atom produces several more bands and destabilizes the {beta} phase.
AB INITIO AND CALPHAD THERMODYNAMICS OF MATERIALS
Turchi, P A
2004-04-14
Ab initio electronic structure methods can supplement CALPHAD in two major ways for subsequent applications to stability in complex alloys. The first one is rather immediate and concerns the direct input of ab initio energetics in CALPHAD databases. The other way, more involved, is the assessment of ab initio thermodynamics {acute a} la CALPHAD. It will be shown how these results can be used within CALPHAD to predict the equilibrium properties of multi-component alloys.
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.
NASA Astrophysics Data System (ADS)
Roemelt, Michael
2015-07-01
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.
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. PMID:26233112
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.
NASA Astrophysics Data System (ADS)
Chalupský, Jakub; Yanai, Takeshi
2013-11-01
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.
Ab initio study of electron-phonon coupling in boron-doped SiC
NASA Astrophysics Data System (ADS)
Margine, E. R.; Blase, X.
2008-11-01
Density functional theory calculations have been used to study the electronic structure, lattice dynamics, and electron-phonon coupling in boron-doped silicon carbide in the cubic phase. Our results provide evidence that the recently discovered superconducting transition in boron-doped silicon carbide can be explained within a standard phonon-mediated mechanism. For the same doping rate, the coupling constant λ in B-doped SiC is very close to that of doped diamond and twice as large as that of B-doped silicon. However, doped silicon carbide differs from its diamond counterpart as most of the electron-phonon coupling originates from low energy vibrational modes.
Chemical trend of exchange coupling in diluted magnetic II-VI semiconductors: Ab initio calculations
NASA Astrophysics Data System (ADS)
Chanier, T.; Virot, F.; Hayn, R.
2009-05-01
We have calculated the chemical trend of magnetic exchange parameters ( Jdd , Nα , and Nβ ) of Zn-based II-VI semiconductors ZnA ( A=O , S, Se, and Te) doped with Co or Mn. We show that a proper treatment of electron correlations by the local spin-density approximation (LSDA)+U method leads to good agreement between experimental and theoretical values of the nearest-neighbor exchange coupling Jdd between localized 3d spins in contrast to the LSDA method. The exchange couplings between localized spins and doped electrons in the conduction band Nα are in good agreement with experiment as well. But the values for Nβ (coupling to doped holes in the valence band) indicate a crossover from weak coupling (for A=Te and Se) to strong coupling (for A=O ) and a localized hole state in ZnO:Mn. This hole localization explains the apparent discrepancy between photoemission and magneto-optical data for ZnO:Mn.
Ab-Initio Coupled-Cluster Study of {sup 16}O
Wloch, M.; Gour, J.R.; Kowalski, K.; Piecuch, P.; Dean, D.J.; Hjorth-Jensen, M.; Papenbrock, T.
2005-06-03
We report converged results for the ground and excited states and matter density of {sup 16}O using realistic two-body nucleon-nucleon interactions and coupled-cluster methods and algorithms developed in quantum chemistry. Most of the binding is obtained with the coupled-cluster singles and doubles approach. Additional binding due to three-body clusters (triples) is minimal. The coupled-cluster method with singles and doubles provides a good description of the matter density, charge radius, charge form factor, and excited states of a one-particle, one-hole nature, but it cannot describe the first-excited 0{sup +} state. Incorporation of triples has no effect on the latter finding.
Ab initio analysis of electron-phonon coupling in molecular devices.
Sergueev, N; Roubtsov, D; Guo, Hong
2005-09-30
We report a first principles analysis of electron-phonon coupling in molecular devices under external bias voltage and during current flow. Our theory and computational framework are based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism. Using a molecular tunnel junction of a 1,4-benzenedithiolate molecule contacted by two aluminum leads as an example, we analyze which molecular vibrational modes are most relevant to charge transport under nonequilibrium conditions. We find that the low-lying modes are most important. As a function of bias voltage, the electron-phonon coupling strength can change drastically while the vibrational spectrum changes at a few percent level. PMID:16241682
Extensive ab initio study of the electronic states of S2 molecule including spin-orbit coupling
NASA Astrophysics Data System (ADS)
Xing, Wei; Shi, Deheng; Sun, Jinfeng; Liu, Hui; Zhu, Zunlue
2013-03-01
The potential energy curves (PECs) of 15 Λ-S states and 24 Ω states generated from the 13 Λ-S bound states of the S2 molecule are investigated in detail using an ab initio quantum chemical method. The PECs are calculated for internuclear separations from 0.12 to 1.10 nm by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification (MRCI + Q). The spin-orbit (SO) coupling effect is accounted for by the Breit-Pauli Hamiltonian. To discuss the effect on the energy splitting by the core-electron correlations, the all-electron basis set, cc-pCVTZ with and without 2s2p correlations, is used for the SO coupling calculations of the A3 ? and B‧3Πg Λ-S states since their measurements can be found in the literature. By comparison, the cc-pCVTZ basis set with 2s2p correlations is chosen for the SO coupling calculations of 13 Λ-S bound states. To improve the quality of PECs, core-valence correlation and scalar relativistic corrections are included. Scalar relativistic correction calculations are made using the third-order Douglas-Kroll Hamiltonian (DKH3) approximation at the level of a cc-pV5Z basis set. Core-valence correlation corrections are taken into account with a cc-pCVTZ basis set. The spectroscopic parameters of 13 Λ-S bound states and 24 Ω states are calculated. With the PECs obtained by the MRCI + Q/aug-cc-pV6Z + CV + DK + SO calculations, the SO coupling splitting energies are 379.25 cm-1 between the A‧3 and A‧2 Ω state, 83.40 cm-1 between the A1 and A0- Ω state and 210.91 cm-1 between the B‧2 and B‧1 Ω state, which agree well with the corresponding measurements of 383, 77.51 and 209 cm-1, respectively. Moreover, other spectroscopic parameters are also in excellent agreement with the measurements. It demonstrates that the spectroscopic parameters of 24 Ω states reported here for the first time can be expected to be
Ab initio quantum chemistry: Methodology and applications
Friesner, Richard A.
2005-01-01
This Perspective provides an overview of state-of-the-art ab initio quantum chemical methodology and applications. The methods that are discussed include coupled cluster theory, localized second-order Moller–Plesset perturbation theory, multireference perturbation approaches, and density functional theory. The accuracy of each approach for key chemical properties is summarized, and the computational performance is analyzed, emphasizing significant advances in algorithms and implementation over the past decade. Incorporation of a condensed-phase environment by means of mixed quantum mechanical/molecular mechanics or self-consistent reaction field techniques, is presented. A wide range of illustrative applications, focusing on materials science and biology, are discussed briefly. PMID:15870212
Ab Initio Coupled-Cluster Approach to Nuclear Structure with Modern Nucleon-Nucleon Interactions
Hagen, Gaute; Papenbrock, T.; Dean, David Jarvis; Hjorth-Jensen, M.
2010-01-01
We perform coupled-cluster calculations for the doubly magic nuclei ^{4}He, ^{16}O, and ^{40,48}Ca for neutron-rich isotopes of oxygen and fluorine, and employ "bare" and secondary renormalized nucleon-nucleon interactions. For the nucleon-nucleon interaction from chiral effective field theory at order next-to-next-to-next-to leading order, we find that the coupled-cluster approximation including triples corrections binds nuclei within 0.4 MeV per nucleon compared to data. We employ interactions from a resolution-scale dependent similarity renormalization group transformation and assess the validity of power-counting estimates in medium-mass nuclei. We find that the missing contributions due to three-nucleon forces are consistent with these estimates. For the unitary correlator model potential, we find a slow convergence with respect to increasing the size of the model space. For the G-matrix approach, we find a weak dependence of ground-state energies on the starting energy combined with a rather slow convergence with respect to increasing model spaces. We also analyze the center-of-mass problem and present a practical and efficient solution.
Ab initio study of vibrational anharmonic coupling effects in oligo(para-phenylenes)
NASA Astrophysics Data System (ADS)
Heimel, G.; Somitsch, D.; Knoll, P.; Zojer, E.
2002-06-01
In this work, we clarify the nature of a previously not precisely identified Fermi dyad in the frequency range around 1600 cm-1 in oligo( para-phenylenes). To this end, we deploy a novel method to calculate third order anharmonic coupling effects in molecules. This Fermi dyad is shown to yield important information on the structural properties of the investigated materials. The nature of all vibrations contributing to this quantum mechanical resonance phenomenon is explained on the basis of a detailed normal coordinate analysis. The anharmonic coupling is then closely investigated by applying our theoretical model. In particular, we discuss the intensity redistribution among the two components of the Fermi doublet as well as their energetic separation. Subsequently, we establish a relation between these features and the structural conformation of the molecules. We show, how oligomer length and planarity of these systems can be determined from experimental Raman spectra by extracting the positions and relative intensities of the two components of the Fermi doublet. Furthermore, this Fermi resonance is shown to be sensitive to chemical modification on the molecules such as deuteration or substitution. Finally, we extend our model to electronically excited states in this class of molecules, as well as to charged species.
Ab initio ballistic conductance with spin-orbit coupling: Application to monoatomic wires
NASA Astrophysics Data System (ADS)
Dal Corso, Andrea; Smogunov, Alexander; Tosatti, Erio
2006-07-01
The approach proposed by Choi and Ihm [Phys. Rev. B 59, 2267 (1999)] for calculating the ballistic conductance of open quantum systems within the Landauer-Büttiker approach is generalized to fully relativistic ultrasoft pseudopotentials, enabling it to deal with ballistic transport in the presence of spin-orbit coupling. As a test case, we present the complex k -vector electronic structure of a perfect monoatomic nonmagnetic Pt wire, and the ballistic conductance of a toy nanocontact model consisting of the same Pt nanowire with one stretched bond. By comparing the fully relativistic and the scalar relativistic results, it is seen that the relative importance of spin-orbit effects can be quite large.
Extensive ab initio study of the electronic states of BSe radical including spin-orbit coupling.
Liu, Siyuan; Zhai, Hongsheng; Liu, Yufang
2016-06-01
The internally contracted multi-reference configuration interaction method (MRCI) with Davidson modification and the Douglas-Kroll scalar relativistic correction has been used to calculate the BSe molecule at the level of aug-cc-pV5Z basis set. The calculated electronic states, including 9 doublet and 6 quartet Λ-S states, are correlated to the dissociation limit of B((2)P(u))+Se((3)P(g)) and B((2)P(u))+Se((1)D(g)). The Spin-orbit coupling (SOC) interaction is taken into account via the state interaction approach with the full Breit-Pauli Hamiltonian operator, which causes the entire 15 Λ-S states to split into 32Ω states. This is the first time that the spin-orbit coupling calculation has been carried out on BSe. The potential energy curves of the Λ-S and Ω electronic states are depicted with the aid of the avoided crossing rule between electronic states of the same symmetry. The spectroscopic constants of the bound Λ-S and Ω states were determined, which are in good agreement with the experimental data. The transition dipole moments (TDMs) and the Franck-Condon factors (FCs) of the transitions from the low-lying bound Ω states A(2)Π(I)3/2, B(2)Π(I)1/2 and C(2)Δ(I)3/2 to the ground state X(2)Σ(+)1/2 have also been presented. Based on the previous calculations, the radiative lifetimes of the A(2)Π(I)3/2, B(2)Π(I)1/2 and C(2)Δ(I)3/2 were evaluated. PMID:26999315
Extensive ab initio study of the electronic states of BSe radical including spin-orbit coupling
NASA Astrophysics Data System (ADS)
Liu, Siyuan; Zhai, Hongsheng; Liu, Yufang
2016-06-01
The internally contracted multi-reference configuration interaction method (MRCI) with Davidson modification and the Douglas-Kroll scalar relativistic correction has been used to calculate the BSe molecule at the level of aug-cc-pV5Z basis set. The calculated electronic states, including 9 doublet and 6 quartet Λ-S states, are correlated to the dissociation limit of B(2Pu) + Se(3Pg) and B(2Pu) + Se(1Dg). The Spin-orbit coupling (SOC) interaction is taken into account via the state interaction approach with the full Breit-Pauli Hamiltonian operator, which causes the entire 15 Λ-S states to split into 32 Ω states. This is the first time that the spin-orbit coupling calculation has been carried out on BSe. The potential energy curves of the Λ-S and Ω electronic states are depicted with the aid of the avoided crossing rule between electronic states of the same symmetry. The spectroscopic constants of the bound Λ-S and Ω states were determined, which are in good agreement with the experimental data. The transition dipole moments (TDMs) and the Franck-Condon factors (FCs) of the transitions from the low-lying bound Ω states A2Π(I)3/2, B2Π(I)1/2 and C2Δ(I)3/2 to the ground state X2Σ+1/2 have also been presented. Based on the previous calculations, the radiative lifetimes of the A2Π(I)3/2, B2Π(I)1/2 and C2Δ(I)3/2 were evaluated.
Renner-Teller nonadiabatic coupling terms: An ab-initio study of the HNH molecule
NASA Astrophysics Data System (ADS)
Halász, G. J.; Vibók, Á.; Baer, R.; Baer, M.
2006-02-01
In this Communication we present the first theoretical/numerical treatment of nonadiabatic coupling terms (NACT) that originate from the Renner-Teller (RT) model, namely, those that follow from the splitting of an electronic level of a linear molecule when it becomes bent. These two newly formed states are characterized by different symmetries and are designated as A and B. Our main findings: (1) The RT NACTs are quantized as long as they are calculated close enough to collinear configuration of the molecule (in this case HNH). Their value is τ =1 (the Jahn-Teller values in similar situations, are τ =1/2). (2) Calculation of RT NACTs at bent configurations (i.e., at a distance from the linear axis) yield decreased values, sometimes by more than 50%. This last finding implies that in strongly bent configurations the two-state Hilbert subspace (formed by the above mentioned A and B states) is affected by upper states, most likely via Jahn-Teller conical intersections. (3) This study has also important practical implications. The fact that the RT NACTs decrease in (strongly) bent situations implies that analyzing spectroscopic data employing only the two Π-states may not be sufficient in order to achieve the required accuracy.
Collective rotation from ab initio theory
NASA Astrophysics Data System (ADS)
Caprio, M. A.; Maris, P.; Vary, J. P.; Smith, R.
2015-08-01
Through ab initio approaches in nuclear theory, we may now seek to quantitatively understand the wealth of nuclear collective phenomena starting from the underlying internucleon interactions. No-core configuration interaction (NCCI) calculations for p-shell nuclei give rise to rotational bands, as evidenced by rotational patterns for excitation energies, electromagnetic moments and electromagnetic transitions. In this review, NCCI calculations of 7-9Be are used to illustrate and explore ab initio rotational structure, and the resulting predictions for rotational band properties are compared with experiment. We highlight the robustness of ab initio rotational predictions across different choices for the internucleon interaction.
Ab initio nuclear structure theory
NASA Astrophysics Data System (ADS)
Negoita, Gianina Alina
Ab initio no core methods have become major tools for understanding the properties of light nuclei based on realistic nucleon-nucleon (NN) and three-nucleon (NNN) interactions. A brief description is provided for the inter-nucleon interactions that fit two-body scattering and bound state data, as well as NNN interactions. Major new progress, including the goal of applying these interactions to solve for properties of nuclei, is limited by convergence issues. That is, with the goal of obtaining high precision solutions of the nuclear many-body Hamiltonian with no core methods (all nucleons treated on the same footing), one needs to proceed to very large basis spaces to achieve a convergence pattern suitable for extrapolation to the exact result. This thesis investigates (1) the similarity renormalization group (SRG) approach to soften the interaction, while preserving its phase shift properties, and (2) adoption of a realistic basis space using Woods-Saxon (WS) single-particle wavefunctions. Both have their advantages and limitations, discussed here. For (1), SRG was demonstrated by applying it to a realistic NN interaction, JISP16, in a harmonic oscillator (HO) representation. The degree of interaction softening achieved through a regulator parameter is examined. For (2), new results are obtained with the realistic JISP16 NN interaction in ab initio calculations of light nuclei 4He, 6He and 12C, using a WS basis optimized to minimize the ground-state energy within the truncated no core shell model. These are numerically-intensive many-body calculations. Finally, to gain insight into the potential for no core investigations of heavier nuclei, an initial investigation was obtained for the odd mass A = 47 - 49 region nuclei straddling 48Ca. The motivation for selecting these nuclei stems from the aim of preparing for nuclear double beta-decay studies of 48Ca. In these heavier systems, phenomenological additions to the realistic NN interaction determined by previous
Ab initio phonon limited transport
NASA Astrophysics Data System (ADS)
Verstraete, Matthieu
We revisit the thermoelectric (TE) transport properties of two champion materials, PbTe and SnSe, using fully first principles methods. In both cases the performance of the material is due to subtle combinations of structural effects, scattering, and phase space reduction. In PbTe anharmonic effects are completely opposite to the predicted quasiharmonic evolution of phonon frequencies and to frequently (and incorrectly) cited extrapolations of experiments. This stabilizes the material at high T, but also tends to enhance its thermal conductivity, in a non linear manner, above 600 Kelvin. This explains why PbTe is in practice limited to room temperature applications. SnSe has recently been shown to be the most efficient TE material in bulk form. This is mainly due to a strongly enhanced carrier concentration and electrical conductivity, after going through a phase transition from 600 to 800 K. We calculate the transport coefficients as well as the defect concentrations ab initio, showing excellent agreement with experiment, and elucidating the origin of the double phase transition as well as the new charge carriers. AH Romero, EKU Gross, MJ Verstraete, and O Hellman PRB 91, 214310 (2015) O. Hellman, IA Abrikosov, and SI Simak, PRB 84 180301 (2011)
Ab initio non-relativistic spin dynamics
Ding, Feizhi; Goings, Joshua J.; Li, Xiaosong; Frisch, Michael J.
2014-12-07
Many magnetic materials do not conform to the (anti-)ferromagnetic paradigm where all electronic spins are aligned to a global magnetization axis. Unfortunately, most electronic structure methods cannot describe such materials with noncollinear electron spin on account of formally requiring spin alignment. To overcome this limitation, it is necessary to generalize electronic structure methods and allow each electron spin to rotate freely. Here, we report the development of an ab initio time-dependent non-relativistic two-component spinor (TDN2C), which is a generalization of the time-dependent Hartree-Fock equations. Propagating the TDN2C equations in the time domain allows for the first-principles description of spin dynamics. A numerical tool based on the Hirshfeld partitioning scheme is developed to analyze the time-dependent spin magnetization. In this work, we also introduce the coupling between electron spin and a homogenous magnetic field into the TDN2C framework to simulate the response of the electronic spin degrees of freedom to an external magnetic field. This is illustrated for several model systems, including the spin-frustrated Li{sub 3} molecule. Exact agreement is found between numerical and analytic results for Larmor precession of hydrogen and lithium atoms. The TDN2C method paves the way for the ab initio description of molecular spin transport and spintronics in the time domain.
Ab initio non-relativistic spin dynamics
NASA Astrophysics Data System (ADS)
Ding, Feizhi; Goings, Joshua J.; Frisch, Michael J.; Li, Xiaosong
2014-12-01
Many magnetic materials do not conform to the (anti-)ferromagnetic paradigm where all electronic spins are aligned to a global magnetization axis. Unfortunately, most electronic structure methods cannot describe such materials with noncollinear electron spin on account of formally requiring spin alignment. To overcome this limitation, it is necessary to generalize electronic structure methods and allow each electron spin to rotate freely. Here, we report the development of an ab initio time-dependent non-relativistic two-component spinor (TDN2C), which is a generalization of the time-dependent Hartree-Fock equations. Propagating the TDN2C equations in the time domain allows for the first-principles description of spin dynamics. A numerical tool based on the Hirshfeld partitioning scheme is developed to analyze the time-dependent spin magnetization. In this work, we also introduce the coupling between electron spin and a homogenous magnetic field into the TDN2C framework to simulate the response of the electronic spin degrees of freedom to an external magnetic field. This is illustrated for several model systems, including the spin-frustrated Li3 molecule. Exact agreement is found between numerical and analytic results for Larmor precession of hydrogen and lithium atoms. The TDN2C method paves the way for the ab initio description of molecular spin transport and spintronics in the time domain.
Li, Zi; Li, Chuanying; Wang, Cong; Zhang, Ping; Kang, Wei
2015-11-15
Ultrafast laser experiments on metals usually induce a high electron temperature and a low ion temperature and, thus, an energy relaxation process. The electron heat capacity and electron-phonon coupling factor are crucial thermal quantities to describe this process. We perform ab initio theoretical studies to determine these thermal quantities and their dependence on density and electron temperature for the metals aluminum and beryllium. The heat capacity shows an approximately linear dependence on the temperature, similar to free electron gas, and the compression only slightly affects the capacity. The electron-phonon coupling factor increases with both temperature and density, and the change observed for beryllium is more obvious than that for aluminum. The connections between thermal quantities and electronic/atomic structures are discussed in detail, and the different behaviors of aluminum and beryllium are well explained.
NASA Astrophysics Data System (ADS)
Li, Zi; Wang, Cong; Kang, Wei; Li, Chuanying; Zhang, Ping
2015-11-01
Ultrafast laser experiments on metals usually induce a high electron temperature and a low ion temperature and, thus, an energy relaxation process. The electron heat capacity and electron-phonon coupling factor are crucial thermal quantities to describe this process. We perform ab initio theoretical studies to determine these thermal quantities and their dependence on density and electron temperature for the metals aluminum and beryllium. The heat capacity shows an approximately linear dependence on the temperature, similar to free electron gas, and the compression only slightly affects the capacity. The electron-phonon coupling factor increases with both temperature and density, and the change observed for beryllium is more obvious than that for aluminum. The connections between thermal quantities and electronic/atomic structures are discussed in detail, and the different behaviors of aluminum and beryllium are well explained.
Kubas, Adam; Blumberger, Jochen; Hoffmann, Felix; Heck, Alexander; Elstner, Marcus; Oberhofer, Harald
2014-03-14
We introduce a database (HAB11) of electronic coupling matrix elements (H{sub ab}) for electron transfer in 11 π-conjugated organic homo-dimer cations. High-level ab inito calculations at the multireference configuration interaction MRCI+Q level of theory, n-electron valence state perturbation theory NEVPT2, and (spin-component scaled) approximate coupled cluster model (SCS)-CC2 are reported for this database to assess the performance of three DFT methods of decreasing computational cost, including constrained density functional theory (CDFT), fragment-orbital DFT (FODFT), and self-consistent charge density functional tight-binding (FODFTB). We find that the CDFT approach in combination with a modified PBE functional containing 50% Hartree-Fock exchange gives best results for absolute H{sub ab} values (mean relative unsigned error = 5.3%) and exponential distance decay constants β (4.3%). CDFT in combination with pure PBE overestimates couplings by 38.7% due to a too diffuse excess charge distribution, whereas the economic FODFT and highly cost-effective FODFTB methods underestimate couplings by 37.6% and 42.4%, respectively, due to neglect of interaction between donor and acceptor. The errors are systematic, however, and can be significantly reduced by applying a uniform scaling factor for each method. Applications to dimers outside the database, specifically rotated thiophene dimers and larger acenes up to pentacene, suggests that the same scaling procedure significantly improves the FODFT and FODFTB results for larger π-conjugated systems relevant to organic semiconductors and DNA.
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
NASA Astrophysics Data System (ADS)
Li, Wu
2015-08-01
We demonstrate the ab initio electrical transport calculation limited by electron-phonon coupling by using the full solution of the Boltzmann transport equation (BTE), which applies equally to metals and semiconductors. Numerical issues are emphasized in this work. We show that the simple linear interpolation of the electron-phonon coupling matrix elements from a relatively coarse grid to an extremely fine grid can ease the calculational burden, which makes the calculation feasible in practice. For the Brillouin zone (BZ) integration of the transition probabilities involving one δ function, the Gaussian smearing method with a physical choice of locally adaptive broadening parameters is employed. We validate the calculation in the cases of n -type Si and Al. The calculated conductivity and mobility are in good agreement with experiments. In the metal case we also demonstrate that the Gaussian smearing method with locally adaptive broadening parameters works excellently for the BZ integration with double δ functions involved in the Eliashberg spectral function and its transport variant. The simpler implementation is the advantage of the Gaussian smearing method over the tetrahedron method. The accuracy of the relaxation time approximation and the approximation made by Allen [Phys. Rev. B 17, 3725 (1978), 10.1103/PhysRevB.17.3725] has been examined by comparing with the exact solution of BTE. We also apply our method to n -type monolayer MoS2, for which a mobility of 150 cm2 v-1 s-1 is obtained at room temperature. Moreover, the mean free paths are less than 9 nm, indicating that in the presence of grain boundaries the mobilities should not be effectively affected if the grain boundary size is tens of nanometers or larger. The ab initio approach demonstrated in this paper can be directly applied to other materials without the need for any a priori knowledge about the electron-phonon scattering processes, and can be straightforwardly extended to study cases with
Ab initio infrared and Raman spectra
NASA Astrophysics Data System (ADS)
Fredkin, Donald R.; Komornicki, Andrew; White, Steven R.; Wilson, Kent R.
1983-06-01
We discuss several ways in which molecular absorption and scattering spectra can be computed ab initio, from the fundamental constants of nature. These methods can be divided into two general categories. In the first, or sequential, type of approach, one first solves the electronic part of the Schrödinger equation in the Born-Oppenheimer approximation, mapping out the potential energy, dipole moment vector (for infrared absorption) and polarizability tensor (for Raman scattering) as functions of nuclear coordinates. Having completed the electronic part of the calculation, one then solves the nuclear part of the problem either classically or quantum mechanically. As an example of the sequential ab initio approach, the infrared and Raman rotational and vibrational-rotational spectral band contours for the water molecule are computed in the simplest rigid rotor, normal mode approximation. Quantum techniques are used to calculate the necessary potential energy, dipole moment, and polarizability information at the equilibrium geometry. A new quick, accurate, and easy to program classical technique involving no reference to Euler angles or special functions is developed to compute the infrared and Raman band contours for any rigid rotor, including asymmetric tops. A second, or simultaneous, type of ab initio approach is suggested for large systems, particularly those for which normal mode analysis is inappropriate, such as liquids, clusters, or floppy molecules. Then the curse of dimensionality prevents mapping out in advance the complete potential, dipole moment, and polarizability functions over the whole space of nuclear positions of all atoms, and a solution in which the electronic and nuclear parts of the Born-Oppenheimer approximation are simultaneously solved is needed. A quantum force classical trajectory (QFCT) molecular dynamic method, based on linear response theory, is described, in which the forces, dipole moment, and polarizability are computed quantum
Ab initio two-component Ehrenfest dynamics
NASA Astrophysics Data System (ADS)
Ding, Feizhi; Goings, Joshua J.; Liu, Hongbin; Lingerfelt, David B.; Li, Xiaosong
2015-09-01
We present an ab initio two-component Ehrenfest-based mixed quantum/classical molecular dynamics method to describe the effect of nuclear motion on the electron spin dynamics (and vice versa) in molecular systems. The two-component time-dependent non-collinear density functional theory is used for the propagation of spin-polarized electrons while the nuclei are treated classically. We use a three-time-step algorithm for the numerical integration of the coupled equations of motion, namely, the velocity Verlet for nuclear motion, the nuclear-position-dependent midpoint Fock update, and the modified midpoint and unitary transformation method for electronic propagation. As a test case, the method is applied to the dissociation of H2 and O2. In contrast to conventional Ehrenfest dynamics, this two-component approach provides a first principles description of the dynamics of non-collinear (e.g., spin-frustrated) magnetic materials, as well as the proper description of spin-state crossover, spin-rotation, and spin-flip dynamics by relaxing the constraint on spin configuration. This method also holds potential for applications to spin transport in molecular or even nanoscale magnetic devices.
Ab initio two-component Ehrenfest dynamics
Ding, Feizhi; Goings, Joshua J.; Liu, Hongbin; Lingerfelt, David B.; Li, Xiaosong
2015-09-21
We present an ab initio two-component Ehrenfest-based mixed quantum/classical molecular dynamics method to describe the effect of nuclear motion on the electron spin dynamics (and vice versa) in molecular systems. The two-component time-dependent non-collinear density functional theory is used for the propagation of spin-polarized electrons while the nuclei are treated classically. We use a three-time-step algorithm for the numerical integration of the coupled equations of motion, namely, the velocity Verlet for nuclear motion, the nuclear-position-dependent midpoint Fock update, and the modified midpoint and unitary transformation method for electronic propagation. As a test case, the method is applied to the dissociation of H{sub 2} and O{sub 2}. In contrast to conventional Ehrenfest dynamics, this two-component approach provides a first principles description of the dynamics of non-collinear (e.g., spin-frustrated) magnetic materials, as well as the proper description of spin-state crossover, spin-rotation, and spin-flip dynamics by relaxing the constraint on spin configuration. This method also holds potential for applications to spin transport in molecular or even nanoscale magnetic devices.
Collective rotation from ab initio theory
NASA Astrophysics Data System (ADS)
Caprio, Mark A.; Maris, Pieter; Vary, James P.
2015-10-01
The challenge of ab initio nuclear theory is to quantitatively predict the complex and highly-correlated behavior of the nuclear many-body system, starting from the underlying internucleon interactions. We may now seek to understand the wealth of nuclear collective phenomena through ab initio approaches. No-core configuration interaction (NCCI) calculations for p-shell nuclei give rise to rotational bands, as evidenced by rotational patterns for excitation energies, electromagnetic moments, and electromagnetic transitions. In this talk, the intrinsic structure of these bands is discussed, and the predicted rotational bands are compared to experiment. Supported by the US DOE under Award Nos. DE-FG02-95ER-40934, DESC0008485 (SciDAC/NUCLEI), and DE-FG02-87ER40371 and the US NSF under Award No. 0904782. Computational resources provided by NERSC (US DOE Contract No. DE-AC02-05CH11231).
Ab initio infrared and Raman spectra
NASA Technical Reports Server (NTRS)
Fredkin, D. R.; White, S. R.; Wilson, K. R.; Komornicki, A.
1983-01-01
It is pointed out that with increased computer power and improved computational techniques, such as the gradients developed in recent years, it is becoming practical to compute spectra ab initio, from the fundamental constants of nature, for systems of increasing complexity. The present investigation has the objective to explore several possible ab initio approaches to spectra, giving particular attention to infrared and nonresonance Raman. Two approaches are discussed. The sequential approach, in which first the electronic part and then later the nuclear part of the Born-Oppenheimer approximation is solved, is appropriate for small systems. The simultaneous approach, in which the electronic and nuclear parts are solved at the same time, is more appropriate for many-atom systems. A review of the newer quantum gradient techniques is provided, and the infrared and Raman spectral band contours for the water molecule are computed.
NASA Astrophysics Data System (ADS)
Thiessen, P. A.; Treder, H.-J.
Jedes initium wird durch experimenta crucis zum eventus. Jedes theoretisch interpretierbare ex-eventu-Resultat führt auf ein neues Initium. Gerade dies ist die gemeinsame Aussage von Atomistik, Quantenmechanik und Relativitätstheorie.Translated AbstractAb initio vel ex eventu. IIEvery initium becomes an eventus by experimenta crucis. Every theoretically interpretable ex-eventu result leads to a new initium. Right this is the joint assertion of atomism, quantum mechanics, and relativity.
Spin-orbit decomposition of ab initio nuclear wave functions
NASA Astrophysics Data System (ADS)
Johnson, Calvin W.
2015-03-01
Although the modern shell-model picture of atomic nuclei is built from single-particle orbits with good total angular momentum j , leading to j -j coupling, decades ago phenomenological models suggested that a simpler picture for 0 p -shell nuclides can be realized via coupling of the total spin S and total orbital angular momentum L . I revisit this idea with large-basis, no-core shell-model calculations using modern ab initio two-body interactions and dissect the resulting wave functions into their component L - and S -components. Remarkably, there is broad agreement with calculations using the phenomenological Cohen-Kurath forces, despite a gap of nearly 50 years and six orders of magnitude in basis dimensions. I suggest that L -S decomposition may be a useful tool for analyzing ab initio wave functions of light nuclei, for example, in the case of rotational bands.
Ab Initio Computation of the Energies of Circular Quantum Dots
Lohne, M. Pedersen; Hagen, Gaute; Hjorth-Jensen, M.; Kvaal, S.; Pederiva, F.
2011-01-01
We perform coupled-cluster and diffusion Monte Carlo calculations of the energies of circular quantum dots up to 20 electrons. The coupled-cluster calculations include triples corrections and a renormalized Coulomb interaction defined for a given number of low-lying oscillator shells. Using such a renormalized Coulomb interaction brings the coupled-cluster calculations with triples correlations in excellent agreement with the diffusion Monte Carlo calculations. This opens up perspectives for doing ab initio calculations for much larger systems of electrons.
Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics
Makhov, Dmitry V.; Shalashilin, Dmitrii V.; Glover, William J.; Martinez, Todd J.
2014-08-07
We present a new algorithm for ab initio quantum nonadiabatic molecular dynamics that combines the best features of ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) methods. In this new method, ab initio multiple cloning (AIMC), the individual trajectory basis functions (TBFs) follow Ehrenfest equations of motion (as in MCE). However, the basis set is expanded (as in AIMS) when these TBFs become sufficiently mixed, preventing prolonged evolution on an averaged potential energy surface. We refer to the expansion of the basis set as “cloning,” in analogy to the “spawning” procedure in AIMS. This synthesis of AIMS and MCE allows us to leverage the benefits of mean-field evolution during periods of strong nonadiabatic coupling while simultaneously avoiding mean-field artifacts in Ehrenfest dynamics. We explore the use of time-displaced basis sets, “trains,” as a means of expanding the basis set for little cost. We also introduce a new bra-ket averaged Taylor expansion (BAT) to approximate the necessary potential energy and nonadiabatic coupling matrix elements. The BAT approximation avoids the necessity of computing electronic structure information at intermediate points between TBFs, as is usually done in saddle-point approximations used in AIMS. The efficiency of AIMC is demonstrated on the nonradiative decay of the first excited state of ethylene. The AIMC method has been implemented within the AIMS-MOLPRO package, which was extended to include Ehrenfest basis functions.
Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics
NASA Astrophysics Data System (ADS)
Makhov, Dmitry V.; Glover, William J.; Martinez, Todd J.; Shalashilin, Dmitrii V.
2014-08-01
We present a new algorithm for ab initio quantum nonadiabatic molecular dynamics that combines the best features of ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) methods. In this new method, ab initio multiple cloning (AIMC), the individual trajectory basis functions (TBFs) follow Ehrenfest equations of motion (as in MCE). However, the basis set is expanded (as in AIMS) when these TBFs become sufficiently mixed, preventing prolonged evolution on an averaged potential energy surface. We refer to the expansion of the basis set as "cloning," in analogy to the "spawning" procedure in AIMS. This synthesis of AIMS and MCE allows us to leverage the benefits of mean-field evolution during periods of strong nonadiabatic coupling while simultaneously avoiding mean-field artifacts in Ehrenfest dynamics. We explore the use of time-displaced basis sets, "trains," as a means of expanding the basis set for little cost. We also introduce a new bra-ket averaged Taylor expansion (BAT) to approximate the necessary potential energy and nonadiabatic coupling matrix elements. The BAT approximation avoids the necessity of computing electronic structure information at intermediate points between TBFs, as is usually done in saddle-point approximations used in AIMS. The efficiency of AIMC is demonstrated on the nonradiative decay of the first excited state of ethylene. The AIMC method has been implemented within the AIMS-MOLPRO package, which was extended to include Ehrenfest basis functions.
Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics.
Makhov, Dmitry V; Glover, William J; Martinez, Todd J; Shalashilin, Dmitrii V
2014-08-01
We present a new algorithm for ab initio quantum nonadiabatic molecular dynamics that combines the best features of ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) methods. In this new method, ab initio multiple cloning (AIMC), the individual trajectory basis functions (TBFs) follow Ehrenfest equations of motion (as in MCE). However, the basis set is expanded (as in AIMS) when these TBFs become sufficiently mixed, preventing prolonged evolution on an averaged potential energy surface. We refer to the expansion of the basis set as "cloning," in analogy to the "spawning" procedure in AIMS. This synthesis of AIMS and MCE allows us to leverage the benefits of mean-field evolution during periods of strong nonadiabatic coupling while simultaneously avoiding mean-field artifacts in Ehrenfest dynamics. We explore the use of time-displaced basis sets, "trains," as a means of expanding the basis set for little cost. We also introduce a new bra-ket averaged Taylor expansion (BAT) to approximate the necessary potential energy and nonadiabatic coupling matrix elements. The BAT approximation avoids the necessity of computing electronic structure information at intermediate points between TBFs, as is usually done in saddle-point approximations used in AIMS. The efficiency of AIMC is demonstrated on the nonradiative decay of the first excited state of ethylene. The AIMC method has been implemented within the AIMS-MOLPRO package, which was extended to include Ehrenfest basis functions. PMID:25106573
Ab initio no core full configuration approach for light nuclei
NASA Astrophysics Data System (ADS)
Kim, Youngman; Shin, Ik Jae; Maris, Pieter; Vary, James P.; Forssén, Christian; Rotureau, Jimmy
2014-07-01
Comprehensive understanding of the structure and reactions of light nuclei poses theoretical and computational challenges. Still, a number of ab initio approaches have been developed to calculate the properties of atomic nuclei using fundamental interactions among nucleons. Among them, we work with the ab initio no core full configuration (NCFC) method and ab initio no core Gamow Shell Model (GSM). We first review these approaches and present some recent results.
Ab initio no core full configuration approach for light nuclei
NASA Astrophysics Data System (ADS)
Kim, Youngman; Shin, Ik Jae; Maris, Pieter; Vary, James P.; Forssén, Christian; Rotureau, Jimmy
2015-10-01
Comprehensive understanding of the structure and reactions of light nuclei poses theoretical and computational challenges. Still, a number of ab initio approaches have been developed to calculate the properties of atomic nuclei using fundamental interactions among nucleons. Among them, we work with the ab initio no core full configuration (NCFC) method and ab initio no core Gamow Shell Model (GSM). We first review these approaches and present some recent results.
Mukherjee, Saikat; Bandyopadhyay, Sudip; Paul, Amit Kumar; Adhikari, Satrajit
2013-04-25
We present the molecular symmetry (MS) adapted treatment of nonadiabatic coupling terms (NACTs) for the excited electronic states (2(2)E' and 1(2)A1') of Na3 cluster, where the adiabatic potential energy surfaces (PESs) and the NACTs are calculated at the MRCI level by using an ab initio quantum chemistry package (MOLPRO). The signs of the NACTs at each point of the configuration space (CS) are determined by employing appropriate irreducible representations (IREPs) arising due to MS group, and such terms are incorporated into the adiabatic to diabatic transformation (ADT) equations to obtain the ADT angles. Since those sign corrected NACTs and the corresponding ADT angles demonstrate the validity of curl condition for the existence of three-state (2(2)E' and 1(2)A1') sub-Hilbert space, it becomes possible to construct the continuous, single-valued, symmetric, and smooth 3 × 3 diabatic Hamiltonian matrix. Finally, nuclear dynamics has been carried out on such diabatic surfaces to explore whether our MS-based treatment of diabatization can reproduce the pattern of the experimental spectrum for system B of Na3 cluster. PMID:23521047
Yamamoto, Takeshi; Kato, Shigeki
2007-06-14
In quantum-mechanical/molecular-mechanical (QM/MM) treatment of chemical reactions in condensed phases, one solves the electronic Schrodinger equation for the solute (or an active site) under the electrostatic field from the environment. This Schrodinger equation depends parametrically on the solute nuclear coordinates R and the external electrostatic potential V. This fact suggests that one may use R and V as natural collective coordinates for describing the entire system, where V plays the role of collective solvent variables. In this paper such an (R,V) representation of the QM/MM canonical ensemble is described, with particular focus on how to treat charge transfer processes in this representation. As an example, the above method is applied to the proton-coupled electron transfer of a ubiquinol analog with phenoxyl radical in acetonitrile solvent. Ab initio free-energy surfaces are calculated as functions of R and V using the reference interaction site model self-consistent field method, the equilibrium points and the minimum free-energy crossing point are located in the (R,V) space, and then the kinetic isotope effects (KIEs) are evaluated approximately. The results suggest that a stiffer proton potential at the transition state may be responsible for unusual KIEs observed experimentally for related systems. PMID:17581070
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 melting curve of osmium
NASA Astrophysics Data System (ADS)
Burakovsky, L.; Burakovsky, N.; Preston, D. L.
2015-11-01
The melting curve of osmium up to a pressure P of 500 GPa is obtained from an extensive suite of ab initio quantum molecular dynamics (QMD) simulations using the Z method. The ab initio P =0 melting point of Os is 3370 ±75 K; this range encompasses all of the available data in the literature and corroborates the conclusion of J. W. Arblaster [Platinum Metals Rev. 49, 166 (2005)], 10.1595/147106705X70264 that the melting temperature of pure Os is 3400 ±50 K and that the 3300 K typically quoted in the literature is the melting point of impure Os. The T =0 equation of state (EOS) of Os and the P dependence of the optimized c /a ratio for the hexagonal unit cell, both to pressures ˜900 GPa, are obtained in the ab initio approach as validation of its use. Although excellent agreement with the available experimental data (P ≲80 GPa) is found, it is the third-order Birch-Murnaghan EOS with B0'=5 rather than the more widely accepted B0'=4 that describes the QMD data to higher pressures, in agreement with the more recent experimental EOS by Godwal et al. The theoretical melting curve of Os obtained earlier by Joshi et al. is shown to be inconsistent with our QMD results, and the possible reason for this discrepancy is suggested. Regularities in the melting curves of Os and five other third-row transition metals (Ta, W, Re, Pt, Au) could be used to estimate the currently unknown melting curves of Hf and Ir.
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 Neutron Drops with Chiral Hamiltonians
NASA Astrophysics Data System (ADS)
Potter, Hugh; Maris, Pieter; Vary, James
2015-04-01
Ab initio calculations for neutron drops are of interest for insights into neutron-rich nuclei and neutron star matter, and for examining the neutron-only sector of nucleon-nucleon and 3-nucleon interactions. I present ab initio results calculated using the no-core shell model with 2- and 3-body chiral Hamiltonians for neutron drops up to 20 neutrons confined in a 10 MeV harmonic trap. I discuss ground state energies, internal energies, radii, and evidence for pairing. In addition, excitation energies can be used to investigate the spin-orbit splittings in the p-shell and sd -shell. Prior Green's Function Monte Carlo calculations using the Argonne v8' potential with added 3-nucleon forces serve as a comparison. Supported by DOE Grants DESC0008485 (SciDAC/NUCLEI), DE-FG02-87ER40371, and NSF Grant 0904782; computational resources provided by the Oak Ridge Leadership Computing Facility (DOE Office of Science Contract DE-AC05-00OR22725) under an INCITE award.
Barrett, B R; Navratil, P; Vary, J P
2011-04-11
A long-standing goal of nuclear theory is to determine the properties of atomic nuclei based on the fundamental interactions among the protons and neutrons (i.e., nucleons). By adopting nucleon-nucleon (NN), three-nucleon (NNN) and higher-nucleon interactions determined from either meson-exchange theory or QCD, with couplings fixed by few-body systems, we preserve the predictive power of nuclear theory. This foundation enables tests of nature's fundamental symmetries and offers new vistas for the full range of complex nuclear phenomena. Basic questions that drive our quest for a microscopic predictive theory of nuclear phenomena include: (1) What controls nuclear saturation; (2) How the nuclear shell model emerges from the underlying theory; (3) What are the properties of nuclei with extreme neutron/proton ratios; (4) Can we predict useful cross sections that cannot be measured; (5) Can nuclei provide precision tests of the fundamental laws of nature; and (6) Under what conditions do we need QCD to describe nuclear structure, among others. Along with other ab initio nuclear theory groups, we have pursued these questions with meson-theoretical NN interactions, such as CD-Bonn and Argonne V18, that were tuned to provide high-quality descriptions of the NN scattering phase shifts and deuteron properties. We then add meson-theoretic NNN interactions such as the Tucson-Melbourne or Urbana IX interactions. More recently, we have adopted realistic NN and NNN interactions with ties to QCD. Chiral perturbation theory within effective field theory ({chi}EFT) provides us with a promising bridge between QCD and hadronic systems. In this approach one works consistently with systems of increasing nucleon number and makes use of the explicit and spontaneous breaking of chiral symmetry to expand the strong interaction in terms of a dimensionless constant, the ratio of a generic small momentum divided by the chiral symmetry breaking scale taken to be about 1 GeV/c. The resulting NN
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)
Henderson, Thomas M.; Cave, Robert J.
1998-11-01
Specific solvent effects on the electronic coupling element for electron transfer are examined using two model donor-acceptor systems (Zn2+ and Li2+) and several model "solvent" species (He, Ne, H2O, and NH3). The effects are evaluated relative to the given donor-acceptor pair without solvent present. The electronic coupling element (Hab) is found to depend strongly on the identity of the intervening solvent, with He atoms decreasing Hab, whereas H2O and NH3 significantly increase Hab. The distance dependence (essentially exponential decay) is weakly affected by a single intervening solvent atom-molecule. However, when the donor-acceptor distance increases in concert with addition of successively greater numbers of solvent species, the decay with distance of Hab is altered appreciably. Effects due to varying the orientation of molecular solvent are found, somewhat surprisingly, to be quite modest.
Li, Xia; Zhang, Xiaomei; Yan, Bing
2015-05-01
Ab initio calculations have been performed on the low-lying excited and ground states of PH(+). The potential energy curves (PECs) of the Λ-S states were calculated with multi-reference configuration interaction (MRCI) method along with the basis sets at 5-ξ level. In order to improve the PECs, the Davidson(+Q) correction and the Scalar relativistic effect are included. The corresponding spectroscopic constants were determined and good agreements with the available measurement were found. The interactions of the A(2)Δ-(4)Π and 1(2)Σ(+)-(4)Π by the spin-orbit coupling (SOC) effect were well described by the spin-orbit matrix elements. The SOC effect makes the original 8 Λ-S states split into 15Ω states. The Ω=1/2 state generated from the X(2)Π state is confirmed to the ground Ω state. And the SOC splitting for the X(2)Π is calculated to be 294cm(-1). The SOC effect has large effect on the PECs of the A(2)Δ and 1(2)Σ(+) states, leading to much more shallow potential wells as well as potential barriers. The analysis of the wavefunction for the Ω states shows that the strong spin-orbit interaction exists near the crossing points of the PECs for the Λ-S states. The transition dipole moments (TDMs) of transitions A(2)Δ-X(2)Π and 1(2)Σ(-)-X(2)Π are evaluated with the MRCI wavefunction. Based on the TDMs along with the calculated Franck-Condon factors, the radiative lifetimes for the selected vibrational levels of A(2)Δ and 1(2)Σ(-) states are predicted at the microseconds (μs). Good agreement with the measurement shows that the lowest vibrational level for A(2)Δ state is almost uninfluenced by the perturbation via the SOC effect. PMID:25688687
NASA Astrophysics Data System (ADS)
Li, Xia; Zhang, Xiaomei; Yan, Bing
2015-05-01
Ab initio calculations have been performed on the low-lying excited and ground states of PH+. The potential energy curves (PECs) of the Λ-S states were calculated with multi-reference configuration interaction (MRCI) method along with the basis sets at 5-ξ level. In order to improve the PECs, the Davidson(+Q) correction and the Scalar relativistic effect are included. The corresponding spectroscopic constants were determined and good agreements with the available measurement were found. The interactions of the A2Δ-4Π and 12Σ+-4Π by the spin-orbit coupling (SOC) effect were well described by the spin-orbit matrix elements. The SOC effect makes the original 8 Λ-S states split into 15 Ω states. The Ω = 1/2 state generated from the X2Π state is confirmed to the ground Ω state. And the SOC splitting for the X2Π is calculated to be 294 cm-1. The SOC effect has large effect on the PECs of the A2Δ and 12Σ+ states, leading to much more shallow potential wells as well as potential barriers. The analysis of the wavefunction for the Ω states shows that the strong spin-orbit interaction exists near the crossing points of the PECs for the Λ-S states. The transition dipole moments (TDMs) of transitions A2Δ-X2Π and 12Σ--X2Π are evaluated with the MRCI wavefunction. Based on the TDMs along with the calculated Franck-Condon factors, the radiative lifetimes for the selected vibrational levels of A2Δ and 12Σ- states are predicted at the microseconds (μs). Good agreement with the measurement shows that the lowest vibrational level for A2Δ state is almost uninfluenced by the perturbation via the SOC effect.
Strong electron-phonon coupling in Be{1-x}B{2}C{2}: ab initio studies
NASA Astrophysics Data System (ADS)
Moudden, A. H.
2008-07-01
Several structures for off-stoichiometric beryllium diboride dicarbide Be{1-x}B2C2 have been designed, and their properties studied from first-principles density functional methods. Among the most stable phases examined, the layered hexagonal structures are shown to exhibit various features in the electronic properties and in the lattice dynamics reminiscent of the superconducting magnesium diboride and alkaline earth-intercalated graphites. For substoichiometric composition x˜ 1/3, the system is found metallic with a moderately strong electron-phonon coupling through a predominant contribution arising from high frequency streching modes modulating the σ-bonding of the B C network, and a weaker contribution at medium frequency range of the phonon spectra, arising from the intercalent motion coupled to the π-bonding states. Further, anharmonicities emerging from the proximity of the Fermi level to the σ-band edge, contributes to reduce the phonon softening hence stabilizing the structure. All these effects appear to combine favourably to produce a high temperature phonon-superconductivity.
Liu, Cong; Assary, Rajeev S; Curtiss, Larry A
2014-06-26
Upgrading furan and small oxygenates obtained from the decomposition of cellulosic materials via formation of carbon-carbon bonds is critical to effective conversion of biomass to liquid transportation fuels. Simulation-driven molecular level understanding of carbon-carbon bond formation is required to design efficient catalysts and processes. Accurate quantum chemical methods are utilized here to predict the reaction energetics for conversion of furan (C4H4O) to C5-C8 ethers and the transformation of furfural (C5H6O2) to C13-C26 alkanes. Furan can be coupled with various C1 to C4 low molecular weight carbohydrates obtained from the pyrolysis via Diels-Alder type reactions in the gas phase to produce C5-C8 cyclic ethers. The computed reaction barriers for these reactions (∼25 kcal/mol) are lower than the cellulose activation or decomposition reactions (∼50 kcal/mol). Cycloaddition of C5-C8 cyclo ethers with furans can also occur in the gas phase, and the computed activation energy is similar to that of the first Diels-Alder reaction. Furfural, obtained from biomass, can be coupled with aldehydes or ketones with α-hydrogen atoms to form longer chain aldol products, and these aldol products can undergo vapor phase hydrocycloaddition (activation barrier of ∼20 kcal/mol) to form the precursors of C26 cyclic hydrocarbons. These thermochemical studies provide the basis for further vapor phase catalytic studies required for upgrading of furans/furfurals to longer chain hydrocarbons. PMID:24902118
Ab initio study of neutron drops with chiral Hamiltonians
NASA Astrophysics Data System (ADS)
Potter, H. D.; Fischer, S.; Maris, P.; Vary, J. P.; Binder, S.; Calci, A.; Langhammer, J.; Roth, R.
2014-12-01
We report ab initio calculations for neutron drops in a 10 MeV external harmonic-oscillator trap using chiral nucleon-nucleon plus three-nucleon interactions. We present total binding energies, internal energies, radii and odd-even energy differences for neutron numbers N = 2- 18 using the no-core shell model with and without importance truncation. Furthermore, we present total binding energies for N = 8 , 16 , 20 , 28 , 40 , 50 obtained in a coupled-cluster approach. Comparisons with quantum Monte Carlo results, where available, using Argonne v8‧ with three-nucleon interactions reveal important dependences on the chosen Hamiltonian.
Guiding ab initio calculations by alchemical derivatives.
to Baben, M; Achenbach, J O; von Lilienfeld, O A
2016-03-14
We assess the concept of alchemical transformations for predicting how a further and not-tested change in composition would change materials properties. This might help to guide ab initio calculations through multidimensional property-composition spaces. Equilibrium volumes, bulk moduli, and relative lattice stability of fcc and bcc 4d transition metals Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag are calculated using density functional theory. Alchemical derivatives predict qualitative trends in lattice stability while equilibrium volumes and bulk moduli are predicted with less than 9% and 28% deviation, respectively. Predicted changes in equilibrium volume and bulk moduli for binary and ternary mixtures of Rh-Pd-Ag are in qualitative agreement even for predicted bulk modulus changes as large as +100% or -50%. Based on these results, it is suggested that alchemical transformations could be meaningful for enhanced sampling in the context of virtual high-throughput materials screening projects. PMID:26979677
Ab initio determination of light hadron masses.
Dürr, S; Fodor, Z; Frison, J; Hoelbling, C; Hoffmann, R; Katz, S D; Krieg, S; Kurth, T; Lellouch, L; Lippert, T; Szabo, K K; Vulvert, G
2008-11-21
More than 99% of the mass of the visible universe is made up of protons and neutrons. Both particles are much heavier than their quark and gluon constituents, and the Standard Model of particle physics should explain this difference. We present a full ab initio calculation of the masses of protons, neutrons, and other light hadrons, using lattice quantum chromodynamics. Pion masses down to 190 mega-electron volts are used to extrapolate to the physical point, with lattice sizes of approximately four times the inverse pion mass. Three lattice spacings are used for a continuum extrapolation. Our results completely agree with experimental observations and represent a quantitative confirmation of this aspect of the Standard Model with fully controlled uncertainties. PMID:19023076
Discovering chemistry with an ab initio nanoreactor
Wang, Lee-Ping; Titov, Alexey; McGibbon, Robert; Liu, Fang; Pande, Vijay S.; Martínez, Todd J.
2014-11-02
Chemical understanding is driven by the experimental discovery of new compounds and reactivity, and is supported by theory and computation that provides detailed physical insight. While theoretical and computational studies have generally focused on specific processes or mechanistic hypotheses, recent methodological and computational advances harken the advent of their principal role in discovery. Here we report the development and application of the ab initio nanoreactor – a highly accelerated, first-principles molecular dynamics simulation of chemical reactions that discovers new molecules and mechanisms without preordained reaction coordinates or elementary steps. Using the nanoreactor we show new pathways for glycine synthesis frommore » primitive compounds proposed to exist on the early Earth, providing new insight into the classic Urey-Miller experiment. Ultimately, these results highlight the emergence of theoretical and computational chemistry as a tool for discovery in addition to its traditional role of interpreting experimental findings.« less
Ab initio molar volumes and Gaussian radii.
Parsons, Drew F; Ninham, Barry W
2009-02-12
Ab initio molar volumes are calculated and used to derive radii for ions and neutral molecules using a spatially diffuse model of the electron distribution with Gaussian spread. The Gaussian radii obtained can be used for computation of nonelectrostatic ion-ion dispersion forces that underlie Hofmeister specific ion effects. Equivalent hard-sphere radii are also derived, and these are in reasonable agreement with crystalline ionic radii. The Born electrostatic self-energy is derived for a Gaussian model of the electronic charge distribution. It is shown that the ionic volumes used in electrostatic calculations of strongly hydrated cosmotropic ions ought best to include the first hydration shell. Ionic volumes for weakly hydrated chaotropic metal cations should exclude electron overlap (in electrostatic calculations). Spherical radii are calculated as well as nonisotropic ellipsoidal radii for nonspherical ions, via their nonisotropic static polarizability tensors. PMID:19140766
Discovering chemistry with an ab initio nanoreactor
NASA Astrophysics Data System (ADS)
Martinez, Todd
Traditional approaches for modeling chemical reaction networks such as those involved in combustion have focused on identifying individual reactions and using theoretical approaches to explore the underlying mechanisms. Recent advances involving graphical processing units (GPUs), commodity products developed for the videogaming industry, have made it possible to consider a distinct approach wherein one attempts to discover chemical reactions and mechanisms. We provide a brief summary of these developments and then discuss the concept behind the ``ab initio nanoreactor'' which explores the space of possible chemical reactions and molecular species for a given stoichiometry. The nanoreactor concept is exemplified with an example to the Urey-Miller reaction network which has been previously advanced as a potential model for prebiotic chemistry. We briefly discuss some of the future directions envisioned for the development of this nanoreactor concept.
Discovering chemistry with an ab initio nanoreactor
Wang, Lee-Ping; Titov, Alexey; McGibbon, Robert; Liu, Fang; Pande, Vijay S.; Martínez, Todd J.
2014-01-01
Chemical understanding is driven by the experimental discovery of new compounds and reactivity, and is supported by theory and computation that provides detailed physical insight. While theoretical and computational studies have generally focused on specific processes or mechanistic hypotheses, recent methodological and computational advances harken the advent of their principal role in discovery. Here we report the development and application of the ab initio nanoreactor – a highly accelerated, first-principles molecular dynamics simulation of chemical reactions that discovers new molecules and mechanisms without preordained reaction coordinates or elementary steps. Using the nanoreactor we show new pathways for glycine synthesis from primitive compounds proposed to exist on the early Earth, providing new insight into the classic Urey-Miller experiment. These results highlight the emergence of theoretical and computational chemistry as a tool for discovery in addition to its traditional role of interpreting experimental findings. PMID:25411881
Guiding ab initio calculations by alchemical derivatives
NASA Astrophysics Data System (ADS)
to Baben, M.; Achenbach, J. O.; von Lilienfeld, O. A.
2016-03-01
We assess the concept of alchemical transformations for predicting how a further and not-tested change in composition would change materials properties. This might help to guide ab initio calculations through multidimensional property-composition spaces. Equilibrium volumes, bulk moduli, and relative lattice stability of fcc and bcc 4d transition metals Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag are calculated using density functional theory. Alchemical derivatives predict qualitative trends in lattice stability while equilibrium volumes and bulk moduli are predicted with less than 9% and 28% deviation, respectively. Predicted changes in equilibrium volume and bulk moduli for binary and ternary mixtures of Rh-Pd-Ag are in qualitative agreement even for predicted bulk modulus changes as large as +100% or -50%. Based on these results, it is suggested that alchemical transformations could be meaningful for enhanced sampling in the context of virtual high-throughput materials screening projects.
Discovering chemistry with an ab initio nanoreactor.
Wang, Lee-Ping; Titov, Alexey; McGibbon, Robert; Liu, Fang; Pande, Vijay S; Martínez, Todd J
2014-12-01
Chemical understanding is driven by the experimental discovery of new compounds and reactivity, and is supported by theory and computation that provide detailed physical insight. Although theoretical and computational studies have generally focused on specific processes or mechanistic hypotheses, recent methodological and computational advances harken the advent of their principal role in discovery. Here we report the development and application of the ab initio nanoreactor--a highly accelerated first-principles molecular dynamics simulation of chemical reactions that discovers new molecules and mechanisms without preordained reaction coordinates or elementary steps. Using the nanoreactor, we show new pathways for glycine synthesis from primitive compounds proposed to exist on the early Earth, which provide new insight into the classic Urey-Miller experiment. These results highlight the emergence of theoretical and computational chemistry as a tool for discovery, in addition to its traditional role of interpreting experimental findings. PMID:25411881
Discovering chemistry with an ab initio nanoreactor
NASA Astrophysics Data System (ADS)
Wang, Lee-Ping; Titov, Alexey; McGibbon, Robert; Liu, Fang; Pande, Vijay S.; Martínez, Todd J.
2014-12-01
Chemical understanding is driven by the experimental discovery of new compounds and reactivity, and is supported by theory and computation that provide detailed physical insight. Although theoretical and computational studies have generally focused on specific processes or mechanistic hypotheses, recent methodological and computational advances harken the advent of their principal role in discovery. Here we report the development and application of the ab initio nanoreactor—a highly accelerated first-principles molecular dynamics simulation of chemical reactions that discovers new molecules and mechanisms without preordained reaction coordinates or elementary steps. Using the nanoreactor, we show new pathways for glycine synthesis from primitive compounds proposed to exist on the early Earth, which provide new insight into the classic Urey-Miller experiment. These results highlight the emergence of theoretical and computational chemistry as a tool for discovery, in addition to its traditional role of interpreting experimental findings.
Ab Initio Modeling of Molecular Radiation
NASA Technical Reports Server (NTRS)
Jaffe, Richard; Schwenke, David
2014-01-01
Radiative emission from excited states of atoms and molecules can comprise a significant fraction of the total heat flux experienced by spacecraft during atmospheric entry at hypersonic speeds. For spacecraft with ablating heat shields, some of this radiative flux can be absorbed by molecular constituents in the boundary layer that are formed by the ablation process. Ab initio quantum mechanical calculations are carried out to predict the strengths of these emission and absorption processes. This talk will describe the methods used in these calculations using, as examples, the 4th positive emission bands of CO and the 1g+ 1u+ absorption in C3. The results of these calculations are being used as input to NASA radiation modeling codes like NeqAir, HARA and HyperRad.
Discovering chemistry with an ab initio nanoreactor
Wang, Lee-Ping; Titov, Alexey; McGibbon, Robert; Liu, Fang; Pande, Vijay S.; Martínez, Todd J.
2014-11-02
Chemical understanding is driven by the experimental discovery of new compounds and reactivity, and is supported by theory and computation that provides detailed physical insight. While theoretical and computational studies have generally focused on specific processes or mechanistic hypotheses, recent methodological and computational advances harken the advent of their principal role in discovery. Here we report the development and application of the ab initio nanoreactor – a highly accelerated, first-principles molecular dynamics simulation of chemical reactions that discovers new molecules and mechanisms without preordained reaction coordinates or elementary steps. Using the nanoreactor we show new pathways for glycine synthesis from primitive compounds proposed to exist on the early Earth, providing new insight into the classic Urey-Miller experiment. Ultimately, these results highlight the emergence of theoretical and computational chemistry as a tool for discovery in addition to its traditional role of interpreting experimental findings.
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 alpha-alpha scattering
NASA Astrophysics Data System (ADS)
Elhatisari, Serdar; Lee, Dean; Rupak, Gautam; Epelbaum, Evgeny; Krebs, Hermann; Lähde, Timo A.; Luu, Thomas; Meißner, Ulf-G.
2015-12-01
Processes such as the scattering of alpha particles (4He), the triple-alpha reaction, and alpha capture play a major role in stellar nucleosynthesis. In particular, alpha capture on carbon determines the ratio of carbon to oxygen during helium burning, and affects subsequent carbon, neon, oxygen, and silicon burning stages. It also substantially affects models of thermonuclear type Ia supernovae, owing to carbon detonation in accreting carbon-oxygen white-dwarf stars. In these reactions, the accurate calculation of the elastic scattering of alpha particles and alpha-like nuclei—nuclei with even and equal numbers of protons and neutrons—is important for understanding background and resonant scattering contributions. First-principles calculations of processes involving alpha particles and alpha-like nuclei have so far been impractical, owing to the exponential growth of the number of computational operations with the number of particles. Here we describe an ab initio calculation of alpha-alpha scattering that uses lattice Monte Carlo simulations. We use lattice effective field theory to describe the low-energy interactions of protons and neutrons, and apply a technique called the ‘adiabatic projection method’ to reduce the eight-body system to a two-cluster system. We take advantage of the computational efficiency and the more favourable scaling with system size of auxiliary-field Monte Carlo simulations to compute an ab initio effective Hamiltonian for the two clusters. We find promising agreement between lattice results and experimental phase shifts for s-wave and d-wave scattering. The approximately quadratic scaling of computational operations with particle number suggests that it should be possible to compute alpha scattering and capture on carbon and oxygen in the near future. The methods described here can be applied to ultracold atomic few-body systems as well as to hadronic systems using lattice quantum chromodynamics to describe the interactions of
Ab initio alpha-alpha scattering.
Elhatisari, Serdar; Lee, Dean; Rupak, Gautam; Epelbaum, Evgeny; Krebs, Hermann; Lähde, Timo A; Luu, Thomas; Meißner, Ulf-G
2015-12-01
Processes such as the scattering of alpha particles ((4)He), the triple-alpha reaction, and alpha capture play a major role in stellar nucleosynthesis. In particular, alpha capture on carbon determines the ratio of carbon to oxygen during helium burning, and affects subsequent carbon, neon, oxygen, and silicon burning stages. It also substantially affects models of thermonuclear type Ia supernovae, owing to carbon detonation in accreting carbon-oxygen white-dwarf stars. In these reactions, the accurate calculation of the elastic scattering of alpha particles and alpha-like nuclei--nuclei with even and equal numbers of protons and neutrons--is important for understanding background and resonant scattering contributions. First-principles calculations of processes involving alpha particles and alpha-like nuclei have so far been impractical, owing to the exponential growth of the number of computational operations with the number of particles. Here we describe an ab initio calculation of alpha-alpha scattering that uses lattice Monte Carlo simulations. We use lattice effective field theory to describe the low-energy interactions of protons and neutrons, and apply a technique called the 'adiabatic projection method' to reduce the eight-body system to a two-cluster system. We take advantage of the computational efficiency and the more favourable scaling with system size of auxiliary-field Monte Carlo simulations to compute an ab initio effective Hamiltonian for the two clusters. We find promising agreement between lattice results and experimental phase shifts for s-wave and d-wave scattering. The approximately quadratic scaling of computational operations with particle number suggests that it should be possible to compute alpha scattering and capture on carbon and oxygen in the near future. The methods described here can be applied to ultracold atomic few-body systems as well as to hadronic systems using lattice quantum chromodynamics to describe the interactions of
NASA Astrophysics Data System (ADS)
Faria Junior, Paulo E.; Campos, Tiago; Bastos, Carlos M. O.; Gmitra, Martin; Fabian, Jaroslav; Sipahi, Guilherme M.
2016-06-01
Semiconductor nanowires based on non-nitride III-V compounds can be synthesized under certain growth conditions to favor the appearance of the wurtzite crystal phase. Despite reports in the literature of ab initio band structures for these wurtzite compounds, we still lack effective multiband models and parameter sets that can be simply used to investigate physical properties of such systems, for instance, under quantum confinement effects. In order to address this deficiency, in this study we calculate the ab initio band structure of bulk InAs and InP in the wurtzite phase and develop an 8 ×8 k .p Hamiltonian to describe the energy bands around the Γ point. We show that our k .p model is robust and can be fitted to describe the important features of the ab initio band structure. The correct description of the spin-splitting effects that arise due to the lack of inversion symmetry in wurtzite crystals is obtained with the k -dependent spin-orbit term in the Hamiltonian, often neglected in the literature. All the energy bands display a Rashba-like spin texture for the in-plane spin expectation value. We also provide the density of states and the carrier density as functions of the Fermi energy. Alternatively, we show an analytical description of the conduction band, valid close to the Γ point. The same fitting procedure is applied to the 6 ×6 valence band Hamiltonian. However, we find that the most reliable approach is the 8 ×8 k .p Hamiltonian for both compounds. The k .p Hamiltonians and parameter sets that we develop in this paper provide a reliable theoretical framework that can be easily applied to investigate electronic, transport, optical, and spin properties of InAs- and InP-based nanostructures.
Ab Initio: And a New Era of Airline Pilot Training.
ERIC Educational Resources Information Center
Gesell, Laurence E.
1995-01-01
Expansion of air transportation and decreasing numbers seeking pilot training point to a shortage of qualified pilots. Ab initio training, in which candidates with no flight time are trained to air transport proficiency, could resolve the problem. (SK)
Phonocatalysis. An ab initio simulation experiment
NASA Astrophysics Data System (ADS)
Kim, Kwangnam; Kaviany, Massoud
2016-06-01
Using simulations, we postulate and show that heterocatalysis on large-bandgap semiconductors can be controlled by substrate phonons, i.e., phonocatalysis. With ab initio calculations, including molecular dynamic simulations, the chemisorbed dissociation of XeF6 on h-BN surface leads to formation of XeF4 and two surface F/h-BN bonds. The reaction pathway and energies are evaluated, and the sorption and reaction emitted/absorbed phonons are identified through spectral analysis of the surface atomic motion. Due to large bandgap, the atomic vibration (phonon) energy transfer channels dominate and among them is the match between the F/h-BN covalent bond stretching and the optical phonons. We show that the chemisorbed dissociation (the pathway activation ascent) requires absorption of large-energy optical phonons. Then using progressively heavier isotopes of B and N atoms, we show that limiting these high-energy optical phonons inhibits the chemisorbed dissociation, i.e., controllable phonocatalysis.
Ab Initio Studies of Calcium Carbonate Hydration.
Lopez-Berganza, Josue A; Diao, Yijue; Pamidighantam, Sudhakar; Espinosa-Marzal, Rosa M
2015-11-25
Ab initio simulations of large hydrated calcium carbonate clusters are challenging due to the existence of multiple local energy minima. Extensive conformational searches around hydrated calcium carbonate clusters (CaCO3·nH2O for n = 1-18) were performed to find low-energy hydration structures using an efficient combination of Monte Carlo searches, density-functional tight binding (DFTB+) method, and density-functional theory (DFT) at the B3LYP level, or Møller-Plesset perturbation theory at the MP2 level. This multilevel optimization yields several low-energy structures for hydrated calcium carbonate. Structural and energetics analysis of the hydration of these clusters revealed a first hydration shell composed of 12 water molecules. Bond-length and charge densities were also determined for different cluster sizes. The solvation of calcium carbonate in bulk water was investigated by placing the explicitly solvated CaCO3·nH2O clusters in a polarizable continuum model (PCM). The findings of this study provide new insights into the energetics and structure of hydrated calcium carbonate and contribute to the understanding of mechanisms where calcium carbonate formation or dissolution is of relevance. PMID:26505205
Isofulminic acid, HONC: Ab initio theory and microwave spectroscopy.
Mladenović, Mirjana; Lewerenz, Marius; McCarthy, Michael C; Thaddeus, Patrick
2009-11-01
Isofulminic acid, HONC, the most energetic stable isomer of isocyanic acid HNCO, higher in energy by 84 kcal/mol, has been detected spectroscopically by rotational spectroscopy supported by coupled cluster electronic structure calculations. The fundamental rotational transitions of the normal, carbon-13, oxygen-18, and deuterium isotopic species have been detected in the centimeter band in a molecular beam by Fourier transform microwave spectroscopy, and rotational constants and nitrogen and deuterium quadrupole coupling constants have been derived. The measured constants agree well with those predicted by ab initio calculations. A number of other electronic and spectroscopic parameters of isofulminic acid, including the dipole moment, vibrational frequencies, infrared intensities, and centrifugal distortion constants have been calculated at a high level of theory. Isofulminic acid is a good candidate for astronomical detection with radio telescopes because it is highly polar and its more stable isomers (HNCO, HOCN, and HCNO) have all been identified in space. PMID:19895013
Ab initio tight-binding Hamiltonian for transition metal dichalcogenides
NASA Astrophysics Data System (ADS)
Fang, Shiang; Kuate Defo, Rodrick; Shirodkar, Sharmila N.; Lieu, Simon; Tritsaris, Georgios A.; Kaxiras, Efthimios
2015-11-01
We present an accurate ab initio tight-binding Hamiltonian for the transition metal dichalcogenides, MoS2, MoSe2, WS2, WSe2, with a minimal basis (the d orbitals for the metal atoms and p orbitals for the chalcogen atoms) based on a transformation of the Kohn-Sham density functional theory Hamiltonian to a basis of maximally localized Wannier functions. The truncated tight-binding Hamiltonian, with only on-site, first, and partial second neighbor interactions, including spin-orbit coupling, provides a simple physical picture and the symmetry of the main band-structure features. Interlayer interactions between adjacent layers are modeled by transferable hopping terms between the chalcogen p orbitals. The full-range tight-binding Hamiltonian can be reduced to hybrid-orbital k .p effective Hamiltonians near the band extrema that capture important low-energy excitations. These ab initio Hamiltonians can serve as the starting point for applications to interacting many-body physics including optical transitions and Berry curvature of bands, of which we give some examples.
Skutterudites under pressure: An ab initio study
Ram, Swetarekha; Kanchana, V.; Valsakumar, M. C.
2014-03-07
Ab initio results on the band structure, density of states, and Fermi surface (FS) properties of LaRu{sub 4}X{sub 12} (X = P, As, Sb) are presented at ambient pressure as well as under compression. The analysis of density of states reveals the major contribution at the Fermi level to be mainly from the Ru-d and X-p states. We have a complicated Fermi surface with both electron and hole characters for all the three compounds which is derived mainly from the Ru-d and X-p states. There is also a simpler FS with hole character derived from the P-p{sub z} orbital for LaRu{sub 4}P{sub 12} and Ru-d{sub z{sup 2}} orbital in the case of As and Sb containing compounds. More interestingly, Fermi surface nesting feature is observed only in the case of the LaRu{sub 4}P{sub 12}. Under compression, we observe the topology of the complicated FS sheet of LaRu{sub 4}As{sub 12} to change around V/V{sub 0} = 0.85, leading to a behaviour similar to that of a multiband superconductor, and in addition, we have two more hole pockets centered around Γ at V/V{sub 0} = 0.8 for the same compound. Apart from this, we find the hole pocket to vanish at V/V{sub 0} = 0.8 in the case of LaRu{sub 4}Sb{sub 12} and the opening of the complicated FS sheet gets reduced. The de Haas van Alphen calculation shows the number of extremal orbits in the complicated sheet to change in As and Sb containing compounds under compression, where we also observe the FS topology to change.
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.
Ab initio water pair potential with flexible monomers.
Jankowski, Piotr; Murdachaew, Garold; Bukowski, Robert; Akin-Ojo, Omololu; Leforestier, Claude; Szalewicz, Krzysztof
2015-03-26
A potential energy surface for the water dimer with explicit dependence on monomer coordinates is presented. The surface was fitted to a set of previously published interaction energies computed on a grid of over a quarter million points in the 12-dimensional configurational space using symmetry-adapted perturbation theory and coupled-cluster methods. The present fit removes small errors in published fits, and its accuracy is critically evaluated. The minimum and saddle-point structures of the potential surface were found to be very close to predictions from direct ab initio optimizations. The computed second virial coefficients agreed well with experimental values. At low temperatures, the effects of monomer flexibility in the virial coefficients were found to be much smaller than the quantum effects. PMID:25687650
An investigation of ab initio shell-model interactions derived by no-core shell model
NASA Astrophysics Data System (ADS)
Wang, XiaoBao; Dong, GuoXiang; Li, QingFeng; Shen, CaiWan; Yu, ShaoYing
2016-09-01
The microscopic shell-model effective interactions are mainly based on the many-body perturbation theory (MBPT), the first work of which can be traced to Brown and Kuo's first attempt in 1966, derived from the Hamada-Johnston nucleon-nucleon potential. However, the convergence of the MBPT is still unclear. On the other hand, ab initio theories, such as Green's function Monte Carlo (GFMC), no-core shell model (NCSM), and coupled-cluster theory with single and double excitations (CCSD), have made many progress in recent years. However, due to the increasing demanding of computing resources, these ab initio applications are usually limited to nuclei with mass up to A = 16. Recently, people have realized the ab initio construction of valence-space effective interactions, which is obtained through a second-time renormalization, or to be more exactly, projecting the full-manybody Hamiltonian into core, one-body, and two-body cluster parts. In this paper, we present the investigation of such ab initio shell-model interactions, by the recent derived sd-shell effective interactions based on effective J-matrix Inverse Scattering Potential (JISP) and chiral effective-field theory (EFT) through NCSM. In this work, we have seen the similarity between the ab initio shellmodel interactions and the interactions obtained by MBPT or by empirical fitting. Without the inclusion of three-body (3-bd) force, the ab initio shell-model interactions still share similar defects with the microscopic interactions by MBPT, i.e., T = 1 channel is more attractive while T = 0 channel is more repulsive than empirical interactions. The progress to include more many-body correlations and 3-bd force is still badly needed, to see whether such efforts of ab initio shell-model interactions can reach similar precision as the interactions fitted to experimental data.
Ab Initio Studies of Stratospheric Ozone Depletion Chemistry
NASA Technical Reports Server (NTRS)
Lee, Timothy J.; Head-Gordon, Martin; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
An overview of the current understanding of ozone depletion chemistry, particularly with regards the formation of the so-called Antarctic ozone hole, will be presented together with an outline as to how ab initio quantum chemistry can be used to further our understanding of stratospheric chemistry. The ability of modern state-of-the art ab initio quantum chemical techniques to characterize reliably the gas-phase molecular structure, vibrational spectrum, electronic spectrum, and thermal stability of fluorine, chlorine, bromine and nitrogen oxide species will be demonstrated by presentation of some example studies. The ab initio results will be shown to be in excellent agreement with the available experimental data, and where the experimental data are either not known or are inconclusive, the theoretical results are shown to fill in the gaps and to resolve experimental controversies. In addition, ab initio studies in which the electronic spectra and the characterization of excited electronic states of halogen oxide species will also be presented. Again where available, the ab initio results are compared to experimental observations, and are used to aid in the interpretation of experimental studies.
THERMODYNAMICS OF MATERIALS: FROM AB INITIO TO PHENOMENOLOGY
Turchi, P A
2004-09-24
Quantum mechanical-based (or ab initio) methods are used to predict the stability properties of materials although their application is limited to relatively simple systems in terms of structures and number of alloy components. However thermodynamics of complex multi-component alloys requires a more versatile approach afforded within the CALPHAD formalism. Despite its success, the lack of experimental data very often prevents the design of robust thermodynamic databases. After a brief survey of ab initio methodologies and CALPHAD, it will be shown how ab initio electronic structure methods can supplement in two ways CALPHAD for subsequent applications. The first one is rather immediate and concerns the direct input of ab initio energetics in CALPHAD databases. The other way, more involved, is the assessment of ab initio thermodynamics '{acute a} la CALPHAD'. It will be shown how these results can be used within CALPHAD to predict the equilibrium properties of multi-component alloys. Finally, comments will be made on challenges and future prospects.
Efficient ab initio free energy calculations by classically assisted trajectory sampling
NASA Astrophysics Data System (ADS)
Wilson, Hugh F.
2015-12-01
A method for efficiently performing ab initio free energy calculations based on coupling constant thermodynamic integration is demonstrated. By the use of Boltzmann-weighted sums over states generated from a classical ensemble, the free energy difference between the classical and ab initio ensembles is readily available without the need for time-consuming integration over molecular dynamics trajectories. Convergence and errors in this scheme are discussed and characterised in terms of a quantity representing the degree of misfit between the classical and ab initio systems. Smaller but still substantial efficiency gains over molecular dynamics are also demonstrated for the calculation of average properties such as pressure and total energy for systems in equilibrium.
Ab initio simulation of transport phenomena in rarefied gases.
Sharipov, Felix; Strapasson, José L
2012-09-01
Ab initio potentials are implemented into the direct simulation Monte Carlo (DSMC) method. Such an implementation allows us to model transport phenomena in rarefied gases without any fitting parameter of intermolecular collisions usually extracted from experimental data. Applying the method proposed by Sharipov and Strapasson [Phys. Fluids 24, 011703 (2012)], the use of ab initio potentials in the DSMC requires the same computational efforts as the widely used potentials such as hard spheres, variable hard sphere, variable soft spheres, etc. At the same time, the ab initio potentials provide more reliable results than any other one. As an example, the transport coefficients of a binary mixture He-Ar, viz., viscosity, thermal conductivity, and thermal diffusion factor, have been calculated for several values of the mole fraction. PMID:23030889
Ab initio computations of photodissociation products of CFC alternatives
Tai, S.; Illinger, K.H.; Kenny, J.E.
1995-12-31
Ab initio computations, have already been used to examine the energetics of the photodissociation of stratospheric chlorofluorocarbons. Our awn research has investigated the ab initio computation of vibrational frequencies and infrared intensities of CF{sub 3}CH{sub 2}F, CF{sub 3}CF{sub 2}H, and CF{sub 3}CH{sub 3}; continuing research will attempt to expand these computations to the energetics of the photodissociation of these molecules, since sane of the most common types of chlorofluorocarbon substitutes are hydrofluoroethanes.
Ab Initio Structure Analysis Using Laboratory Powder Diffraction Data
NASA Astrophysics Data System (ADS)
Sasaki, Akito
Today, laboratory X-ray diffractometers are seeing increasingly wide use in the ab initio crystal structure analysis of organic powder samples. This is because optics and optical devices have been improved, making it possible to obtain precise integrated intensities of reflections in high 2-theta ranges. Another reason is that one can use direct-space methods, which do not require “high-resolution diffraction data”, much more easily than before. Described here are some key points to remember when performig ab initio crystal structure analysis using powder diffraction data from organic compounds.
Ab Initio Simulation of the Photoelectron Spectrum for Methoxy Radical
NASA Astrophysics Data System (ADS)
Cheng, Lan; Weichman, Marissa L.; Kim, Jongjin B.; Ichino, Takatoshi; Neumark, Daniel; Stanton, John F.
2015-06-01
A theoretical simulation of the photoelectron spectrum for the ground state of methoxy radical is reported based on the quasidiabatic model Hamiltonian originally proposed by Köppel, Domcke, and Cederbaum. The parameters in the model Hamiltonian have been obtained from ab initio coupled-cluster calculations. The linear and quadratic force constants have been calculated using equation-of-motion coupled-cluster ionization potential method with the singles, doubles, and triples (EOMIP-CCSDT) truncation scheme together with atomic natural orbital basis sets of triple-zeta quality (ANO1). The cubic and quartic force constants have been obtained from EOMIP-CCSD calculations with ANO basis sets of double-zeta quality (ANO0), and the spin-orbit coupling constant has been computed at the EOMIP-CCSD/pCVTZ level. The nuclear Schroedinger equation has been solved using the Lanzcos algorithm to obtain vibronic energy levels as well as the corresponding intensities. The simulated spectrum compares favorably with the recent high-resolution slow electron velocity-map imaging experiment for vibronic levels up to 2000 cm-1.
Hammouri, Mahmoud; Fohtung, Edwin; Vasiliev, Igor
2016-10-01
Multiferroic heterostructures composed of thin layers of ferromagnetic and ferroelectric perovskites have attracted considerable attention in recent years. We apply ab initio computational methods based on density functional theory to study the magnetoelectric coupling at the (0 0 1) interface between [Formula: see text] (LSMO) and [Formula: see text] (PZT). Our study demonstrates that the ferroelectric polarization of PZT has a strong influence on the distribution of magnetization in LSMO. The presence of polarized PZT changes the balance between the ferromagnetic and antiferromagnetic states of LSMO. The observed interfacial magnetoelectric effect can be explained by the variation of the charge density across the LSMO/PZT interface and by the change of the magnetic order in the LSMO layer adjacent to PZT. PMID:27494690
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.
Towards SiC Surface Functionalization: An Ab Initio Study
Cicero, G; Catellani, A
2005-01-28
We present a microscopic model of the interaction and adsorption mechanism of simple organic molecules on SiC surfaces as obtained from ab initio molecular dynamics simulations. Our results open the way to functionalization of silicon carbide, a leading candidate material for bio-compatible devices.
Ab initio pseudopotential band calculation of organic conductors
Ishibashi, Shoji; Kohyama, Masanori
1999-12-01
The authors have calculated the band structures of organic conductors TTF-TCNQ and {beta}-(BEDT-TTF){sub 2}I{sub 3} using the ab initio plane-wave pseudopotential method within the local-density approximation (LDA). The Fermi-surface shape and the origin of bands near the Fermi level are investigated for each compound.
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.
Takahasi, S. ); Curtiss, L.A.; Gosztola, D.; Koura, N. ); Loong, C.K.; Saboungi, M.L. . Materials Science Div.)
1993-04-01
The Raman and neutron scattering spectra of 46 mol% AlCl[sub 3] -54 mol% 1-ethyl-3-methyl imidazolium chloride (EMIC) and 67 mol% AlCl[sub 3] - 33 mol% EMIC melts are presented. Ab initio molecular orbital calculations have been carried out on structures of chloroaluminate anion and EMI cation and the interaction between anion and cation.
Accurate ab initio vibrational energies of methyl chloride
Owens, Alec; Yurchenko, Sergei N.; Yachmenev, Andrey; Tennyson, Jonathan; Thiel, Walter
2015-06-28
Two new nine-dimensional potential energy surfaces (PESs) have been generated using high-level ab initio theory for the two main isotopologues of methyl chloride, CH{sub 3}{sup 35}Cl and CH{sub 3}{sup 37}Cl. The respective PESs, CBS-35{sup HL}, and CBS-37{sup HL}, are based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set (CBS) limit, and incorporate a range of higher-level (HL) additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, scalar relativistic effects, and diagonal Born-Oppenheimer corrections. Variational calculations of the vibrational energy levels were performed using the computer program TROVE, whose functionality has been extended to handle molecules of the form XY {sub 3}Z. Fully converged energies were obtained by means of a complete vibrational basis set extrapolation. The CBS-35{sup HL} and CBS-37{sup HL} PESs reproduce the fundamental term values with root-mean-square errors of 0.75 and 1.00 cm{sup −1}, respectively. An analysis of the combined effect of the HL corrections and CBS extrapolation on the vibrational wavenumbers indicates that both are needed to compute accurate theoretical results for methyl chloride. We believe that it would be extremely challenging to go beyond the accuracy currently achieved for CH{sub 3}Cl without empirical refinement of the respective PESs.
Accurate ab initio vibrational energies of methyl chloride.
Owens, Alec; Yurchenko, Sergei N; Yachmenev, Andrey; Tennyson, Jonathan; Thiel, Walter
2015-06-28
Two new nine-dimensional potential energy surfaces (PESs) have been generated using high-level ab initio theory for the two main isotopologues of methyl chloride, CH3 (35)Cl and CH3 (37)Cl. The respective PESs, CBS-35( HL), and CBS-37( HL), are based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set (CBS) limit, and incorporate a range of higher-level (HL) additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, scalar relativistic effects, and diagonal Born-Oppenheimer corrections. Variational calculations of the vibrational energy levels were performed using the computer program TROVE, whose functionality has been extended to handle molecules of the form XY 3Z. Fully converged energies were obtained by means of a complete vibrational basis set extrapolation. The CBS-35( HL) and CBS-37( HL) PESs reproduce the fundamental term values with root-mean-square errors of 0.75 and 1.00 cm(-1), respectively. An analysis of the combined effect of the HL corrections and CBS extrapolation on the vibrational wavenumbers indicates that both are needed to compute accurate theoretical results for methyl chloride. We believe that it would be extremely challenging to go beyond the accuracy currently achieved for CH3Cl without empirical refinement of the respective PESs. PMID:26133427
Accurate ab initio vibrational energies of methyl chloride
NASA Astrophysics Data System (ADS)
Owens, Alec; Yurchenko, Sergei N.; Yachmenev, Andrey; Tennyson, Jonathan; Thiel, Walter
2015-06-01
Two new nine-dimensional potential energy surfaces (PESs) have been generated using high-level ab initio theory for the two main isotopologues of methyl chloride, CH335Cl and CH337Cl. The respective PESs, CBS-35 HL, and CBS-37 HL, are based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set (CBS) limit, and incorporate a range of higher-level (HL) additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, scalar relativistic effects, and diagonal Born-Oppenheimer corrections. Variational calculations of the vibrational energy levels were performed using the computer program TROVE, whose functionality has been extended to handle molecules of the form XY 3Z. Fully converged energies were obtained by means of a complete vibrational basis set extrapolation. The CBS-35 HL and CBS-37 HL PESs reproduce the fundamental term values with root-mean-square errors of 0.75 and 1.00 cm-1, respectively. An analysis of the combined effect of the HL corrections and CBS extrapolation on the vibrational wavenumbers indicates that both are needed to compute accurate theoretical results for methyl chloride. We believe that it would be extremely challenging to go beyond the accuracy currently achieved for CH3Cl without empirical refinement of the respective PESs.
Ab initio molecular crystal structures, spectra, and phase diagrams.
Hirata, So; Gilliard, Kandis; He, Xiao; Li, Jinjin; Sode, Olaseni
2014-09-16
Conspectus Molecular crystals are chemists' solids in the sense that their structures and properties can be understood in terms of those of the constituent molecules merely perturbed by a crystalline environment. They form a large and important class of solids including ices of atmospheric species, drugs, explosives, and even some organic optoelectronic materials and supramolecular assemblies. Recently, surprisingly simple yet extremely efficient, versatile, easily implemented, and systematically accurate electronic structure methods for molecular crystals have been developed. The methods, collectively referred to as the embedded-fragment scheme, divide a crystal into monomers and overlapping dimers and apply modern molecular electronic structure methods and software to these fragments of the crystal that are embedded in a self-consistently determined crystalline electrostatic field. They enable facile applications of accurate but otherwise prohibitively expensive ab initio molecular orbital theories such as Møller-Plesset perturbation and coupled-cluster theories to a broad range of properties of solids such as internal energies, enthalpies, structures, equation of state, phonon dispersion curves and density of states, infrared and Raman spectra (including band intensities and sometimes anharmonic effects), inelastic neutron scattering spectra, heat capacities, Gibbs energies, and phase diagrams, while accounting for many-body electrostatic (namely, induction or polarization) effects as well as two-body exchange and dispersion interactions from first principles. They can fundamentally alter the role of computing in the studies of molecular crystals in the same way ab initio molecular orbital theories have transformed research practices in gas-phase physical chemistry and synthetic chemistry in the last half century. In this Account, after a brief summary of formalisms and algorithms, we discuss applications of these methods performed in our group as compelling
Ab initio description of the exotic unbound 7He nucleus
Baroni, Simone; Navratil, Petr; Quaglioni, Sofia
2013-01-11
In this study, the neutron-rich unbound 7He nucleus has been the subject of many experimental investigations. While the ground-state 3/2– resonance is well established, there is a controversy concerning the excited 1/2– resonance reported in some experiments as low lying and narrow (ER~1 MeV, Γ≤1 MeV) while in others as very broad and located at a higher energy. This issue cannot be addressed by ab initio theoretical calculations based on traditional bound-state methods. We introduce a new unified approach to nuclear bound and continuum states based on the coupling of the no-core shell model, a bound-state technique, with the no-coremore » shell model combined with the resonating-group method, a nuclear scattering technique. Our calculations describe the ground-state resonance in agreement with experiment and, at the same time, predict a broad 1/2– resonance above 2 MeV.« less
Ab Initio Investigation of NH_3-O_2 Exciplex
NASA Astrophysics Data System (ADS)
Haupert, L. M.; Simpson, G.; Slipchenko, L. V.
2010-06-01
In their recent investigation of fluorescence from poly(amido amine) (PAMAM) dendrimers, Chu and Imae suggested an exciplex composed of tertiary amine and oxygen molecules might be responsible for fluorescence in PAMAM dendrimers. In this work, we present an ab initio investigation of the electronic structure of a possible ammonia-oxygen exciplex model system using equation-of-motion coupled cluster techniques. Geometry optimization of the triplet ground state produced a weakly bound state with an equilibrium separation of ˜ 3.5 Å, and an excited state geometry scan revealed a bound, excited triplet state with an equilibrium separation of 2.02 Å, consistent with results of earlier PM3 work by Juranic et al. The energy gap between the triplet ground state and first triplet excited state of the exciplex at 2.02 Å is 412.8 nm, lending support to the exciplex hypothesis. C.-C. Chu, and T. Imae, Macromol. Rapid. Commun., 30, 89-93 (2009). I. Juranic, H. S. Rzepa, and Y. MinYan, J. Chem. Soc. Perkin Trans., 2 (1990)
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 molecular dynamics: concepts, recent developments, and future trends.
Iftimie, Radu; Minary, Peter; Tuckerman, Mark E
2005-05-10
The methodology of ab initio molecular dynamics, wherein finite-temperature dynamical trajectories are generated by using forces computed "on the fly" from electronic structure calculations, has had a profound influence in modern theoretical research. Ab initio molecular dynamics allows chemical processes in condensed phases to be studied in an accurate and unbiased manner, leading to new paradigms in the elucidation of microscopic mechanisms, rationalization of experimental data, and testable predictions of new phenomena. The purpose of this work is to give a brief introduction to the technique and to review several important recent developments in the field. Several illustrative examples showing the power of the technique have been chosen. Perspectives on future directions in the field also will be given. PMID:15870204
Recent progress in ab initio density matrix renormalization group methodology
NASA Astrophysics Data System (ADS)
Hachmann, Johannes; Dorando, Jonathan J.; Kin-Lic Chan, Garnet
2008-03-01
We present some recent developments in the ab initio density matrix renormalization group (DMRG) method for quantum chemical problems, in particular our local, quadratic scaling algorithm [1] for low dimensional systems. This method is particularly suited for the description of strong nondynamic correlation, and allows us to compute numerically exact (FCI) correlated energies for large active spaces, up to one order of magnitude larger then can be done by conventional CASCI techniques. Other features of this method are its inherent multireference nature, compactness, variational results, size-consistency and size-extensivity. In addition we will review the problems (predominantly organic electronic materials) on which we applied the ab initio DMRG: 1) metal-insulator transition in hydrogen chains [1] 2) all-trans polyacetylene [1] 3) acenes [2] 4) polydiacetylenes [3]. References [1] Hachmann, Cardoen, Chan, JCP 125 (2006), 144101. [2] Hachmann, Dorando, Avil'es, Chan, JCP 127 (2007), 134309. [3] unpublished.
Understanding phonon transport in thermoelectric materials using ab initio approaches
NASA Astrophysics Data System (ADS)
Broido, David
Good thermoelectric materials have low phonon thermal conductivity, kph. Accurate theories to describe kph are important components in developing predictive models of thermoelectric efficiency that can help guide synthesis and measurement efforts. We have developed ab initio approaches to calculate kph, in which phonon modes and phonon scattering rates are computed using interatomic force constants determined from density functional theory, and a full solution of the Boltzmann transport equation for phonons is implemented. A recent approach to calculate interatomic force constants using ab initio molecular dynamics has yielded a good description of the thermal properties of Bi2Te3. But, the complexity of new promising candidate thermoelectric materials introduces computational challenges in assessing their thermal properties. An example is germanane, a germanium based hydrogen-terminated layered semiconductor, which we will discuss in this talk.
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.
Exploring complex chemical reactions by ab-initio simulation
NASA Astrophysics Data System (ADS)
Parrinello, Michele
1998-03-01
Recent progress in the ab-initio molecular dynamics method and the power of parallel computing, allow the detailed study of complex chemical reaction of great industrial relevance. We illustrate this unprecedented capability by investigating the second generation Ziegler-Natta catalytic process. In this inhomogeneous catalyst, a polymerization reaction is induced by TiCl4 molecules deposited on an MgCl2 solid support. A density functional based ab-initio molecular dynamics calculation conducted with a minimum of initial assumption allows to understand the nature of the catalytic center and to determine the reaction path with the associated free energy barrier. Furthermore our calculation can explain in a nontrivial way the stereo-selectivity of the process.
Ab Initio Electronic Relaxation Times and Transport in Noble Metals
NASA Astrophysics Data System (ADS)
Mustafa, Jamal I.; Bernardi, Marco; Neaton, Jeffrey B.; Louie, Steven G.
Relaxation times employed to study electron transport in metals are typically assumed to be constants and obtained empirically using the Drude model. Here, we employ ab initio calculations to compute the electron-phonon relaxation times of Cu, Ag, and Au, and find that they vary significantly on the Fermi surface, spanning ~15 -45 fs. We compute room temperature resistivities in excellent agreement with experiment by combining GW bandstructures, Wannier-interpolated band velocities, and ab initio relaxation times. Our calculations are compared to other approximations used for the relaxation times. Additionally, an importance sampling scheme is introduced to speed up the convergence of resistivity and transport calculations by sampling directly points on the Fermi surface. This work was supported by NSF Grant No. DMR15-1508412 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at LBNL's NERSC facility.
Ab initio Monte Carlo investigation of small lithium clusters.
Srinivas, S.
1999-06-16
Structural and thermal properties of small lithium clusters are studied using ab initio-based Monte Carlo simulations. The ab initio scheme uses a Hartree-Fock/density functional treatment of the electronic structure combined with a jump-walking Monte Carlo sampling of nuclear configurations. Structural forms of Li{sub 8} and Li{sub 9}{sup +} clusters are obtained and their thermal properties analyzed in terms of probability distributions of the cluster potential energy, average potential energy and configurational heat capacity all considered as a function of the cluster temperature. Details of the gradual evolution with temperature of the structural forms sampled are examined. Temperatures characterizing the onset of structural changes and isomer coexistence are identified for both clusters.
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.
GAUSSIAN 76: An ab initio Molecular Orbital Program
DOE R&D Accomplishments Database
Binkley, J. S.; Whiteside, R.; Hariharan, P. C.; Seeger, R.; Hehre, W. J.; Lathan, W. A.; Newton, M. D.; Ditchfield, R.; Pople, J. A.
1978-01-01
Gaussian 76 is a general-purpose computer program for ab initio Hartree-Fock molecular orbital calculations. It can handle basis sets involving s, p and d-type Gaussian functions. Certain standard sets (STO-3G, 4-31G, 6-31G*, etc.) are stored internally for easy use. Closed shell (RHF) or unrestricted open shell (UHF) wave functions can be obtained. Facilities are provided for geometry optimization to potential minima and for limited potential surface scans.
Thermochemical data for CVD modeling from ab initio calculations
Ho, P.; Melius, C.F.
1993-12-31
Ab initio electronic-structure calculations are combined with empirical bond-additivity corrections to yield thermochemical properties of gas-phase molecules. A self-consistent set of heats of formation for molecules in the Si-H, Si-H-Cl, Si-H-F, Si-N-H and Si-N-H-F systems is presented, along with preliminary values for some Si-O-C-H species.
The study of molecular spectroscopy by ab initio methods
NASA Technical Reports Server (NTRS)
Bauschlicher, Charles W., Jr.; Langhoff, Stephen R.
1991-01-01
This review illustrates the potential of theory for solving spectroscopic problems. The accuracy of approximate techniques for including electron correlation have been calibrated by comparison with full configuration-interaction calculations. Examples of the application of ab initio calculations to vibrational, rotational, and electronic spectroscopy are given. It is shown that the state-averaged, complete active space self-consistent field, multireference configuration-interaction procedure provides a good approach for treating several electronic states accurately in a common molecular orbital basis.
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.
Hyperfine Parameters for Aluminum Hydride: An ab Initio Molecular Orbital Study
NASA Astrophysics Data System (ADS)
Gee, Myrlene; Wasylishen, Roderick E.
2001-06-01
An extensive ab initio molecular orbital study of the 27Al nuclear spin-rotation and nuclear quadrupolar coupling constants in aluminum hydride, AlH, has been performed. The 27Al nuclear spin-rotation constant (C⊥), calculated to be approximately 300 kHz, was neglected in a previous analysis of the hyperfine structure in the microwave spectrum (M. Goto and S. Saito, Astrophys. J.452, L147-148 (1995)). Unfortunately, the ab initio calculations do not provide a definitive value for the aluminum nuclear quadrupolar coupling constant, but suggest a value of -49±4 MHz. It is apparent that the microwave study of AlH should be repeated.
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 Defect Properties in YPO4
Gao, Fei; Xiao, Haiyan Y.; Zhou, Yungang; Devanathan, Ramaswami; Hu, Shenyang Y.; Li, Yulan; Sun, Xin; Khaleel, Mohammad A.
2012-03-01
Ab initio methods based on density functional theory have been used to calculate the formation energies of intrinsic defects, including vacancies, interstitials, antisites and Frenkel pairs in YPO4 under the O-rich and Y2O3-rich, and the O-rich and Y-rich conditions. The larger size of the yttrium atom may give rise to higher formation energy of the phosphorus antisite defect. In general, the formation energies of anion interstitials are much smaller than those of cation interstitials for both conditions considered. It is of greatly interest to find that the relative stabilities among the same types of interstitials are independent of the reference states. The most stable configuration for oxygen interstitials is an O-O split interstitial near the Ta site, while the most stable configuration for cation interstitials is a tetrahedral interstitial near the Ta site. The cation split interstitials are unfavorable in YPO4, with much higher formation energies. Furthermore, the properties of Frenkel pairs are compared with those calculated using empirical potentials. The results reveal that both ab initio and empirical potential calculations show a similar trend in the formation energies of Frenkel pairs, but the formation energies obtained by empirical potentials are much larger than those calculated by ab initio method.
A Complete and Accurate Ab Initio Repeat Finding Algorithm.
Lian, Shuaibin; Chen, Xinwu; Wang, Peng; Zhang, Xiaoli; Dai, Xianhua
2016-03-01
It has become clear that repetitive sequences have played multiple roles in eukaryotic genome evolution including increasing genetic diversity through mutation, changes in gene expression and facilitating generation of novel genes. However, identification of repetitive elements can be difficult in the ab initio manner. Currently, some classical ab initio tools of finding repeats have already presented and compared. The completeness and accuracy of detecting repeats of them are little pool. To this end, we proposed a new ab initio repeat finding tool, named HashRepeatFinder, which is based on hash index and word counting. Furthermore, we assessed the performances of HashRepeatFinder with other two famous tools, such as RepeatScout and Repeatfinder, in human genome data hg19. The results indicated the following three conclusions: (1) The completeness of HashRepeatFinder is the best one among these three compared tools in almost all chromosomes, especially in chr9 (8 times of RepeatScout, 10 times of Repeatfinder); (2) in terms of detecting large repeats, HashRepeatFinder also performed best in all chromosomes, especially in chr3 (24 times of RepeatScout and 250 times of Repeatfinder) and chr19 (12 times of RepeatScout and 60 times of Repeatfinder); (3) in terms of accuracy, HashRepeatFinder can merge the abundant repeats with high accuracy. PMID:26272474
Internal dynamics in azetidine: A microwave and ab initio study
NASA Astrophysics Data System (ADS)
López, Juan C.; Blanco, Susana; Lesarri, Alberto; Alonso, José L.
2001-02-01
The internal dynamics of interconversion between equivalent conformations due to the coupling between ring puckering and NH inversion in azetidine has been investigated by rotational spectroscopy and ab initio computations. Analysis of the rotational spectra in the 8-220 GHz region has been completed for the ground state and first four excited states of the ring-puckering vibration. Rotational transitions exhibit a characteristic doubling originated by tunneling between equivalent conformations through a C2v barrier, which is related to symmetric (A1) and antisymmetric (B1) inversion states. Additionally, nuclear quadrupole hyperfine structure arising from the N nucleus could be resolved for low-J transitions. Accurate rotational and centrifugal distortion parameters together with the energy difference between inversion states derived from μc-type inversion transitions have been derived for each ring-puckering state using a two-state Hamiltonian. An effective monodimensional reduced potential function for the ring-puckering vibration V(X)=10.82(X4+14.29X-8.93X2-0.28X3) has been found consistent with the observed experimental variation of the rotational and centrifugal distortion constants with ring-puckering. This asymmetric single minimum potential function supports the existence of only one stable equatorial form. The barrier to interconversion between equivalent equatorial conformers, related to the C2v conformation of azetidine in which the ring atoms and the NH group are coplanar, has been estimated to range between 1900 and 2600 cm-1. The strong dependence of the dipole moment and quadrupole coupling constants with ring-puckering vibrational state evidence structural changes that occur along the ring-puckering coordinate.
Cirtog, M; Asselin, P; Soulard, P; Tremblay, B; Madebène, B; Alikhani, M E; Georges, R; Moudens, A; Goubet, M; Huet, T R; Pirali, O; Roy, P
2011-03-31
A series of hydrogen bonded complexes involving oxirane and water molecules have been studied. In this paper we report on the vibrational study of the oxirane-water complex (CH(2))(2)O-H(2)O. Neon matrix experiments and ab initio anharmonic vibrational calculations have been performed, providing a consistent set of vibrational frequencies and anharmonic coupling constants. The implementation of a new large flow supersonic jet coupled to the Bruker IFS 125 HR spectrometer at the infrared AILES beamline of the French synchrotron SOLEIL (Jet-AILES) enabled us to record first jet-cooled Fourier transform infrared spectra of oxirane-water complexes at different resolutions down to 0.2 cm(-1). Rovibrational parameters and a lower bound of the predissociation lifetime of 25 ps for the v(OH)(b) = 1 state have been derived from the rovibrational analysis of the ν(OH)(b) band contour recorded at respective rotational temperatures of 12 K (Jet-AILES) and 35 K (LADIR jet). PMID:21381647
NASA Astrophysics Data System (ADS)
George, D. X. F.; Kumar, Sanjay
2010-08-01
Ab initio global adiabatic as well as quasidiabatic potential energy surfaces for the ground and the first excited electronic states of the H + + CO system have been computed as a function of the Jacobi coordinates ( R, r, γ) using Dunning's cc-pVTZ basis set at the internally contracted multi-reference (single and double) configuration interaction level of accuracy. In addition, nonadiabatic coupling matrix elements arising from radial motion, mixing angle and coupling potential have been computed using the ab initio procedure [Simah et al. (1999) [66
Chiroptical properties of unsubstituted carbohydrates: Ab initio and semiempirical studies
NASA Astrophysics Data System (ADS)
Parra C., Alejandro
Ab initio calculations support assignment of the vacuum ultraviolet circular dichroism (CD) of simple saccharides to 11A 1 --> 21B1 and 11A 1 --> 11A2 transitions centered on the oxygen atoms of the acetal group treated as two weakly coupled ether chromophores. The calculations are consistent with assignments previously made on the basis of a deconvolution of CD spectra. Estimates of the oxygen centered contributions to magnetic transition dipole moments were made. Semiempirical calculations were performed to model the NaD molar optical rotation of 1,6- and 3,6- anhydrosugars. For 1,6-anhydrosugars, current parameters produce reasonable agreement with experimental values. For 3,6-anhydrosugars, modifications to the ether parameters had to be introduced. The most relevant included a reorientation of the bond-centered s-->s* transition dipole charges in the ether chromophore to a C2v orientation, and a shift from prolate polarizability ellipsoids to general ellipsoids. These changes result in good agreement with experimental Na D molar rotations for 3,6-anhydrosugars. A low energy CD band arises in 3,6- and 1,6-anhydrosugars when agreement with the experimental NaD molar rotations is achieved. It is proposed that this band is a real feature in the spectrum. The origin of the band is primarily the interaction between b1 symmetry- oriented transition dipoles in the COC groups with other transition dipoles in the molecule. Comparison with experimental spectra leads to an assignment of this band to 11A1 --> 21B1 transitions centered on the COC groups.
Ab initio DFT calculations of vibrational properties
NASA Astrophysics Data System (ADS)
Story, S. M.; Vila, F. D.; Kas, J. J.; Rehr, J. J.
2014-03-01
Vibrational properties such as EXAFS and crystallographic Debye-Waller factors, vibrational free energies, phonon self-energies, and phonon contributions to the electron spectral function, are key to understanding many aspects of materials beyond ground state electronic structure. Thus, their simulation using first principles methods is of particular importance. Many of these vibrational properties can be calculated from the dynamical matrix and electron-phonon coupling coefficients obtained from DFT calculations. Here we present a code DMVP that calculates these properties from the output of electronic structure codes such as ABINIT, Gaussian, Quantum Espresso and VASP. Our modular interfacing tool AI2PS allows us to translate the different outputs into a DMVP compatible format and generate vibrational properties in an automated way. Finally, we present some current applications that take advantage of the modular form of AI2PS to extend its capabilities to the calculation of coefficients of thermal expansion and other properties of interest such as infrared spectra. This work was supported by DOE Grant DE-FG02-97ER45623.
Ab Initio Modelling of Steady Rotating Stars
NASA Astrophysics Data System (ADS)
Rieutord, Michel; Espinosa Lara, Francisco
Modelling isolated rotating stars at any rotation rate is a challenge for the next generation of stellar models. These models will couple dynamical aspects of rotating stars, like angular momentum and chemicals transport, with classical chemical evolution, gravitational contraction or mass-loss. Such modelling needs to be achieved in two dimensions, combining the calculation of the structure of the star, its mean flows and the time-evolution of the whole. We present here a first step in this challenging programme. It leads to the first self-consistent two-dimensional models of rotating stars in a steady state generated by the ESTER code. In these models the structure (pressure, density and temperature) and the flow fields are computed in a self-consistent way allowing the prediction of the differential rotation and the associated meridian circulation of the stars. After a presentation of the physical properties of such models and the numerical methods at work, we give the first grid of such models describing massive and intermediate-mass stars for a selection of rotation rates up to 90 % of the breakup angular velocity.
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.
Properties of metals during the heating by intense laser irradiation using ab initio simulations
NASA Astrophysics Data System (ADS)
Holst, Bastian; Recoules, Vanina; Torrent, Marc; Mazevet, Stephane
2011-10-01
Ultrashort laser pulses irradiating a target heat the electrons to very high temperatures. In contrast, the ionic lattice is unaffected on the time scale of the laser pulse since the heat capacity of electrons is much smaller than that of the lattice. This non-equilibrium system can be described as a composition of two subsystems: one consisting of hot electrons and the other of an ionic lattice at low temperature. We studied the effect of this intense electronic excitations on the optical properties of gold using ab initio simulations. We additionally use ab initio linear response to compute the phonon spectrum and the electron-phonon coupling constant within Density Functional Theory for several electronic temperatures of few eV. LULI, Ecole Polytechnique, CNRS, CEA, UPMC, 91128 Palaiseau, France.
NASA Astrophysics Data System (ADS)
Bock, Charles W.; Trachtman, Mendel; George, Philip
1982-12-01
The lengths of the terminal double bonds and the central CC single bond, and the cross-coupling constant involving the double bonds, calculated ab initio using the unscaled 4-31G basis set, are reported for 15 planar conformers of conjugated dienes containing the OCC'C″ grouping, 9 planar conformers containing the OCC'N grouping, and 6 planar conformers containing the OCC'O'grouping. The larger values for the CC bond length tend to be associated with the smaller values for the double-bond lengths, and vice versa. A linear relationship holds fairly well between rcc and the sum of the double-bond lengths. The sign of the cross-coupling constant is the same for each type of conjugated diene, negative for foc.c'c″ and foc.c'N' but positive for foc.c'o'. The values of foc.c'c″ and f oc.c'N are larger the smaller rcc' in accord with a linear relationship. For all three types of diene the values for pairs of cis and trans conformers do not differ very much, the ratio fcis/ ftrans being a little greater than 1 in ten cases and a little less than 1 in six cases. This lack of any marked difference between cis and trans conformers suggests that the coupling between the double bonds occurs in the main via the bonded framework of the molecule, and that "through-space" interactions, which would be a special feature of cis conformers, are of relatively little importance.
NASA Technical Reports Server (NTRS)
Allen, B. Danette; Alexandrov, Natalia
2016-01-01
Incremental approaches to air transportation system development inherit current architectural constraints, which, in turn, place hard bounds on system capacity, efficiency of performance, and complexity. To enable airspace operations of the future, a clean-slate (ab initio) airspace design(s) must be considered. This ab initio National Airspace System (NAS) must be capable of accommodating increased traffic density, a broader diversity of aircraft, and on-demand mobility. System and subsystem designs should scale to accommodate the inevitable demand for airspace services that include large numbers of autonomous Unmanned Aerial Vehicles and a paradigm shift in general aviation (e.g., personal air vehicles) in addition to more traditional aerial vehicles such as commercial jetliners and weather balloons. The complex and adaptive nature of ab initio designs for the future NAS requires new approaches to validation, adding a significant physical experimentation component to analytical and simulation tools. In addition to software modeling and simulation, the ability to exercise system solutions in a flight environment will be an essential aspect of validation. The NASA Langley Research Center (LaRC) Autonomy Incubator seeks to develop a flight simulation infrastructure for ab initio modeling and simulation that assumes no specific NAS architecture and models vehicle-to-vehicle behavior to examine interactions and emergent behaviors among hundreds of intelligent aerial agents exhibiting collaborative, cooperative, coordinative, selfish, and malicious behaviors. The air transportation system of the future will be a complex adaptive system (CAS) characterized by complex and sometimes unpredictable (or unpredicted) behaviors that result from temporal and spatial interactions among large numbers of participants. A CAS not only evolves with a changing environment and adapts to it, it is closely coupled to all systems that constitute the environment. Thus, the ecosystem that
Macromolecular ab initio phasing enforcing secondary and tertiary structure
Millán, Claudia; Sammito, Massimo; Usón, Isabel
2015-01-01
Ab initio phasing of macromolecular structures, from the native intensities alone with no experimental phase information or previous particular structural knowledge, has been the object of a long quest, limited by two main barriers: structure size and resolution of the data. Current approaches to extend the scope of ab initio phasing include use of the Patterson function, density modification and data extrapolation. The authors’ approach relies on the combination of locating model fragments such as polyalanine α-helices with the program PHASER and density modification with the program SHELXE. Given the difficulties in discriminating correct small substructures, many putative groups of fragments have to be tested in parallel; thus calculations are performed in a grid or supercomputer. The method has been named after the Italian painter Arcimboldo, who used to compose portraits out of fruit and vegetables. With ARCIMBOLDO, most collections of fragments remain a ‘still-life’, but some are correct enough for density modification and main-chain tracing to reveal the protein’s true portrait. Beyond α-helices, other fragments can be exploited in an analogous way: libraries of helices with modelled side chains, β-strands, predictable fragments such as DNA-binding folds or fragments selected from distant homologues up to libraries of small local folds that are used to enforce nonspecific tertiary structure; thus restoring the ab initio nature of the method. Using these methods, a number of unknown macromolecules with a few thousand atoms and resolutions around 2 Å have been solved. In the 2014 release, use of the program has been simplified. The software mediates the use of massive computing to automate the grid access required in difficult cases but may also run on a single multicore workstation (http://chango.ibmb.csic.es/ARCIMBOLDO_LITE) to solve straightforward cases. PMID:25610631
The implementation of ab initio quantum chemistry calculations on transporters.
Cooper, M D; Hillier, I H
1991-06-01
The RHF and geometry optimization sections of the ab initio quantum chemistry code, GAMESS, have been optimized for a network of parallel microprocessors, Inmos T800-20 transputers, using both indirect and direct SCF techniques. The results indicate great scope for implementation of such codes on small parallel computer systems, very high efficiencies having been achieved, particularly in the cases of direct SCF and geometry optimization with large basis sets. The work, although performed upon one particular parallel system, the Meiko Computing Surface, is applicable to a wide range of parallel systems with both shared and distributed memory. PMID:1919615
Ab initio electronic properties of dual phosphorus monolayers in silicon
2014-01-01
In the midst of the epitaxial circuitry revolution in silicon technology, we look ahead to the next paradigm shift: effective use of the third dimension - in particular, its combination with epitaxial technology. We perform ab initio calculations of atomically thin epitaxial bilayers in silicon, investigating the fundamental electronic properties of monolayer pairs. Quantitative band splittings and the electronic density are presented, along with effects of the layers’ relative alignment and comments on disordered systems, and for the first time, the effective electronic widths of such device components are calculated. PMID:25246862
Ab initio vibrational and dielectric properties of Y V O
NASA Astrophysics Data System (ADS)
Vali, R.
2009-10-01
For the yttrium orthovanadate Y V O with a tetragonal zircon-type structure, the first complete set of Raman-active and IR-active phonon modes has been calculated using ab initio density functional perturbation theory. The calculated IR reflectivity spectra are in good agreement with available experimental data. We report the calculated frequencies of three Raman-active modes that could not be detected experimentally and a new assignment of the experimental Raman data. The contributions of each IR-active phonon modes to static dielectric tensor have been determined.
Potential Dependence of Electrochemical Barriers from ab Initio Calculations.
Chan, Karen; Nørskov, Jens K
2016-05-01
We present a simple and computationally efficient method to determine the potential dependence of the activation energies for proton-electron transfer from a single ab initio barrier calculation. We show that the potential dependence of the activation energy is given by the partial charge transferred at the transition state. The method is evaluated against the potential dependence determined explicitly through multiple calculations at varying potential. We show that the transfer coefficient is given by the charge transferred from the initial to transition state, which has significant implications for electrochemical kinetics. PMID:27088442
Ab-Initio Shell Model with a Core
Lisetskiy, A F; Barrett, B R; Kruse, M; Navratil, P; Stetcu, I; Vary, J P
2008-06-04
We construct effective 2- and 3-body Hamiltonians for the p-shell by performing 12{h_bar}{Omega} ab initio no-core shell model (NCSM) calculations for A=6 and 7 nuclei and explicitly projecting the many-body Hamiltonians onto the 0{h_bar}{Omega} space. We then separate these effective Hamiltonians into 0-, 1- and 2-body contributions (also 3-body for A=7) and analyze the systematic behavior of these different parts as a function of the mass number A and size of the NCSM basis space. The role of effective 3- and higher-body interactions for A > 6 is investigated and discussed.
Ab initio Study of He Stability in hcp-Ti
Dai, Yunya; Yang, Li; Peng, SM; Long, XG; Gao, Fei; Zu, Xiaotao T.
2010-12-20
The stability of He in hcp-Ti was studied using ab initio method based on density functional theory. The results indicate that a single He atom prefers to occupy the tetrahedral site rather than the octahedral site. The interaction of He defects with Ti atoms has been used to explain the relative stabilities of He point defects in hcp-Ti. The relative stability of He defects in hcp-Ti is useful for He clustering and bubble nucleation in metal tritides, which provides the basis for development of improved atomistic models.
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 study of hydrogen on beryllium surfaces
NASA Astrophysics Data System (ADS)
Bachurin, D. V.; Vladimirov, P. V.
2015-11-01
Static ab initio calculations were performed for five principal hexagonal close-packed beryllium surfaces: basal, prismatic (type I and II) and pyramidal (type I and II). The basal plane was found to be the most energetically favorable, while the energies of the prismatic (type I) and pyramidal (type I) planes were slightly higher followed by the type II planes. Beryllium is known to show extreme interlayer distance relaxation near the surface. Up to five outermost atomic layers were involved in surface relaxation. The presence of hydrogen on the beryllium surfaces led to a noticeable reduction of the surface energy.
An improved ab initio structure for fluorine peroxide (FOOF)
NASA Astrophysics Data System (ADS)
Mack, Hans-Georg; Oberhammer, Heinz
1988-03-01
Ab initio calculations with the 6-31G* and Dunning (9s5p/4s2p) basis sets augmented with p and d functions at various levels of theory (RHF, MP2, MP3, and MP4) were carried out on F 2O 2. The best result was obtained at the MP2 level with the Dunning basis plus one set of d functions on fluorine and two sets of d functions on oxygen. These calculations reproduce the experimental bond lengths to within 0.01 Å and the angles to within the experimental uncertainties.
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.
NASA Astrophysics Data System (ADS)
Curchod, Basile F. E.; Rauer, Clemens; Marquetand, Philipp; González, Leticia; Martínez, Todd J.
2016-03-01
Full multiple spawning is a formally exact method to describe the excited-state dynamics of molecular systems beyond the Born-Oppenheimer approximation. However, it has been limited until now to the description of radiationless transitions taking place between electronic states with the same spin multiplicity. This Communication presents a generalization of the full and ab initio multiple spawning methods to both internal conversion (mediated by nonadiabatic coupling terms) and intersystem crossing events (triggered by spin-orbit coupling matrix elements) based on a spin-diabatic representation. The results of two numerical applications, a model system and the deactivation of thioformaldehyde, validate the presented formalism and its implementation.
NASA Astrophysics Data System (ADS)
Kedziera, Dariusz; Mentel, Łukasz; Żuchowski, Piotr S.; Knoop, Steven
2015-06-01
We have obtained accurate ab initio +4Σ quartet potentials for the diatomic metastable triplet helium+alkali-metal (Li, Na, K, Rb) systems, using all-electron restricted open-shell coupled cluster singles and doubles with noniterative triples corrections CCSD(T) calculations and accurate calculations of the long-range C6 coefficients. These potentials provide accurate ab initio quartet scattering lengths, which for these many-electron systems is possible, because of the small reduced masses and shallow potentials that result in a small amount of bound states. Our results are relevant for ultracold metastable triplet helium+alkali-metal mixture experiments.
Muñoz, D; de Graaf, C; Illas, F
2004-07-30
The influence of the basis set size and computational method in the calculation of the magnetic coupling constant J is evaluated using a series of cuprate superconductor parent compounds as a case study. The variational DDCI method and an iterative modification, the IDDCI method, are tested, as well as the perturbative CASPT2 method, with two different reference wave functions. Results show that the DDCI magnetic coupling constant is in rather good agreement with the experiment, although it shows a moderate basis set dependency. The IDDCI results are less dependent on the size of the basis set, but slightly overestimate the magnetic coupling constant. CASPT2 results are nearly independent of the chosen basis set. With a minimal active space values are obtained that are about 20% smaller than the DDCI results. The experimental coupling constant can be reproduced when an extended reference wave function is used. PMID:15139036
In pursuit of the ab initio limit for conformational energy prototypes
NASA Astrophysics Data System (ADS)
Császár, Attila G.; Allen, Wesley D.; Schaefer, Henry F.
1998-06-01
The convergence of ab initio predictions to the one- and n-particle limits has been systematically explored for several conformational energy prototypes: the inversion barriers of ammonia, water, and isocyanic acid, the torsional barrier of ethane, the E/Z rotamer separation of formic acid, and the barrier to linearity of silicon dicarbide. Explicit ab initio results were obtained with atomic-orbital basis sets as large as [7s6p5d4f3g2h1i/6s5p4d3f2g1h] and electron correlation treatments as extensive as fifth-order Møller-Plesset perturbation theory (MP5), the full coupled-cluster method through triple excitations (CCSDT), and Brueckner doubles theory including perturbational corrections for both triple and quadruple excitations [BD(TQ)]. Subsequently, basis set and electron correlation extrapolation schemes were invoked to gauge any further variations in arriving at the ab initio limit. Physical effects which are tacitly neglected in most theoretical work have also been quantified by computations of non-Born-Oppenheimer (BODC), relativistic, and core correlation shifts of relative energies. Instructive conclusions are drawn for the pursuit of spectroscopic accuracy in theoretical conformational analyses, and precise predictions for the key energetic quantities of the molecular prototypes are advanced.
Ab initio study of hot electrons in GaAs.
Bernardi, Marco; Vigil-Fowler, Derek; Ong, Chin Shen; Neaton, Jeffrey B; Louie, Steven G
2015-04-28
Hot carrier dynamics critically impacts the performance of electronic, optoelectronic, photovoltaic, and plasmonic devices. Hot carriers lose energy over nanometer lengths and picosecond timescales and thus are challenging to study experimentally, whereas calculations of hot carrier dynamics are cumbersome and dominated by empirical approaches. In this work, we present ab initio calculations of hot electrons in gallium arsenide (GaAs) using density functional theory and many-body perturbation theory. Our computed electron-phonon relaxation times at the onset of the Γ, L, and X valleys are in excellent agreement with ultrafast optical experiments and show that the ultrafast (tens of femtoseconds) hot electron decay times observed experimentally arise from electron-phonon scattering. This result is an important advance to resolve a controversy on hot electron cooling in GaAs. We further find that, contrary to common notions, all optical and acoustic modes contribute substantially to electron-phonon scattering, with a dominant contribution from transverse acoustic modes. This work provides definitive microscopic insight into hot electrons in GaAs and enables accurate ab initio computation of hot carriers in advanced materials. PMID:25870287
Ab initio dynamics of the cytochrome P450 hydroxylation reaction
Elenewski, Justin E.; Hackett, John C
2015-01-01
The iron(IV)-oxo porphyrin π-cation radical known as Compound I is the primary oxidant within the cytochromes P450, allowing these enzymes to affect the substrate hydroxylation. In the course of this reaction, a hydrogen atom is abstracted from the substrate to generate hydroxyiron(IV) porphyrin and a substrate-centered radical. The hydroxy radical then rebounds from the iron to the substrate, yielding the hydroxylated product. While Compound I has succumbed to theoretical and spectroscopic characterization, the associated hydroxyiron species is elusive as a consequence of its very short lifetime, for which there are no quantitative estimates. To ascertain the physical mechanism underlying substrate hydroxylation and probe this timescale, ab initio molecular dynamics simulations and free energy calculations are performed for a model of Compound I catalysis. Semiclassical estimates based on these calculations reveal the hydrogen atom abstraction step to be extremely fast, kinetically comparable to enzymes such as carbonic anhydrase. Using an ensemble of ab initio simulations, the resultant hydroxyiron species is found to have a similarly short lifetime, ranging between 300 fs and 3600 fs, putatively depending on the enzyme active site architecture. The addition of tunneling corrections to these rates suggests a strong contribution from nuclear quantum effects, which should accelerate every step of substrate hydroxylation by an order of magnitude. These observations have strong implications for the detection of individual hydroxylation intermediates during P450 catalysis. PMID:25681906
Three-cluster dynamics within an ab initio framework
Quaglioni, Sofia; Romero-Redondo, Carolina; Navratil, Petr
2013-09-26
In this study, we introduce a fully antisymmetrized treatment of three-cluster dynamics within the ab initio framework of the no-core shell model/resonating-group method. Energy-independent nonlocal interactions among the three nuclear fragments are obtained from realistic nucleon-nucleon interactions and consistent ab initio many-body wave functions of the clusters. The three-cluster Schrödinger equation is solved with bound-state boundary conditions by means of the hyperspherical-harmonic method on a Lagrange mesh. We discuss the formalism in detail and give algebraic expressions for systems of two single nucleons plus a nucleus. Using a soft similarity-renormalization-group evolved chiral nucleon-nucleon potential, we apply the method to amore » 4He+n+n description of 6He and compare the results to experiment and to a six-body diagonalization of the Hamiltonian performed within the harmonic-oscillator expansions of the no-core shell model. Differences between the two calculations provide a measure of core (4He) polarization effects.« less
Oxidation of GaN: An ab initio thermodynamic approach
NASA Astrophysics Data System (ADS)
Jackson, Adam J.; Walsh, Aron
2013-10-01
GaN is a wide-band-gap semiconductor used in high-efficiency light-emitting diodes and solar cells. The solid is produced industrially at high chemical purities by deposition from a vapor phase, and oxygen may be included at this stage. Oxidation represents a potential path for tuning its properties without introducing more exotic elements or extreme processing conditions. In this work, ab initio computational methods are used to examine the energy potentials and electronic properties of different extents of oxidation in GaN. Solid-state vibrational properties of Ga, GaN, Ga2O3, and a single substitutional oxygen defect have been studied using the harmonic approximation with supercells. A thermodynamic model is outlined which combines the results of ab initio calculations with data from experimental literature. This model allows free energies to be predicted for arbitrary reaction conditions within a wide process envelope. It is shown that complete oxidation is favorable for all industrially relevant conditions, while the formation of defects can be opposed by the use of high temperatures and a high N2:O2 ratio.
Ab initio thermodynamic model for magnesium carbonates and hydrates.
Chaka, Anne M; Felmy, Andrew R
2014-09-01
An ab initio thermodynamic framework for predicting properties of hydrated magnesium carbonate minerals has been developed using density-functional theory linked to macroscopic thermodynamics through the experimental chemical potentials for MgO, water, and CO2. Including semiempirical dispersion via the Grimme method and small corrections to the generalized gradient approximation of Perdew, Burke, and Ernzerhof for the heat of formation yields a model with quantitative agreement for the benchmark minerals brucite, magnesite, nesquehonite, and hydromagnesite. The model shows how small differences in experimental conditions determine whether nesquehonite, hydromagnesite, or magnesite is the result of laboratory synthesis from carbonation of brucite, and what transformations are expected to occur on geological time scales. Because of the reliance on parameter-free first-principles methods, the model is reliably extensible to experimental conditions not readily accessible to experiment and to any mineral composition for which the structure is known or can be hypothesized, including structures containing defects, substitutions, or transitional structures during solid state transformations induced by temperature changes or processes such as water, CO2, or O2 diffusion. Demonstrated applications of the ab initio thermodynamic framework include an independent means to evaluate differences in thermodynamic data for lansfordite, predicting the properties of Mg analogues of Ca-based hydrated carbonates monohydrocalcite and ikaite, which have not been observed in nature, and an estimation of the thermodynamics of barringtonite from the stoichiometry and a single experimental observation. PMID:24679248
Ab Initio Thermodynamic Model for Magnesium Carbonates and Hydrates
Chaka, Anne M.; Felmy, Andrew R.
2014-03-28
An ab initio thermodynamic framework for predicting properties of hydrated magnesium carbonate minerals has been developed using density-functional theory linked to macroscopic thermodynamics through the experimental chemical potentials for MgO, water, and CO2. Including semiempirical dispersion via the Grimme method and small corrections to the generalized gradient approximation of Perdew, Burke, and Ernzerhof for the heat of formation yields a model with quantitative agreement for the benchmark minerals brucite, magnesite, nesquehonite, and hydromagnesite. The model shows how small differences in experimental conditions determine whether nesquehonite, hydromagnesite, or magnesite is the result of laboratory synthesis from carbonation of brucite, and what transformations are expected to occur on geological time scales. Because of the reliance on parameter-free first principles methods, the model is reliably extensible to experimental conditions not readily accessible to experiment and to any mineral composition for which the structure is known or can be hypothesized, including structures containing defects, substitutions, or transitional structures during solid state transformations induced by temperature changes or processes such as water, CO2, or O2 diffusion. Demonstrated applications of the ab initio thermodynamic framework include an independent means to evaluate differences in thermodynamic data for lansfordite, predicting the properties of Mg analogs of Ca-based hydrated carbonates monohydrocalcite and ikaite which have not been observed in nature, and an estimation of the thermodynamics of barringtonite from the stoichiometry and a single experimental observation.
Ab initio prediction of the critical thickness of a precipitate
NASA Astrophysics Data System (ADS)
Sampath, S.; Janisch, R.
2013-09-01
Segregation and precipitation of second phases in metals and metallic alloys is an important phenomenon that has a strong influence on the mechanical properties of the material. Models exist that describe the growth of coherent, semi-coherent and incoherent precipitates. One important parameter of these models is the energy of the interface between matrix and precipitate. In this work we apply ab initio density functional theory calculations to obtain this parameter and to understand how it depends on chemical composition and mechanical strain at the interface. Our example is a metastable Mo-C phase, the body-centred tetragonal structure, which exists as a semi-coherent precipitate in body-centred cubic molybdenum. The interface of this precipitate is supposed to change from coherent to semi-coherent during the growth of the precipitate. We predict the critical thickness of the precipitate by calculating the different contributions to a semi-coherent interface energy by means of ab initio density functional theory calculations. The parameters in our model include the elastic strain energy stored in the precipitate, as well as a misfit dislocation energy that depends on the dislocation core width and the dislocation spacing. Our predicted critical thickness agrees well with experimental observations.
Ab initio dynamics of the cytochrome P450 hydroxylation reaction
NASA Astrophysics Data System (ADS)
Elenewski, Justin E.; Hackett, John C.
2015-02-01
The iron(IV)-oxo porphyrin π-cation radical known as Compound I is the primary oxidant within the cytochromes P450, allowing these enzymes to affect the substrate hydroxylation. In the course of this reaction, a hydrogen atom is abstracted from the substrate to generate hydroxyiron(IV) porphyrin and a substrate-centered radical. The hydroxy radical then rebounds from the iron to the substrate, yielding the hydroxylated product. While Compound I has succumbed to theoretical and spectroscopic characterization, the associated hydroxyiron species is elusive as a consequence of its very short lifetime, for which there are no quantitative estimates. To ascertain the physical mechanism underlying substrate hydroxylation and probe this timescale, ab initio molecular dynamics simulations and free energy calculations are performed for a model of Compound I catalysis. Semiclassical estimates based on these calculations reveal the hydrogen atom abstraction step to be extremely fast, kinetically comparable to enzymes such as carbonic anhydrase. Using an ensemble of ab initio simulations, the resultant hydroxyiron species is found to have a similarly short lifetime, ranging between 300 fs and 3600 fs, putatively depending on the enzyme active site architecture. The addition of tunneling corrections to these rates suggests a strong contribution from nuclear quantum effects, which should accelerate every step of substrate hydroxylation by an order of magnitude. These observations have strong implications for the detection of individual hydroxylation intermediates during P450 catalysis.
Ab initio dynamics of the cytochrome P450 hydroxylation reaction
Elenewski, Justin E.; Hackett, John C
2015-02-14
The iron(IV)-oxo porphyrin π-cation radical known as Compound I is the primary oxidant within the cytochromes P450, allowing these enzymes to affect the substrate hydroxylation. In the course of this reaction, a hydrogen atom is abstracted from the substrate to generate hydroxyiron(IV) porphyrin and a substrate-centered radical. The hydroxy radical then rebounds from the iron to the substrate, yielding the hydroxylated product. While Compound I has succumbed to theoretical and spectroscopic characterization, the associated hydroxyiron species is elusive as a consequence of its very short lifetime, for which there are no quantitative estimates. To ascertain the physical mechanism underlying substrate hydroxylation and probe this timescale, ab initio molecular dynamics simulations and free energy calculations are performed for a model of Compound I catalysis. Semiclassical estimates based on these calculations reveal the hydrogen atom abstraction step to be extremely fast, kinetically comparable to enzymes such as carbonic anhydrase. Using an ensemble of ab initio simulations, the resultant hydroxyiron species is found to have a similarly short lifetime, ranging between 300 fs and 3600 fs, putatively depending on the enzyme active site architecture. The addition of tunneling corrections to these rates suggests a strong contribution from nuclear quantum effects, which should accelerate every step of substrate hydroxylation by an order of magnitude. These observations have strong implications for the detection of individual hydroxylation intermediates during P450 catalysis.
AB INITIO SIMULATIONS FOR MATERIAL PROPERTIES ALONG THE JUPITER ADIABAT
French, Martin; Becker, Andreas; Lorenzen, Winfried; Nettelmann, Nadine; Bethkenhagen, Mandy; Redmer, Ronald; Wicht, Johannes
2012-09-15
We determine basic thermodynamic and transport properties of hydrogen-helium-water mixtures for the extreme conditions along Jupiter's adiabat via ab initio simulations, which are compiled in an accurate and consistent data set. In particular, we calculate the electrical and thermal conductivity, the shear and longitudinal viscosity, and diffusion coefficients of the nuclei. We present results for associated quantities like the magnetic and thermal diffusivity and the kinematic shear viscosity along an adiabat that is taken from a state-of-the-art interior structure model. Furthermore, the heat capacities, the thermal expansion coefficient, the isothermal compressibility, the Grueneisen parameter, and the speed of sound are calculated. We find that the onset of dissociation and ionization of hydrogen at about 0.9 Jupiter radii marks a region where the material properties change drastically. In the deep interior, where the electrons are degenerate, many of the material properties remain relatively constant. Our ab initio data will serve as a robust foundation for applications that require accurate knowledge of the material properties in Jupiter's interior, e.g., models for the dynamo generation.
Entropy of Liquid Water from Ab Initio Molecular Dynamics
NASA Astrophysics Data System (ADS)
Spanu, Leonardo; Zhang, Cui; Galli, Giulia
2012-02-01
The debate on the structural properties of water has been mostly based on the calculation of pair correlation functions. However, the simulation of thermodynamic and spectroscopic quantities may be of great relevance for the characterization of liquid water properties. We have computed the entropy of liquid water using a two-phase thermodynamic model and trajectories generated by ab initio molecular dynamics simulations [1]. In an attempt to better understand the performance of several density functionals in simulating liquid water, we have performed ab initio molecular dynamics using semilocal, hybrid [2] and van der Waals density functionals [3]. We show that in all cases, at the experimental equilibrium density and at temperatures in the vicinity of 300 K, the computed entropies are underestimated, with respect to experiment, and the liquid exhibits a degree of tetrahedral order higher than in experiments. We also discuss computational strategies to simulate spectroscopic properties of water, including infrared and Raman spectra.[4pt] [1] C.Zhang, L.Spanu and G.Galli, J.Phys.Chem. B 2011 (in press)[0pt] [2] C.Zhang, D.Donadio, F.Gygi and G.Galli, J. Chem. Theory Comput. 7, 1443 (2011)[0pt] [3] C.Zhang, J.Wu, G.Galli and F.Gygi, J. Chem. Theory Comput. 7, 3061 (2011)
Unified ab initio approaches to nuclear structure and reactions
NASA Astrophysics Data System (ADS)
Navrátil, Petr; Quaglioni, Sofia; Hupin, Guillaume; Romero-Redondo, Carolina; Calci, Angelo
2016-05-01
The description of nuclei starting from the constituent nucleons and the realistic interactions among them has been a long-standing goal in nuclear physics. In addition to the complex nature of the nuclear forces, with two-, three- and possibly higher many-nucleon components, 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. After a brief overview of the field, we focus on ab initio many-body approaches—built upon the no-core shell model—that are capable of simultaneously describing both bound and scattering nuclear states, and present results for resonances in light nuclei, reactions important for astrophysics and fusion research. In particular, we review recent calculations of resonances in the 6He halo nucleus, of five- and six-nucleon scattering, and an investigation of the role of chiral three-nucleon interactions in the structure of 9Be. Further, we discuss applications to the 7Be {({{p}},γ )}8{{B}} radiative capture. Finally, we highlight our efforts to describe transfer reactions including the 3H{({{d}},{{n}})}4He fusion.
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.
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.
Development of Novel Analytical Method for Ab Initio Powder Structural Analysis
NASA Astrophysics Data System (ADS)
Sakata, Makoto; Nishibori, Eiji; Sawa, Hiroshi
Genetic Algorithm (GA) applied to ab initio structure determination from synchrotron powder diffraction is described. It seems to have an advantage over other real space methods for ab initio structure determination because of the existence of schema theorem. As an example, the case of Prednisolone Succinate is shown in some detail. Future development of GA in crystallography is briefly described.
NASA Astrophysics Data System (ADS)
Hadipour, N. L.; Elmi, F.
2003-03-01
Nuclear quadrupole coupling constants ( χ) of 27Al, 35Cl and 81Br in AlX 3 monomers as well as Al 2X 6 (X=Cl, Br) dimers are calculated at the RHF/6-311G* and B 3LYP/6-311G* levels, using G AUSSIAN 98 package. Correlations are made between χ and dihedral angles θ, of Al 2X 6. These θs are produced through the ring puckering motions about the hinge line which joins the two bridge halogens (X b). Nuclear quadrupole coupling constants of 35Cl, 81Br and 27Al are used as probes for monitoring the departure of the symmetry of Al 2X 6 from a high symmetry point group D 2h to a lower one. The χs of chlorine nuclei of AlCl 3 differ significantly from those of Al 2Cl 6. These differences appear negligible for AlBr 3 in comparison to Al 2Br 6. This work demonstrates the considerable sensitivity of nuclear quadrupole resonance in distinguishing between Al 2X 6 conformers. This is in comparison to the usage of energy differences which is customarily employed.
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.
NASA Astrophysics Data System (ADS)
Kądzielawa, Andrzej P.; Bielas, Agata; Acquarone, Marcello; Biborski, Andrzej; Maśka, Maciej M.; Spałek, Józef
2014-12-01
The hydrogen molecules H2 and {{≤ft( {{H}2} \\right)}2} are analyzed with electronic correlations taken into account between the 1s electrons in an exact manner. The optimal single-particle Slater orbitals are evaluated in the correlated state of H2 by combining their variational determination with the diagonalization of the full Hamiltonian in the second-quantization language. All electron-ion coupling constants are determined explicitly and their relative importance is discussed. Sizable zero-point motion amplitude and the corresponding energy are then evaluated by taking into account the anharmonic contributions up to the ninth order in the relative displacement of the ions from their static equilibrium value. The applicability of the model to solid molecular hydrogen is briefly analyzed by calculating intermolecular microscopic parameters for the 2× {{H}2} rectangular configuration, as well its ground state energy.
Electronic Structures of Anti-Ferromagnetic Tetraradicals: Ab Initio and Semi-Empirical Studies.
Zhang, Dawei; Liu, Chungen
2016-04-12
The energy relationships and electronic structures of the lowest-lying spin states in several anti-ferromagnetic tetraradical model systems are studied with high-level ab initio and semi-empirical methods. The Full-CI method (FCI), the complete active space second-order perturbation theory (CASPT2), and the n-electron valence state perturbation theory (NEVPT2) are employed to obtain reference results. By comparing the energy relationships predicted from the Heisenberg and Hubbard models with ab initio benchmarks, the accuracy of the widely used Heisenberg model for anti-ferromagnetic spin-coupling in low-spin polyradicals is cautiously tested in this work. It is found that the strength of electron correlation (|U/t|) concerning anti-ferromagnetically coupled radical centers could range widely from strong to moderate correlation regimes and could become another degree of freedom besides the spin multiplicity. Accordingly, the Heisenberg-type model works well in the regime of strong correlation, which reproduces well the energy relationships along with the wave functions of all the spin states. In moderately spin-correlated tetraradicals, the results of the prototype Heisenberg model deviate severely from those of multi-reference electron correlation ab initio methods, while the extended Heisenberg model, containing four-body terms, can introduce reasonable corrections and maintains its accuracy in this condition. In the weak correlation regime, both the prototype Heisenberg model and its extended forms containing higher-order correction terms will encounter difficulties. Meanwhile, the Hubbard model shows balanced accuracy from strong to weak correlation cases and can reproduce qualitatively correct electronic structures, which makes it more suitable for the study of anti-ferromagnetic coupling in polyradical systems. PMID:26963572
Zhao, Pengfei; Liang, Chongyun; Gong, Xiwen; Gao, Ran; Liu, Jiwei; Wang, Min; Che, Renchao
2013-09-01
Monodispersed manganese oxide (Mn1-xCox)3O4 (0 ≤ x ≤ 0.5) nanoparticles, less than 10 nm size, are respectively synthesized via a facile thermolysis method at a rather low temperature, ranging from 90 to 100 °C, without any inertia gas for protection. The influences of the Co dopant content on the critical reaction temperature required for the nanoparticle formation, electronic band structures, magnetic properties, and the microwave absorption capability of (Mn1-xCox)3O4 are comprehensively investigated by means of both experimental and theoretical approaches including powder X-ray diffraction (XRD), electron energy loss spectroscopy (EELS), super conductivity quantum interference device (SQUID) examination, and first-principle simulations. Co is successfully doped into the Mn atomic sites of the (Mn1-xCox)3O4 lattice, which is further confirmed by EELS data acquired from one individual nanoparticle. Therefore, continuous solid solutions of well-crystallized (Mn1-xCox)3O4 products are achieved without any impurity phase or phase separation. With increases in the Co dopant concentration x from 0 to 0.5, the lattice parameters change systemically, where the overall saturation magnetization at 30 K increases due to the more intense coupling of the 3d electrons between Mn and Co, as revealed by simulations. The microwave absorption properties of the (Mn1-xCox)3O4 nanoparticles are examined between 2 and 18 GHz. The maximum absorption peak -11.0 dB of the x = 0 sample is enhanced to -11.5 dB for x = 0.2, -12.7 dB for x = 0.25, -15.6 dB for x = 0.33, and -24.0 dB for x = 0.5 respectively, suggesting the Co doping effects. Our results might provide novel insights into the understanding of the influences of metallic ion doping on the electromagnetic properties of metallic oxide nanomaterials. PMID:23868450
NASA Astrophysics Data System (ADS)
Wu, Dong-lan; Tan, Bin; Wen, Yu-feng; Zeng, Xue-feng; Xie, An-dong; Yan, Bing
2016-05-01
Accurate theoretical calculations on the MgBr radical have been carried out by using the high-level relativistic multireference configuration interaction method with Davidson correction (MRCI + Q) using correlation-consistent Quintuple-ζ quality basis set. The potential energy curves (PECs) of the 14 Λ-S states of MgBr have been computed. In order to improve the PECs, the core-valence correlation, scalar relativistic effect, and spin-orbit coupling effect are taken into account in the computations. The spectroscopic constants of the bound states have been determined from the computed PECs. The results of the ground state X2Σ+ and the first excited state A2Π are in good agreement with those from the available experiments, while spectroscopic constants of the other electronic states are firstly reported. The low-lying ion-pair state B2Σ+ correlated to ion-pair dissociation limit Mg+ (2Sg) + Br- (1Sg) is characterized. The permanent dipole moments (PDMs) of Λ-S states and the R-dependent spin-orbit (SO) matrix elements are computed. The results indicate that the abrupt changes of PDMs and the SO matrix elements are attributed to the changes of the electronic configurations near the avoided crossing point. After taking the SOC effect into account, the 14 Λ-S states split into 30 Ω states, and the SOC splitting for the A2Π is calculated to be 102.58 cm- 1. The SOC effect, leading to the double-well potential of the Ω = (3)1/2 state, is found to be substantial for MgBr. In order to further illustrate the SOC effect and the avoided crossing phenomenon of the PECs, the Λ-S compositions in the Ω state wavefunctions are analyzed in detail. Finally, the transition dipole moments (TDMs) of several transitions from upper Ω states to the ground X2Σ+1/2 state and the corresponding radiative lifetimes have been studied. It is shown that the (1)3/2-X2Σ+1/2 and (2)3/2-X2Σ+1/2 are particularly important to the observed transitions A2Π-X2Σ+ and C2Π-X2Σ+. The
Wu, Dong-Lan; Tan, Bin; Wen, Yu-Feng; Zeng, Xue-Feng; Xie, An-Dong; Yan, Bing
2016-05-15
Accurate theoretical calculations on the MgBr radical have been carried out by using the high-level relativistic multireference configuration interaction method with Davidson correction (MRCI+Q) using correlation-consistent Quintuple-ζ quality basis set. The potential energy curves (PECs) of the 14 Λ-S states of MgBr have been computed. In order to improve the PECs, the core-valence correlation, scalar relativistic effect, and spin-orbit coupling effect are taken into account in the computations. The spectroscopic constants of the bound states have been determined from the computed PECs. The results of the ground state X(2)Σ(+) and the first excited state A(2)Π are in good agreement with those from the available experiments, while spectroscopic constants of the other electronic states are firstly reported. The low-lying ion-pair state B(2)Σ(+) correlated to ion-pair dissociation limit Mg(+) ((2)Sg)+Br(-) ((1)Sg) is characterized. The permanent dipole moments (PDMs) of Λ-S states and the R-dependent spin-orbit (SO) matrix elements are computed. The results indicate that the abrupt changes of PDMs and the SO matrix elements are attributed to the changes of the electronic configurations near the avoided crossing point. After taking the SOC effect into account, the 14 Λ-S states split into 30Ω states, and the SOC splitting for the A(2)Π is calculated to be 102.58cm(-1). The SOC effect, leading to the double-well potential of the Ω=(3)1/2 state, is found to be substantial for MgBr. In order to further illustrate the SOC effect and the avoided crossing phenomenon of the PECs, the Λ-S compositions in the Ω state wavefunctions are analyzed in detail. Finally, the transition dipole moments (TDMs) of several transitions from upper Ω states to the ground X(2)Σ(+)1/2 state and the corresponding radiative lifetimes have been studied. It is shown that the (1)3/2-X(2)Σ(+)1/2 and (2)3/2-X(2)Σ(+)1/2 are particularly important to the observed transitions A(2)Π-X(2)
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.
Site occupancy trend of Co in Ni2MnIn: Ab initio approach
NASA Astrophysics Data System (ADS)
Pal, Soumyadipta; Mahadevan, Priya; Biswas, C.
2015-06-01
The trend of site occupation of Co at Ni sites of Ni2MnIn system is studied in austenitic phase having L21 structure by ab initio density functional theory (DFT) calculation. The Co atoms prefer to be at Ni sites rather than Mn site and are ferromagetically coupled with Ni and Mn. The ground state has tetragonal structure for Ni1.5Co0.5MnIn and Ni1.25Co0.75MnIn. The Co tends to form cluster.
The ab initio potential energy surface and spectroscopic constants of HOCl
NASA Astrophysics Data System (ADS)
Koput, Jacek; Peterson, Kirk A.
1998-02-01
The potential energy surface of hypochlorous acid, HOCl, has been determined from large-scale ab initio calculations using the coupled-cluster method CCSD(T), with basis sets of quadruple- and quintuple-zeta quality. The effect of core-electron correlation on the calculated structural parameters has been investigated. The vibrational-rotational energy levels of the three isotopic species of HOCl have then been calculated using the variational method and have been further characterized by the spectroscopic constants determined using the perturbational approach. The spectroscopic constants determined, are found to be in excellent agreement with experimental data.
Charge carrier motion in disordered conjugated polymers: a multiscale ab-initio study
Vukmirovic, Nenad; Wang, Lin-Wang
2009-11-10
We developed an ab-initio multiscale method for simulation of carrier transport in large disordered systems, based on direct calculation of electronic states and electron-phonon coupling constants. It enabled us to obtain the never seen before rich microscopic details of carrier motion in conjugated polymers, which led us to question several assumptions of phenomenological models, widely used in such systems. The macroscopic mobility of disordered poly(3- hexylthiophene) (P3HT) polymer, extracted from our simulation, is in agreement with experimental results from the literature.
NASA Astrophysics Data System (ADS)
Zhao, Pengfei; Liang, Chongyun; Gong, Xiwen; Gao, Ran; Liu, Jiwei; Wang, Min; Che, Renchao
2013-08-01
Monodispersed manganese oxide (Mn1-xCox)3O4 (0 <= x <= 0.5) nanoparticles, less than 10 nm size, are respectively synthesized via a facile thermolysis method at a rather low temperature, ranging from 90 to 100 °C, without any inertia gas for protection. The influences of the Co dopant content on the critical reaction temperature required for the nanoparticle formation, electronic band structures, magnetic properties, and the microwave absorption capability of (Mn1-xCox)3O4 are comprehensively investigated by means of both experimental and theoretical approaches including powder X-ray diffraction (XRD), electron energy loss spectroscopy (EELS), super conductivity quantum interference device (SQUID) examination, and first-principle simulations. Co is successfully doped into the Mn atomic sites of the (Mn1-xCox)3O4 lattice, which is further confirmed by EELS data acquired from one individual nanoparticle. Therefore, continuous solid solutions of well-crystallized (Mn1-xCox)3O4 products are achieved without any impurity phase or phase separation. With increases in the Co dopant concentration x from 0 to 0.5, the lattice parameters change systemically, where the overall saturation magnetization at 30 K increases due to the more intense coupling of the 3d electrons between Mn and Co, as revealed by simulations. The microwave absorption properties of the (Mn1-xCox)3O4 nanoparticles are examined between 2 and 18 GHz. The maximum absorption peak -11.0 dB of the x = 0 sample is enhanced to -11.5 dB for x = 0.2, -12.7 dB for x = 0.25, -15.6 dB for x = 0.33, and -24.0 dB for x = 0.5 respectively, suggesting the Co doping effects. Our results might provide novel insights into the understanding of the influences of metallic ion doping on the electromagnetic properties of metallic oxide nanomaterials.Monodispersed manganese oxide (Mn1-xCox)3O4 (0 <= x <= 0.5) nanoparticles, less than 10 nm size, are respectively synthesized via a facile thermolysis method at a rather low
The ab-initio density matrix renormalization group in practice
Olivares-Amaya, Roberto; Hu, Weifeng; Sharma, Sandeep; Yang, Jun; Chan, Garnet Kin-Lic; Nakatani, Naoki
2015-01-21
The ab-initio density matrix renormalization group (DMRG) is a tool that can be applied to a wide variety of interesting problems in quantum chemistry. Here, we examine the density matrix renormalization group from the vantage point of the quantum chemistry user. What kinds of problems is the DMRG well-suited to? What are the largest systems that can be treated at practical cost? What sort of accuracies can be obtained, and how do we reason about the computational difficulty in different molecules? By examining a diverse benchmark set of molecules: π-electron systems, benchmark main-group and transition metal dimers, and the Mn-oxo-salen and Fe-porphine organometallic compounds, we provide some answers to these questions, and show how the density matrix renormalization group is used in practice.
Ab initio engineering of materials with stacked hexagonal tin frameworks.
Shao, Junping; Beaufils, Clément; Kolmogorov, Aleksey N
2016-01-01
The group-IV tin has been hypothesized to possess intriguing electronic properties in an atom-thick hexagonal form. An attractive pathway of producing sizable 2D crystallites of tin is based on deintercalation of bulk compounds with suitable tin frameworks. Here, we have identified a new synthesizable metal distannide, NaSn2, with a 3D stacking of flat hexagonal layers and examined a known compound, BaSn2, with buckled hexagonal layers. Our ab initio results illustrate that despite being an exception to the 8-electron rule, NaSn2 should form under pressures easily achievable in multi-anvil cells and remain (meta)stable under ambient conditions. Based on calculated Z2 invariants, the predicted NaSn2 may display topologically non-trivial behavior and the known BaSn2 could be a strong topological insulator. PMID:27387140
Highly anisotropic thermal conductivity of arsenene: An ab initio study
NASA Astrophysics Data System (ADS)
Zeraati, Majid; Vaez Allaei, S. Mehdi; Abdolhosseini Sarsari, I.; Pourfath, Mahdi; Donadio, Davide
2016-02-01
Elemental two-dimensional (2D) materials exhibit intriguing heat transport and phononic properties. Here we have investigated the lattice thermal conductivity of newly proposed arsenene, the 2D honeycomb structure of arsenic, using ab initio calculations. Solving the Boltzmann transport equation for phonons, we predict a highly anisotropic thermal conductivity of 30.4 and 7.8 W/mK along the zigzag and armchair directions, respectively, at room temperature. Our calculations reveal that phonons with mean free paths between 20 nm and 1 μ m provide the main contribution to the large thermal conductivity in the zigzag direction; mean free paths of phonons contributing to heat transport in the armchair directions range between 20 and 100 nm. The obtained anisotropic thermal conductivity and feasibility of synthesis, in addition to high electron mobility reported elsewhere, make arsenene a promising material for nanoelectronic applications and thermal management.
Ab initio study of II-(VI)2 dichalcogenides.
Olsson, P; Vidal, J; Lincot, D
2011-10-12
The structural stabilities of the (Zn,Cd)(S,Se,Te)(2) dichalcogenides have been determined ab initio. These compounds are shown to be stable in the pyrite phase, in agreement with available experiments. Structural parameters for the ZnTe(2) pyrite semiconductor compound proposed here are presented. The opto-electronic properties of these dichalcogenide compounds have been calculated using quasiparticle GW theory. Bandgaps, band structures and effective masses are proposed as well as absorption coefficients and refraction indices. The compounds are all indirect semiconductors with very flat conduction band dispersion and high absorption coefficients. The work functions and surface properties are predicted. The Te and Se based compounds could be of interest as absorber materials in photovoltaic applications. PMID:21937783
Ab initio study of guanine damage by hydroxyl radical.
Chaban, Galina M; Wang, Dunyou; Huo, Winifred M
2015-01-15
Multiconfigurational ab initio methods are used in this study to examine two initial reactions that take place during the OH radical attack of the DNA base guanine: a ring opening reaction and a hydrogen transfer reaction. The same reactions are also studied in the presence of a single water molecule. The ring opening reaction has a moderate barrier height of ∼20-25 kcal/mol that is relatively insensitive to the presence of water. The barrier of the H-transfer reaction, on the other hand, is lowered from ∼50 to ∼22 kcal/mol when one water molecule is added, thus becoming comparable to the barrier height of the ring opening reaction. PMID:25517252
Vibrational and ab initio molecular dynamics studies of bradykinin
NASA Astrophysics Data System (ADS)
Święch, Dominika; Kubisiak, Piotr; Andrzejak, Marcin; Borowski, Piotr; Proniewicz, Edyta
2016-07-01
In this study, the comprehensive theoretical and experimental investigations of Raman (RS) and infrared absorption (IR) spectra of bradykinin (BK) are presented. The ab initio Born-Oppenheimer molecular dynamics (BOMD) calculations, in the presence of water molecules that form the first coordination sphere, were used for conformational analysis of the BK structure. Based on the Density Functional Theory (DFT) calculations at the B3LYP/6-311G(d) level the vibrational spectra were interpreted. The calculated frequencies were scaled by means of the effective scaling frequency factor (ESFF) method. The theoretical data, which confirm the compact structure of BK in the presence of the water molecules revealed the remarkable effect of the intermolecular hydrogen bonding on the BK structural properties.
Reactive Monte Carlo sampling with an ab initio potential
NASA Astrophysics Data System (ADS)
Leiding, Jeff; Coe, Joshua D.
2016-05-01
We present the first application of reactive Monte Carlo in a first-principles context. The algorithm samples in a modified NVT ensemble in which the volume, temperature, and total number of atoms of a given type are held fixed, but molecular composition is allowed to evolve through stochastic variation of chemical connectivity. We discuss general features of the method, as well as techniques needed to enhance the efficiency of Boltzmann sampling. Finally, we compare the results of simulation of NH3 to those of ab initio molecular dynamics (AIMD). We find that there are regions of state space for which RxMC sampling is much more efficient than AIMD due to the "rare-event" character of chemical reactions.
Efficient Ab initio Modeling of Random Multicomponent Alloys
NASA Astrophysics Data System (ADS)
Jiang, Chao; Uberuaga, Blas P.
2016-03-01
We present in this Letter a novel small set of ordered structures (SSOS) method that allows extremely efficient ab initio modeling of random multicomponent alloys. Using inverse II-III spinel oxides and equiatomic quinary bcc (so-called high entropy) alloys as examples, we demonstrate that a SSOS can achieve the same accuracy as a large supercell or a well-converged cluster expansion, but with significantly reduced computational cost. In particular, because of this efficiency, a large number of quinary alloy compositions can be quickly screened, leading to the identification of several new possible high-entropy alloy chemistries. The SSOS method developed here can be broadly useful for the rapid computational design of multicomponent materials, especially those with a large number of alloying elements, a challenging problem for other approaches.
Ab initio H2O in realistic hydrophilic confinement.
Allolio, Christoph; Klameth, Felix; Vogel, Michael; Sebastiani, Daniel
2014-12-15
A protocol for the ab initio construction of a realistic cylindrical pore in amorphous silica, serving as a geometric nanoscale confinement for liquids and solutions, is presented. Upon filling the pore with liquid water at different densities, the structure and dynamics of the liquid inside the confinement can be characterized. At high density, the pore introduces long-range oscillations into the water density profile, which makes the water structure unlike that of the bulk across the entire pore. The tetrahedral structure of water is also affected up to the second solvation shell of the pore wall. Furthermore, the effects of the confinement on hydrogen bonding and diffusion, resulting in a weakening and distortion of the water structure at the pore walls and a slowdown in diffusion, are characterized. PMID:25208765
High-throughput ab-initio dilute solute diffusion database
Wu, Henry; Mayeshiba, Tam; Morgan, Dane
2016-01-01
We demonstrate automated generation of diffusion databases from high-throughput density functional theory (DFT) calculations. A total of more than 230 dilute solute diffusion systems in Mg, Al, Cu, Ni, Pd, and Pt host lattices have been determined using multi-frequency diffusion models. We apply a correction method for solute diffusion in alloys using experimental and simulated values of host self-diffusivity. We find good agreement with experimental solute diffusion data, obtaining a weighted activation barrier RMS error of 0.176 eV when excluding magnetic solutes in non-magnetic alloys. The compiled database is the largest collection of consistently calculated ab-initio solute diffusion data in the world. PMID:27434308
Ab initio engineering of materials with stacked hexagonal tin frameworks
Shao, Junping; Beaufils, Clément; Kolmogorov, Aleksey N.
2016-01-01
The group-IV tin has been hypothesized to possess intriguing electronic properties in an atom-thick hexagonal form. An attractive pathway of producing sizable 2D crystallites of tin is based on deintercalation of bulk compounds with suitable tin frameworks. Here, we have identified a new synthesizable metal distannide, NaSn2, with a 3D stacking of flat hexagonal layers and examined a known compound, BaSn2, with buckled hexagonal layers. Our ab initio results illustrate that despite being an exception to the 8-electron rule, NaSn2 should form under pressures easily achievable in multi-anvil cells and remain (meta)stable under ambient conditions. Based on calculated Z2 invariants, the predicted NaSn2 may display topologically non-trivial behavior and the known BaSn2 could be a strong topological insulator. PMID:27387140
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.
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. PMID:26671378
Ab initio calculation of thermodynamic properties of silicon
NASA Astrophysics Data System (ADS)
Wei, Siqing; Li, Changlin; Chou, M. Y.
1994-11-01
We present a fully ab initio calculation of the thermodynamic properties for silicon within the quasiharmonic approximation, making use of volume-dependent phonon frequencies obtained from pseudopotential local-density calculations. The temperature dependence of the thermal-expansion coefficient, specific heat (at constant volume), and other related quantities are studied. We confirm that the thermal-expansion coefficient behaves differently in three temperature regions: positive for temperature below 15 K, negative between 15 and 125 K, and positive again above 125 K. This finding agrees with experiment. The abnormal (negative) thermal-expansion coefficient at low temperatures is explained through a detailed study of mode Grüneisen parameters. Both specific-heat and thermal-expansion-coefficient values calculated are in excellent agreement with experiment up to a few hundred kelvin.
XMVB: a program for ab initio nonorthogonal valence bond computations.
Song, Lingchun; Mo, Yirong; Zhang, Qianer; Wu, Wei
2005-04-15
An ab initio nonorthogonal valence bond program, called XMVB, is described in this article. The XMVB package uses Heitler-London-Slater-Pauling (HLSP) functions as state functions, and calculations can be performed with either all independent state functions for a molecule or preferably a few selected important state functions. Both our proposed paired-permanent-determinant approach and conventional Slater determinant expansion algorithm are implemented for the evaluation of the Hamiltonian and overlap matrix elements among VB functions. XMVB contains the capabilities of valence bond self-consistent field (VBSCF), breathing orbital valence bond (BOVB), and valence bond configuration interaction (VBCI) computations. The VB orbitals, used to construct VB functions, can be defined flexibly in the calculations depending on particular applications and focused problems, and they may be strictly localized, delocalized, or bonded-distorted (semidelocalized). The parallel version of XMVB based on MPI (Message Passing Interface) is also available. PMID:15704237
Ab initio theory of NMR chemical shifts in solids
Louie, S.G. |
1997-12-31
A new formalism for ab initio calculation of the orbital magnetic susceptibility and the NMR chemical shifts in solids and liquids is presented. The approach can be applied to periodic systems such as crystals, surfaces or polymers, and with a supercell technique, to nonperiodic systems such as amorphous materials, liquids, or solids with defects. The formalism is based on the density functional theory in the local density approximation and makes use of a generalized f-sum rule to eliminate the divergent terms that plagued previous theories. Calculations have been successfully carried out for the diamagnetic susceptibility of a number of insulators and for the NMR chemical shifts of a variety of systems including free molecules, ionic crystals, hydrogen-bonded materials and amorphous carbon.
Ab initio Potential Energy Surface for H-H2
NASA Technical Reports Server (NTRS)
Partridge, Harry; Bauschlicher, Charles W., Jr.; Stallcop, James R.; Levin, Eugene
1993-01-01
Ab initio calculations employing large basis sets are performed to determine an accurate potential energy surface for H-H2 interactions for a broad range of separation distances. At large distances, the spherically averaged potential determined from the calculated energies agrees well with the corresponding results determined from dispersion coefficients; the van der Waals well depth is predicted to be 75 +/- (mu)E(sub h). Large basis sets have also been applied to reexamine the accuracy of theoretical repulsive potential energy surfaces. Multipolar expansions of the computed H-H2 potential energy surface are reported for four internuclear separation distances (1.2, 1.401, 1.449, and 1.7a(sub 0) of the hydrogen molecule. The differential elastic scattering cross section calculated from the present results is compared with the measurements from a crossed beam experiment.
Ab initio calculations of grain boundaries in bcc metals
NASA Astrophysics Data System (ADS)
Scheiber, Daniel; Pippan, Reinhard; Puschnig, Peter; Romaner, Lorenz
2016-03-01
In this study, we compute grain boundary (GB) properties for a large set of GBs in bcc transition metals with a special focus on W, Mo and Fe using ab initio density functional theory (DFT) and semi-empirical second nearest neighbour modified embedded atom method (2NN-MEAM) potentials. The GB properties include GB energies, surface energies, GB excess volume and work of separation, which we analyse and then compare to experimental data. We find that the used 2NN-MEAM potentials can predict general trends of GB properties, but do not always reproduce the GB ground state structure and energy found with DFT. In particular, our results explain the experimental finding that W and Mo prefer intergranular fracture, while other bcc metals prefer transgranular cleavage.
Transport coefficients in diamond from ab-initio calculations
NASA Astrophysics Data System (ADS)
Löfâs, Henrik; Grigoriev, Anton; Isberg, Jan; Ahuja, Rajeev
2013-03-01
By combining the Boltzmann transport equation with ab-initio electronic structure calculations, we obtain transport coefficients for boron-doped diamond. We find the temperature dependence of the resistivity and the hall coefficients in good agreement with experimental measurements. Doping in the samples is treated via the rigid band approximation and scattering is treated in the relaxation time approximation. In contrast to previous results, the acoustic phonon scattering is the dominating scattering mechanism for the considered doping range. At room temperature, we find the thermopower, S, in the range 1-1.6 mV/K and the power factor, S2σ, in the range 0.004-0.16 μW /cm K2.
Ab initio potential energy surface and rovibrational states of HBO
NASA Astrophysics Data System (ADS)
Ha, Tae-Kyu; Makarewicz, Jan
1999-01-01
The potential energy surface describing the large-amplitude motion of H around the BO core in the HBO molecule has been determined from ab initio calculations. This surface has been sampled by a set of 170 grid points from a two-dimensional space defined by the stretching and the bending coordinates of the H nucleus. At each grid point, the BO bond length has been optimized using the second-order Møller-Plesset perturbation theory with the basis set aug-cc-pVTZ. The surface has a local minimum for the linear as well as the bent configuration of HBO. A low energy barrier to the linear configuration BOH causes a large-amplitude motion and a strong rovibrational interaction in the molecule. Its rovibrational dynamics is different from the dynamics in bent or quasilinear triatomic molecules.
Ab initio engineering of materials with stacked hexagonal tin frameworks
NASA Astrophysics Data System (ADS)
Shao, Junping; Beaufils, Clément; Kolmogorov, Aleksey N.
2016-07-01
The group-IV tin has been hypothesized to possess intriguing electronic properties in an atom-thick hexagonal form. An attractive pathway of producing sizable 2D crystallites of tin is based on deintercalation of bulk compounds with suitable tin frameworks. Here, we have identified a new synthesizable metal distannide, NaSn2, with a 3D stacking of flat hexagonal layers and examined a known compound, BaSn2, with buckled hexagonal layers. Our ab initio results illustrate that despite being an exception to the 8-electron rule, NaSn2 should form under pressures easily achievable in multi-anvil cells and remain (meta)stable under ambient conditions. Based on calculated Z2 invariants, the predicted NaSn2 may display topologically non-trivial behavior and the known BaSn2 could be a strong topological insulator.
Ab initio electronic stopping power of protons in bulk materials
NASA Astrophysics Data System (ADS)
Shukri, Abdullah Atef; Bruneval, Fabien; Reining, Lucia
2016-01-01
The electronic stopping power is a crucial quantity for ion irradiation: it governs the deposited heat, the damage profile, and the implantation depth. Whereas experimental data are readily available for elemental solids, the data are much more scarce for compounds. Here we develop a fully ab initio computational scheme based on linear response time-dependent density-functional theory to predict the random electronic stopping power (RESP) of materials without any empirical fitting. We show that the calculated RESP compares well with experimental data, when at full convergence, with the inclusion of the core states and of the exchange correlation. We evaluate the unexpectedly limited magnitude of the nonlinear terms in the RESP by comparing with other approaches based on the time propagation of time-dependent density-functional theory. Finally, we check the validity of a few empirical rules of thumbs that are commonly used to estimate the electronic stopping power.
Ab initio study of helium behavior in titanium tritides
Liang, J. H.; Dai, Yunya; Yang, Li; Peng, SM; Fan, K. M.; Long, XG; Zhou, X. S.; Zu, Xiaotao; Gao, Fei
2013-03-01
Ab initio calculations based on density functional theory have been performed to investigate the relative stability of titanium tritides and the helium behavior in stable titanium tritides. The results show that the β-phase TiT1.5 without two tritium along the [100] direction (TiT1.5[100]) is more stable than other possible structures. The stability of titanium tritides decrease with the increased generation of helium in TiT1.5[100]. In addition, helium generated by tritium decay prefers locating at a tetrahedral site, and favorably migrates between two neighbor vacant tetrahedral sites through an intermediate octahedral site in titanium tritides, with a migration energy of 0.23 eV. Furthermore, helium is easily accumulated on a (100) plane in β-phase TiT1.5[100].
High-throughput ab-initio dilute solute diffusion database.
Wu, Henry; Mayeshiba, Tam; Morgan, Dane
2016-01-01
We demonstrate automated generation of diffusion databases from high-throughput density functional theory (DFT) calculations. A total of more than 230 dilute solute diffusion systems in Mg, Al, Cu, Ni, Pd, and Pt host lattices have been determined using multi-frequency diffusion models. We apply a correction method for solute diffusion in alloys using experimental and simulated values of host self-diffusivity. We find good agreement with experimental solute diffusion data, obtaining a weighted activation barrier RMS error of 0.176 eV when excluding magnetic solutes in non-magnetic alloys. The compiled database is the largest collection of consistently calculated ab-initio solute diffusion data in the world. PMID:27434308
Efficient Ab initio Modeling of Random Multicomponent Alloys.
Jiang, Chao; Uberuaga, Blas P
2016-03-11
We present in this Letter a novel small set of ordered structures (SSOS) method that allows extremely efficient ab initio modeling of random multicomponent alloys. Using inverse II-III spinel oxides and equiatomic quinary bcc (so-called high entropy) alloys as examples, we demonstrate that a SSOS can achieve the same accuracy as a large supercell or a well-converged cluster expansion, but with significantly reduced computational cost. In particular, because of this efficiency, a large number of quinary alloy compositions can be quickly screened, leading to the identification of several new possible high-entropy alloy chemistries. The SSOS method developed here can be broadly useful for the rapid computational design of multicomponent materials, especially those with a large number of alloying elements, a challenging problem for other approaches. PMID:27015491
Ab initio correlated calculations of rare-gas dimer quadrupoles
NASA Astrophysics Data System (ADS)
Donchev, Alexander G.
2007-10-01
This paper reports ab initio calculations of rare gas ( RG=Kr , Ar, Ne, and He) dimer quadrupoles at the second order of Møller-Plesset perturbation theory (MP2). The study reveals the crucial role of the dispersion contribution to the RG2 quadrupole in the neighborhood of the equilibrium dimer separation. The magnitude of the dispersion quadrupole is found to be much larger than that predicted by the approximate model of Hunt. As a result, the total MP2 quadrupole moment is significantly smaller than was assumed in virtually all previous related studies. An analytical model for the distance dependence of the RG2 quadrupole is proposed. The model is based on the effective-electron approach of Jansen, but replaces the original Gaussian approximation to the electron density in an RG atom by an exponential one. The role of the nonadditive contribution in RG3 quadrupoles is discussed.
Ab Initio Screening of CO2-philic Groups.
Tian, Ziqi; Saito, Tomonori; Jiang, De-En
2015-04-23
Ab initio calculations were used to identify CO2-philic groups. Over 55 neutral molecules were screened for CO2 affinity via binding energetics. It is found that poly(ethylene oxide)s (PEO) oligomers with more than three repeating units are good CO2-binding groups, consistent with the high-performance of PEO-based materials for CO2/N2 separation. More interestingly, two triazole groups linked with a methylene chain are also excellent for CO2 binding with a favorable interaction of more than 28 kJ/mol, indicating that polymers or covalent-organic frameworks (COFs) with triazoles may be utilized for CO2 capture. This work provides a useful guide to introduce promising organic groups into polymeric membranes and COFs for CO2/N2 separation media. PMID:25825811
Ab initio methods for nuclear properties - a computational physics approach
NASA Astrophysics Data System (ADS)
Maris, Pieter
2011-04-01
A microscopic theory for the structure and reactions of light nuclei poses formidable challenges for high-performance computing. Several ab-initio methods have now emerged that provide nearly exact solutions for some nuclear properties. The ab-initio no-core full configuration (NCFC) approach is based on basis space expansion methods and uses Slater determinants of single-nucleon basis functions to express the nuclear wave function. In this approach, the quantum many-particle problem becomes a large sparse matrix eigenvalue problem. The eigenvalues of this matrix give us the binding energies, and the corresponding eigenvectors the nuclear wave functions. These wave functions can be employed to evaluate experimental quantities. In order to reach numerical convergence for fundamental problems of interest, the matrix dimension often exceeds 1 billion, and the number of nonzero matrix elements may saturate available storage on present-day leadership class facilities. I discuss different strategies for distributing and solving this large sparse matrix on current multicore computer architectures, including methods to deal with with memory bottleneck. Several of these strategies have been implemented in the code MFDn, which is a parallel fortran code for nuclear structure calculations. I will show scaling behavior and compare the performance of the pure MPI version with the hybrid MPI/OpenMP code on Cray XT4 and XT5 platforms. For large core counts (typically 5,000 and above), the hybrid version is more efficient than pure MPI. With this code, we have been able to predict properties of the unstable nucleus 14F, which have since been confirmed by experiments. I will also give an overview of other recent results for nuclei in the A = 6 to 16 range with 2- and 3-body interactions. Supported in part by US DOE Grant DE-FC02-09ER41582.
An Ab Initio Approach Towards Engineering Fischer-Tropsch Surface Chemistry
Matthew Neurock; David A. Walthall
2006-05-07
One of the greatest societal challenges over the next decade is the production of cheap, renewable energy for the 10 billion people that inhabit the earth. This will require the development of various different energy sources potentially including fuels derived from methane, coal, and biomass and alternatives sources such as solar, wind and nuclear energy. One approach will be to synthesize gasoline and other fuels from simpler hydrocarbons such as CO derived from methane or other U.S. based sources such as coal. Syngas (CO and H{sub 2}) can be readily converted into higher molecular weight hydrocarbons through Fischer-Tropsch synthesis. Fischer-Tropsch synthesis involves the initiation or activation of CO and H{sub 2} bonds, the subsequent propagation steps including hydrogenation and carbon-carbon coupling, followed by chain termination reactions. Commercially viable catalysts include supported Co and Co-alloys. Over the first two years of this project we have used ab initio methods to determine the adsorption energies for all reactants, intermediates, and products along with the overall reaction energies and their corresponding activation barriers over the Co(0001) surface. Over the third year of the project we developed and advanced an ab initio-based kinetic Monte Carlo simulation code to simulate Fischer Tropsch synthesis. This report details our work over the last year which has focused on the derivation of kinetic parameters for the elementary steps involved in FT synthesis from ab initio density functional theoretical calculations and the application of the kinetic Monte Carlo algorithm to simulate the initial rates of reaction for FT over the ideal Co(0001) surface. The results from our simulations over Co(0001) indicate the importance of stepped surfaces for the activation of adsorbed CO. In addition, they demonstrate that the dominant CH{sub x}* surface intermediate under steady state conditions is CH*. This strongly suggests that hydrocarbon coupling
4He Thermophysical Properties: New Ab Initio Calculations
Hurly, John J.; Mehl, James B.
2007-01-01
Since 2000, atomic physicists have reduced the uncertainty of the helium-helium “ab initio” potential; for example, from approximately 0.6 % to 0.1 % at 4 bohr, and from 0.8 % to 0.1 % at 5.6 bohr. These results led us to: (1) construct a new inter-atomic potential ϕ07, (2) recalculate values of the second virial coefficient, the viscosity, and the thermal conductivity of 4He from 1 K to 10,000 K, and (3), analyze the uncertainties of the thermophysical properties that propagate from the uncertainty of ϕ07 and from the Born-Oppenheimer approximation of the electron-nucleon quantum mechanical system. We correct minor errors in a previous publication [J. J. Hurly and M. R. Moldover, J. Res. Nat. Inst. Standards Technol. 105, 667 (2000)] and compare our results with selected data published after 2000. The ab initio results tabulated here can serve as standards for the measurement of thermophysical properties. PMID:27110456
Operator evolution for ab initio theory of light nuclei
NASA Astrophysics Data System (ADS)
Schuster, Micah; Quaglioni, Sofia; Johnson, Calvin; Jurgenson, Eric; Navrátil, Petr
2014-09-01
The past two decades have seen a revolution in ab initio calculations of nuclear properties. One key element has been the development of a rigorous effective interaction theory, applying unitary transformations to soften the nuclear Hamiltonian and hence accelerate the convergence as a function of the model space size. For consistency, however, one ought to apply the same transformation to other operators when calculating transitions and mean values from the eigenstates of the renormalized Hamiltonian. Working in a translationally invariant harmonic oscillator basis for the two- and three-nucleon systems, we evolve the Hamiltonian, square radius, and total dipole strength operators by the similarity renormalization group (SRG). The inclusion of up to three-body matrix elements in the 4He nucleus all but completely restores the invariance of the expectation values under the transformation. We also consider a Gaussian operator with adjustable range; short ranges have the largest absolute renormalization when including two- and three-body induced terms, while at long ranges the induced three-body contribution takes on increased relative importance. The past two decades have seen a revolution in ab initio calculations of nuclear properties. One key element has been the development of a rigorous effective interaction theory, applying unitary transformations to soften the nuclear Hamiltonian and hence accelerate the convergence as a function of the model space size. For consistency, however, one ought to apply the same transformation to other operators when calculating transitions and mean values from the eigenstates of the renormalized Hamiltonian. Working in a translationally invariant harmonic oscillator basis for the two- and three-nucleon systems, we evolve the Hamiltonian, square radius, and total dipole strength operators by the similarity renormalization group (SRG). The inclusion of up to three-body matrix elements in the 4He nucleus all but completely restores
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 calculation and anharmonic force field of hypochlorous acid, HOCl
NASA Astrophysics Data System (ADS)
Halonen, L.; Ha, T.-K.
1988-03-01
Ab initio calculations on HOCl have been performed at the third-order Møller-Plesset perturbation theory level to determine the equilibrium structure and the anharmonic force field. An empirical anharmonic force field based on the ab initio results is obtained using available experimental vibration-rotation data. Four of the six harmonic and six of the ten cubic force constants have been determined experimentally, the remaining values being fixed at the ab initio values. A good fit to the experimental vibration-rotation data of four isotopic species is obtained.
Liu, Hui; Shi, Deheng; Sun, Jinfeng; Zhu, Zunlue
2013-01-15
The potential energy curves (PECs) of 15 Ω states generated from five Λ-S states (A2Π, 1(4)Σ+, 1(4)Π, 2(4)Π and 1(6)Σ+) of AlO radical are studied in detail using high level ab initio quantum chemical method for the first time. All the PEC calculations are made by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification (MRCI+Q). The spin-orbit coupling effect is included by the Breit-Pauli Hamiltonian with the aug-cc-pCVTZ basis set. Convergent behavior is discussed and excellent convergence has been observed with respect to the basis sets and level of theory. To improve the quality of PECs, core-valence correlation and scalar relativistic corrections are taken into account. Core-valence correlation corrections are included employing a cc-pCVQZ basis set. Scalar relativistic corrections are calculated by the third-order Douglas-Kroll Hamiltonian approximation at the level of a cc-pV5Z basis set. All the PECs are extrapolated to the complete basis set limit by the total-energy extrapolation scheme. With these PECs including all the corrections used here, on the one hand, the spectroscopic parameters of all the Λ-S and Ω states are calculated, which are in reasonable agreement with the experimental and other theoretical results; on the other hand, the vibrational levels and inertial rotation constants of X2Σ+, A2Π, B2Σ+ Λ-S states as well as A2Π3/2 and A2Π1/2 Ω states are determined, which also agree well with the measurements. The vibrational levels and inertial rotation constants of A(2)Π3/2 and A2Π1/2 Ω states as well as the spectroscopic parameters of four Λ-S states (1(4)Σ+, 1(4)Π, 2(4)Π and 1(6)Σ+) and their corresponding 13 Ω states can be expected to be reliable predicted ones. PMID:23143209
Towards an ab initio description of correlated materials
NASA Astrophysics Data System (ADS)
Yee, Chuck-Hou
Strongly-correlated materials are a rich playground for physical phenomena, exhibiting complex phase diagrams with many competing orders. Ab initio insights into materials combined with physical ideas provide the ability to identify the organizing principles driving the correlated electronic behavior and pursue first-principles design of new compounds. Realistic modeling of correlated materials is an active area of research, especially with the recent merger of density functional theory (DFT) with dynamical mean-field theory (DMFT). This thesis is structured in two parts. The first describes the methods and algorithmic developments which drive advances in DFT+DMFT. In Ch. 2 and 3, we provide an overview of the two foundational theories, DMFT and DFT. In the second half of Ch. 3, we describe some of the principles guiding the combination of the two theories to form DFT+DMFT. In Ch. 4, we describe the algorithm lying at the heart of modern DFT+DMFT implementations, the hybridization expansion formulation of continuous-time quantum monte carlo (CTQMC) for the general Anderson impurity problem, as well as a fast rejection algorithm for speeding-up the local trace evaluation. The final chapter in the methods section describes an algorithm for direct sampling of the partition function, and thus the free energy and entropy, of simple Anderson impurity models within CTQMC. The second part of the thesis is a collection of applications of our ab initio approach to key correlated materials. We first apply our method to plutonium binary alloys (Ch. 6), which when supplemented with slave-boson mean-field theory, allows us to understand the observed photoemission spectra. Ch. 7 describes the computation of spectra and optical conductivity for rare-earth nickelates grown as epitaxial thin films. In the final two chapters, we turn our attention to the high-temperature superconductors. In the first, we show that the charge-transfer energy is a key chemical variable which controls
Ab Initio Studies of Halogen and Nitrogen Oxide Species of Interest in Stratospheric Chemistry
NASA Technical Reports Server (NTRS)
Lee, Timothy J.; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
The ability of modern state-of-the art ab initio quantum chemical techniques to characterize reliably the gas-phase molecular structure, vibrational spectrum, electronic spectrum, and thermal stability of fluorine, chlorine, bromine and nitrogen oxide species will be demonstrated by presentation of some example studies. The ab initio results are shown to be in excellent agreement with the available experimental data, and where the experimental data are either not known or are inconclusive, the theoretical results are shown to fill in the gaps and to resolve experimental controversies. In addition, ab initio studies in which the electronic spectra and the characterization of excited electronic states of halogen oxide species will also be presented. Again where available, the ab initio results are compared to experimental observations, and are used to aid in the interpretation of experimental studies.
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.
Ab initio studies of niobium defects in uranium
Xiang, S; Huang, H; Hsiung, L
2007-06-01
Uranium (U), with the addition of small amount of niobium (Nb), is stainless. The Nb is fully miscible with the high temperature phase of U and tends to segregate upon cooling below 647 C. The starting point of segregation is the configuration of Nb substitutional or interstitial defects. Using density-functional-theory based ab initio calculations, the authors find that the formation energy of a single vacancy is 1.08 eV, that of Nb substitution is 0.59 eV, that of Nb interstitial at octahedral site is 1.58 eV, and that of Nb interstitial at tetrahedral site is 2.35 eV; all with reference to a reservoir of {gamma} phase U and pure Nb. The formation energy of Nb defects correlates with the local perturbation of electron distribution; higher formation energy to larger perturbation. Based on this study, Nb atoms thermodynamically prefer to occupy substitutional sites in {gamma} phase U, and they prefer to be in individual substitutional defects than clusters.
Melting curves of metals by ab initio calculations
NASA Astrophysics Data System (ADS)
Minakov, Dmitry; Levashov, Pavel
2015-06-01
In this work we used several ab initio approaches to reproduce melting curves and discussed their abilities, advantages and drawbacks. We used quasiharmonic appoximation and Lindemann criterion to build melting curves in wide region of pressures. This approach allows to calculate the total free energy of electrons and phonons, so it is possible to obtain all thermodynamic properties in the crystalline state. We also used quantum molecular dynamics simulations to investigate melting at various pressures. We explored the size-effect of the heat until it melts (HUM) method in detail. Special attention was paid to resolve the boundaries of the melting region on density. All calculations were performed for aluminum, copper and gold. Results were in good agreement with available experimental data. Also we studied the influence of electronic temperature on melting curves. It turned out that the melting temperature increased with the rise of electron temperature at normal density and had non-monotonic behavior at higher densities. This work is supported by the Ministry of Education and Science of the Russian Federation (Project No. 3.522.2014/K).
Ab initio simulation of gap discrete breathers in strained graphene
NASA Astrophysics Data System (ADS)
Lobzenko, I. P.; Chechin, G. M.; Bezuglova, G. S.; Baimova, Yu. A.; Korznikova, E. A.; Dmitriev, S. V.
2016-03-01
The methods of the density functional theory were used for the first time for the simulation of discrete breathers in graphene. It is demonstrated that breathers can exist with frequencies lying in the gap of the phonon spectrum, induced by uniaxial tension of a monolayer graphene sheet in the "zigzag" direction (axis X), polarized in the "armchair" direction (axis Y). The found gap breathers are highly localized dynamic objects, the core of which is formed by two adjacent carbon atoms located on the Y axis. The atoms surrounding the core vibrate at much lower amplitudes along both the axes ( X and Y). The dependence of the frequency of these breathers on amplitude is found, which shows a soft type of nonlinearity. No breathers of this type were detected in the gap induced by stretching along the Y axis. It is shown that the breather vibrations may be approximated by the Morse oscillators, the parameters of which are determined from ab initio calculations. The results are of fundamental importance, as molecular dynamics calculations based on empirical potentials cannot serve as a reliable proof of the existence of breathers in crystals.
Volumic omit maps in ab initio dual-space phasing.
Oszlányi, Gábor; Sütő, András
2016-07-01
Alternating-projection-type dual-space algorithms have a clear construction, but are susceptible to stagnation and, thus, inefficient for solving the phase problem ab initio. To improve this behaviour new omit maps are introduced, which are real-space perturbations applied periodically during the iteration process. The omit maps are called volumic, because they delete some predetermined subvolume of the unit cell without searching for atomic regions or analysing the electron density in any other way. The basic algorithms of positivity, histogram matching and low-density elimination are tested by their solution statistics. It is concluded that, while all these algorithms based on weak constraints are practically useless in their pure forms, appropriate volumic omit maps can transform them to practically useful methods. In addition, the efficiency of the already useful reflector-type charge-flipping algorithm can be further improved. It is important that these results are obtained by using non-sharpened structure factors and without any weighting scheme or reciprocal-space perturbation. The mathematical background of volumic omit maps and their expected applications are also discussed. PMID:27357850
Ab initio study of optical excitations in VO2
NASA Astrophysics Data System (ADS)
Coulter, John; Gali, Adam; Manousakis, Efstratios
2014-03-01
Motivated by recent experimental efforts to fabricate p-n junctions from transition metal oxides (TMOs) and a recent theoretical study claiming TMOs to be good absorbers and promising materials for efficient carrier multiplication, we study the optical properties of a prototypical TMO, the insulator M1 phase of vanadium dioxide (VO2), by ab initio methods. We applied the Bethe-Salpeter equations (BSE) to calculate the optical properties, starting from self-consistent GW quasi-particle energy levels and states. In contrast to expectations, the exciton binding energy obtained by BSE is in good agreement with the experiment. We find that the electron-electron interaction is very strong which makes this material promising for efficient carrier multiplication that might lead to an enhanced efficiency in photo-voltaics applications. To illustrate this more quantitatively, we calculated the impact ionization rate within the independent quasiparticle approximation, and find that the rate is significantly higher than silicon in the region of highest solar intensity, due to the strong multiple carrier excitations.
Ab initio studies of phoshorene island single electron transistor.
Ray, S J; Venkata Kamalakar, M; Chowdhury, R
2016-05-18
Phosphorene is a newly unveiled two-dimensional crystal with immense potential for nanoelectronic and optoelectronic applications. Its unique electronic structure and two dimensionality also present opportunities for single electron devices. Here we report the behaviour of a single electron transistor (SET) made of a phosphorene island, explored for the first time using ab initio calculations. We find that the band gap and the charging energy decrease monotonically with increasing layer numbers due to weak quantum confinement. When compared to two other novel 2D crystals such as graphene and MoS2, our investigation reveals larger adsorption energies of gas molecules on phosphorene, which indicates better a sensing ability. The calculated charge stability diagrams show distinct changes in the presence of an individual molecule which can be applied to detect the presence of different molecules with sensitivity at a single molecular level. The higher charging energies of the molecules within the SET display operational viability at room temperature, which is promising for possible ultra sensitive detection applications. PMID:27093536
Ab initio studies of phosphorene island single electron transistor
NASA Astrophysics Data System (ADS)
Ray, S. J.; Venkata Kamalakar, M.; Chowdhury, R.
2016-05-01
Phosphorene is a newly unveiled two-dimensional crystal with immense potential for nanoelectronic and optoelectronic applications. Its unique electronic structure and two dimensionality also present opportunities for single electron devices. Here we report the behaviour of a single electron transistor (SET) made of a phosphorene island, explored for the first time using ab initio calculations. We find that the band gap and the charging energy decrease monotonically with increasing layer numbers due to weak quantum confinement. When compared to two other novel 2D crystals such as graphene and MoS2, our investigation reveals larger adsorption energies of gas molecules on phosphorene, which indicates better a sensing ability. The calculated charge stability diagrams show distinct changes in the presence of an individual molecule which can be applied to detect the presence of different molecules with sensitivity at a single molecular level. The higher charging energies of the molecules within the SET display operational viability at room temperature, which is promising for possible ultra sensitive detection applications.
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.
Ab initio investigation of grain boundary cohesion in Al alloys
NASA Astrophysics Data System (ADS)
Zhang, Shengjun; Kontsevoi, Oleg Y.; Freeman, A. J.; Olson, G. B.
2010-03-01
Strength and hardness of aluminum alloys can be substantially increased by alloying with Mg, Zn, Cu, Si, and other elements. The main drawback of Al alloys is their susceptibility to stress corrosion cracking, which is caused by alloying impurities segregated at grain boundaries. We investigated the embrittling and cohesion-enhancing effects of impurities on a σ5(012)[100] grain boundary in Al by means of the full-potential linearized augmented plane-wave (FLAPW) method within the framework of the Rice-Wang thermodynamic model and within the ab initio tensile test approach. We calculated segregation energies, analyzed local atomic configurations, electronic structures and spatial charge density distributions around segregated impurities, and identified the roles of atomic size and the bonding behavior of the impurity with the surrounding Al atoms. The results show that He, H and Na are strong embrittlers, Zn is a weak embrittler, while Sc, B, Cu and Mg are cohesion enhancers. We further evaluated the effect of co-alloying with two or more elements on grain boundary strength. This work provides a fundamental basis for the design of high strength Al alloys.
Exploring the free energy surface using ab initio molecular dynamics.
Samanta, Amit; Morales, Miguel A; Schwegler, Eric
2016-04-28
Efficient exploration of configuration space and identification of metastable structures in condensed phase systems are challenging from both computational and algorithmic perspectives. In this regard, schemes that utilize a set of pre-defined order parameters to sample the relevant parts of the configuration space [L. Maragliano and E. Vanden-Eijnden, Chem. Phys. Lett. 426, 168 (2006); J. B. Abrams and M. E. Tuckerman, J. Phys. Chem. B 112, 15742 (2008)] have proved useful. Here, we demonstrate how these order-parameter aided temperature accelerated sampling schemes can be used within the Born-Oppenheimer and the Car-Parrinello frameworks of ab initio molecular dynamics to efficiently and systematically explore free energy surfaces, and search for metastable states and reaction pathways. We have used these methods to identify the metastable structures and reaction pathways in SiO2 and Ti. In addition, we have used the string method [W. E, W. Ren, and E. Vanden-Eijnden, Phys. Rev. B 66, 052301 (2002); L. Maragliano et al., J. Chem. Phys. 125, 024106 (2006)] within the density functional theory to study the melting pathways in the high pressure cotunnite phase of SiO2 and the hexagonal closed packed to face centered cubic phase transition in Ti. PMID:27131525
TOPICAL REVIEW: Ab initio symplectic no-core shell model
NASA Astrophysics Data System (ADS)
Dytrych, T.; Sviratcheva, K. D.; Draayer, J. P.; Bahri, C.; Vary, J. P.
2008-12-01
The no-core shell model (NCSM) is a prominent ab initio method that yields a good description of the low-lying states in few-nucleon systems as well as in more complex p-shell nuclei. Nevertheless, its applicability is limited by the rapid growth of the many-body basis with larger model spaces and increasing number of nucleons. The symplectic no-core shell model (Sp-NCSM) aspires to extend the scope of the NCSM beyond the p-shell region by augmenting the conventional spherical harmonic oscillator basis with the physically relevant symplectic \\SpR{3} symmetry-adapted configurations of the symplectic shell model that describe naturally the monopole-quadrupole vibrational and rotational modes, and also partially incorporate α-cluster correlations. In this review, the models underpinning the Sp-NCSM approach, namely, the NCSM, the Elliott SU(3) model and the symplectic shell model, are discussed. Following this, a prescription for constructing translationally invariant symplectic configurations in the spherical harmonic oscillator basis is given. This prescription is utilized to unveil the extent to which symplectic configurations enter into low-lying states in 12C and 16O nuclei calculated within the framework of the NCSM with the JISP16 realistic nucleon-nucleon interaction. The outcomes of this proof-of-principle study are presented in detail.
Microsolvation of methyl hydrogen peroxide: Ab initio quantum chemical approach
NASA Astrophysics Data System (ADS)
Kulkarni, Anant D.; Rai, Dhurba; Bartolotti, Libero J.; Pathak, Rajeev K.
2009-08-01
Methyl hydrogen peroxide (MHP), one of the simplest organic hydroperoxides, is a strong oxidant, with enhanced activity in aqueous ambience. The present study investigates, at the molecular level, the role of hydrogen bonding that is conducive to cluster formation of MHP with water molecules from its peroxide end, with the methyl group remaining hydrophobic for up to five water molecules. Ab initio quantum chemical computations on MHP⋯(H2O)n, [n =1-5] are performed at second order Møller-Plesset (MP2) perturbation theory employing the basis sets 6-31G(d,p) and 6-311++G(2d,2p) to study the cluster formation of MHP with water molecules from its peroxide end and hydrophobic hydration due to the methyl group. Successive addition of water molecules alters the hydrogen bonding pattern, which leads to changes in overall cluster geometry and in turn to IR vibrational frequency shifts. Molecular co-operativity in these clusters is gauged directly through a detailed many-body interaction energy analysis. Molecular electrostatic potential maps are shown to have a bearing on predicting further growth of these clusters, which is duly corroborated through sample calculations for MHP⋯(H2O)8. Further, a continuum solvation model calculation for energetically stable clusters suggests that this study should serve as a precursor for pathways to aqueous solvation of MHP.
Ab initio study of MoS2 nanotube bundles
NASA Astrophysics Data System (ADS)
Verstraete, Matthieu; Charlier, Jean-Christophe
2003-07-01
Recently, the synthesis of a new phase of MoS2I1/3 stoichiometry was reported [M. Remskar, A. Mrzel, Z. Skraba, A. Jesih, M. Ceh, J. Demšar, P. Stadelmann, F. Lévy, and D. Mihailovic, Science 292, 479 (2001)]. Electron microscope images and diffraction data were interpreted to indicate bundles of sub-nanometer-diameter single-wall MoS2 nanotubes. After experimental characterization, the structure was attributed to an assembly of “armchair” nanotubes with interstitial iodine. Using first-principles total-energy calculations, bundles of MoS2 nanotubes with different topologies and stoichiometries are investigated. All of the systems are strongly metallic. Configurations with “zigzag” structures are found to be more stable energetically than the “armchair” ones, though all of the structures have similar stabilities. After relaxation, there remain several candidates which give a lattice parameter in relative agreement with experiment. Further, spin-polarized calculations indicate that a structure with armchair tubes iodine atoms in their center acquires a very large spontaneous magnetic moment of 12μB, while the other structures are nonmagnetic. Our ab initio calculations show that in most of the other structures, the tubes are very strongly bound together, and that the compounds should be considered as a crystal, rather than as a bundle of tubes in the habitual sense.
Ab initio modelling of methane hydrate thermophysical properties.
Jendi, Z M; Servio, P; Rey, A D
2016-04-21
The key thermophysical properties of methane hydrate were determined using ab initio modelling. Using density functional theory, the second-order elastic constants, heat capacity, compressibility, and thermal expansion coefficient were calculated. A wide and relevant range of pressure-temperature conditions were considered, and the structures were assessed for stability using the mean square displacement and radial distribution functions. Methane hydrate was found to be elastically isotropic with a linear dependence of the bulk modulus on pressure. Equally significant, multi-body interactions were found to be important in hydrates, and water-water interactions appear to strongly influence compressibility like in ice Ih. While the heat capacity of hydrate was found to be higher than that of ice, the thermal expansion coefficient was significantly lower, most likely due to the lower rigidity of hydrates. The mean square displacement gave important insight into stability, heat capacity, and elastic moduli, and the radial distribution functions further confirmed stability. The presented results provide a much needed atomistic thermoelastic characterization of methane hydrates and are essential input for the large-scale applications of hydrate detection and production. PMID:27019976
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.
Ab initio simulations of pseudomorphic silicene and germanene bidimensional heterostructures
NASA Astrophysics Data System (ADS)
Debernardi, Alberto; Marchetti, Luigi
2016-06-01
Among the novel two-dimensional (2D) materials, silicene and germanene, which are two honeycomb crystal structures composed of a monolayer of Si and Ge, respectively, have attracted the attention of material scientists because they combine the advantages of the new 2D ultimate-scaled electronics with their compatibility with industrial processes presently based on Si and Ge. We envisage pseudomorphic lateral heterostructures based on ribbons of silicene and germanene, which are the 2D analogs of conventional 3D Si/Ge superlattices and quantum wells. In spite of the considerable lattice mismatch (˜4 % ) between free-standing silicene and germanene, our ab initio simulations predict that, considering striped 2D lateral heterostructures made by alternating silicene and germanene ribbons of constant width, the silicene/germanene junction remains pseudomorphic—i.e., it maintains lattice-matched edges—up to critical ribbon widths that can reach some tens of nanometers. Such critical widths are one order of magnitude larger than the critical thickness measured in 3D pseudomorphic Si/Ge heterostructures and the resolution of state-of-the-art lithography, thus enabling the possibility of lithography patterned silicene/germanene junctions. We computed how the strain produced by the pseudomorphic growth modifies the crystal structure and electronic bands of the ribbons, providing a mechanism for band-structure engineering. Our results pave the way for lithography patterned lateral heterostructures that can serve as the building blocks of novel 2D electronics.
Exploring the free energy surface using ab initio molecular dynamics
NASA Astrophysics Data System (ADS)
Samanta, Amit; Morales, Miguel A.; Schwegler, Eric
2016-04-01
Efficient exploration of configuration space and identification of metastable structures in condensed phase systems are challenging from both computational and algorithmic perspectives. In this regard, schemes that utilize a set of pre-defined order parameters to sample the relevant parts of the configuration space [L. Maragliano and E. Vanden-Eijnden, Chem. Phys. Lett. 426, 168 (2006); J. B. Abrams and M. E. Tuckerman, J. Phys. Chem. B 112, 15742 (2008)] have proved useful. Here, we demonstrate how these order-parameter aided temperature accelerated sampling schemes can be used within the Born-Oppenheimer and the Car-Parrinello frameworks of ab initio molecular dynamics to efficiently and systematically explore free energy surfaces, and search for metastable states and reaction pathways. We have used these methods to identify the metastable structures and reaction pathways in SiO2 and Ti. In addition, we have used the string method [W. E, W. Ren, and E. Vanden-Eijnden, Phys. Rev. B 66, 052301 (2002); L. Maragliano et al., J. Chem. Phys. 125, 024106 (2006)] within the density functional theory to study the melting pathways in the high pressure cotunnite phase of SiO2 and the hexagonal closed packed to face centered cubic phase transition in Ti.
Ab Initio Potential Energy Surface for H-H2
NASA Technical Reports Server (NTRS)
Patridge, Harry; Bauschlicher, Charles W., Jr.; Stallcop, James R.; Levin, Eugene
1993-01-01
Ab initio calculations employing large basis sets are performed to determine an accurate potential energy surface for H-H2 interactions for a broad range of separation distances. At large distances, the spherically averaged potential determined from the calculated energies agrees well with the corresponding results determined from dispersion coefficients; the van der Waals well depth is predicted to be 75 +/- 3 micro E(h). Large basis sets have also been applied to reexamine the accuracy of theoretical repulsive potential energy surfaces (25-70 kcal/mol above the H-H2 asymptote) at small interatomic separations; the Boothroyd, Keogh, Martin, and Peterson (BKMP) potential energy surface is found to agree with results of the present calculations within the expected uncertainty (+/- 1 kcal/mol) of the fit. Multipolar expansions of the computed H-H2 potential energy surface are reported for four internuclear separation distances (1.2, 1.401, 1.449, and 1.7a(0)) of the hydrogen molecule. The differential elastic scattering cross section calculated from the present results is compared with the measurements from a crossed beam experiment.
Ab initio description of p-shell hypernuclei.
Wirth, Roland; Gazda, Daniel; Navrátil, Petr; Calci, Angelo; Langhammer, Joachim; Roth, Robert
2014-11-01
We present the first ab initio calculations for p-shell single-Λ hypernuclei. For the solution of the many-baryon problem, we develop two variants of the no-core shell model with explicit Λ and Σ(+),Σ(0),Σ(-) hyperons including Λ-Σ conversion, optionally supplemented by a similarity renormalization group transformation to accelerate model-space convergence. In addition to state-of-the-art chiral two- and three-nucleon interactions, we use leading-order chiral hyperon-nucleon interactions and a recent meson-exchange hyperon-nucleon interaction. We validate the approach for s-shell hypernuclei and apply it to p-shell hypernuclei, in particular to (Λ)(7)Li, (Λ)(9)Be, and (Λ)(13)C. We show that the chiral hyperon-nucleon interactions provide ground-state and excitation energies that generally agree with experiment within the cutoff dependence. At the same time we demonstrate that hypernuclear spectroscopy provides tight constraints on the hyperon-nucleon interactions. PMID:25415901
Ab initio SCF calculations on hydrogen bonded cresol isomers
NASA Astrophysics Data System (ADS)
Pohl, M.; Kleinermanns, K.
1988-12-01
Ab initio GAUSSIAN 80 calculations with two different basis sets (STO-3G and 4 31 G*) were performed on hydrogen bonded cresol isomers for comparison with experimental data from free jet fluorescence excitation spectroscopy. For m-cresol, the calculated barriers for hindered internal rotation of the OH-group and the CH3-group are in good agreement with experiment. The calculations show the trans-linear configuration of p-cresol· B-clusters ( B = H2O, CH3OH) to be more stable than the all-planar configuration. This agrees with CI calculations and microwave spectroscopic investigations of the water dimer. Calculations of both the intermolecular stretch and bend frequencies of p-cresol· B-clusters show little dependence on the all-planar or trans-linear configuration but a strong dependence on the choice of the basis set. With the minimal basis set STO-3G, the vibrational energies are generally too high. The agreement between the calculated vibrational frequencies from the 4 31 G* basis set and the experimental values is fair.
Predicting lattice thermal conductivity with help from ab initio methods
NASA Astrophysics Data System (ADS)
Broido, David
2015-03-01
The lattice thermal conductivity is a fundamental transport parameter that determines the utility a material for specific thermal management applications. Materials with low thermal conductivity find applicability in thermoelectric cooling and energy harvesting. High thermal conductivity materials are urgently needed to help address the ever-growing heat dissipation problem in microelectronic devices. Predictive computational approaches can provide critical guidance in the search and development of new materials for such applications. Ab initio methods for calculating lattice thermal conductivity have demonstrated predictive capability, but while they are becoming increasingly efficient, they are still computationally expensive particularly for complex crystals with large unit cells . In this talk, I will review our work on first principles phonon transport for which the intrinsic lattice thermal conductivity is limited only by phonon-phonon scattering arising from anharmonicity. I will examine use of the phase space for anharmonic phonon scattering and the Grüneisen parameters as measures of the thermal conductivities for a range of materials and compare these to the widely used guidelines stemming from the theory of Liebfried and Schölmann. This research was supported primarily by the NSF under Grant CBET-1402949, and by the S3TEC, an Energy Frontier Research Center funded by the US DOE, office of Basic Energy Sciences under Award No. DE-SC0001299.
Ab initio calculation of double ionization of atoms
Serov, V. V.
2013-02-15
The Solov'ev-Vinitsky method was used to perform an ab initio calculation of the triple-differential cross section for the double single-photon photoionization of helium for the case of equal emitted-electron energies. A Gaussian width {gamma} describing angular electron-electron correlations at the total electron energy E taking values in range between 0.1 and 100 eV was obtained for this cross section. The results agree with available experimental data, but they raise a doubt as to whether the well-known Wannier law {gamma} {proportional_to} E{sup 1/4} is applicable at experimentally accessible energies. The Gaussian width {gamma} was investigated as a function of the total emitted-electron energy for targets that have a strongly asymmetric configuration of the initial state-specifically, a negative atomic-hydrogen ion H{sup -} and heliumin the 1s2s{sup 1}S and 1s3s{sup 1}S excited states. It was found that this function, {gamma}(E), had a maximum at low energies. It was also shown that, at low energies, the dependence of the double-differential cross section on the angle between the emitted-electron momenta for the targets indicated above differed substantially from the Gaussian dependence, featuring maxima whose number was equal to the number of radial nodes in the initial state. This opens new possibilities for a qualitative analysis of the electron structure of targets.
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.
Ab initio simulations of peptide-mineral interactions
NASA Astrophysics Data System (ADS)
Hug, Susanna; Hunter, Graeme K.; Goldberg, Harvey; Karttunen, Mikko
We performed Car-Parrinello Molecular Dynamics (CPMD) simulations of two amino acids, aspartic acid (Asp) and phophoserine (pSer), on a calcium oxalate monohydrate (COM) surface as a model of the interactions of phosphoproteins with biominerals. In our earlier work using in vitro experiments and classical Molecular Dynamics (MD) simulations we have demonstrated the importance of phosphorylation of serine on the interactions of osteopontin (OPN) with COM. We used configurations from our previous classical MD simulations as a starting point for the ab initio simulations. In the case of Asp we found that the α-carboxyl and amine groups form temporary close contacts with the surface. For the dipeptide Asp-pSer the carboxyl groups form permanent close contacts with the surface and the distances of its other functional groups do not vary much. We show how the interaction of carboxyl groups with COM crystal is established and confirm the importance of phosphorylation in mediating the interactions between COM surfaces and OPN.
Ab initio study of the phenol-water cation radical
NASA Astrophysics Data System (ADS)
Hobza, Pavel; Burcl, Rudolf; Špirko, Vladimír; Dopfer, Otto; Müller-Dethlefs, Klaus; Schlag, Edward W.
1994-07-01
The phenol-water cation radical has been investigated by ab initio theory using the spin-restricted open-shell Hartree-Fock and spin-restricted open-shell second-order Møller-Plesset theories with 3-21G*(O) and 6-31G* basis sets. The full geometrical optimization was performed for several hydrogen-bonded structures and one hemibonded structure. Clearly, the most stable structure has been found for Cs symmetry with the linear hydrogen bond between the proton of the OH group of the phenol cation radical and the oxygen of the water, and the water hydrogens pointing away from the phenyl ring. For this structure harmonic (and for some intermolecular modes anharmonic) vibrational frequencies have been computed for various isotopic complexes. The computed shifts of phenol-localized intramolecular modes on complexation and on deuteration as well as the calculated intermolecular frequencies of the different isotopic complexes allow for an assignment of vibrational frequencies observed in the experimental zero-kinetic-energy (ZEKE) photoelectron spectra. Five out of a possible six intermolecular vibrations and several intramolecular modes have been assigned, including the 18b vibration which shows a strong blue shift in frequency upon complexation. Structure and properties of the phenol-water cation radical are compared with those of the corresponding neutral complex.
An efficient approach to ab initio Monte Carlo simulation
Leiding, Jeff; Coe, Joshua D.
2014-01-21
We present a Nested Markov chain Monte Carlo (NMC) scheme for building equilibrium averages based on accurate potentials such as density functional theory. Metropolis sampling of a reference system, defined by an inexpensive but approximate potential, was used to substantially decorrelate configurations at which the potential of interest was evaluated, thereby dramatically reducing the number needed to build ensemble averages at a given level of precision. The efficiency of this procedure was maximized on-the-fly through variation of the reference system thermodynamic state (characterized here by its inverse temperature β{sup 0}), which was otherwise unconstrained. Local density approximation results are presented for shocked states of argon at pressures from 4 to 60 GPa, where—depending on the quality of the reference system potential—acceptance probabilities were enhanced by factors of 1.2–28 relative to unoptimized NMC. The optimization procedure compensated strongly for reference potential shortcomings, as evidenced by significantly higher speedups when using a reference potential of lower quality. The efficiency of optimized NMC is shown to be competitive with that of standard ab initio molecular dynamics in the canonical ensemble.
Ab initio study of the bonding in diatomic nickel
Noell, J.O.; Newton, M.D.; Hay, P.J.; Martin, R.L.; Bobrowicz, F.W.
1980-09-01
Hartree--Fock, GVB, and configuration interaction calculations were performed for diatomic nickel using an ab initio effective core potential. A basis set specifically optimized for the /sup 3/D state of atomic nickel is found to be far superior to the more common basis obtained from the /sup 3/F atomic state. Correlation effects are found to be significant in determining the bond energy. In particular, the two electrons of the s--s bond must be appropriately correlated. In addition, correlation effects which one would interpret as being principally intra-atomic in character are found to have a marked effect on the molecular properties. The theoretically predicted bond dissociation energy (D/sub e/) of 43.4 kcal/mol is significantly lower than the experimental estimate of 55 +- 5 kcal/mol. However, molecular partition functions calculated using the present results indicate that the experimental value should be revised downward to a value of approx.46 +- 5 kcal/mol, in good agreement with our calculations. An interatomic distance of 4.27 bohr is computed and compared with experimental estimates. Spectroscopic parameters for dipole-allowed transitions from the ground state were determined from SCF and GVB calculations and discussed in relation to the experimentally observed visible and ultraviolet spectra attributed to Ni/sub 2/.
Ab initio effective interactions for s d -shell valence nucleons
NASA Astrophysics Data System (ADS)
Dikmen, E.; Lisetskiy, A. F.; Barrett, B. R.; Maris, P.; Shirokov, A. M.; Vary, J. P.
2015-06-01
We perform ab initio no-core shell-model calculations for A =18 and 19 nuclei in a 4 ℏ Ω , or Nmax=4 , model space by using the effective JISP16 and chiral N3LO nucleon-nucleon potentials and transform the many-body effective Hamiltonians into the 0 ℏ Ω model space to construct the A -body effective Hamiltonians in the s d shell. We separate the A -body effective Hamiltonians with A =18 and A =19 into inert core, one-, and two-body components. Then we use these core, one-, and two-body components to perform standard shell-model calculations for the A =18 and A =19 systems with valence nucleons restricted to the s d shell. Finally, we compare the standard shell-model results in the 0 ℏ Ω model space with the exact no-core shell-model results in the 4 ℏ Ω model space for the A =18 and A =19 systems and find good agreement.
Lead-Chalcogenides Under Pressure: Ab-Initio Study
NASA Astrophysics Data System (ADS)
Gupta, Dinesh C.; Hamid, Idris
ab-initio calculations using fully relativistic pseudo-potential have been performed to investigate the high pressure phase transition, elastic and electronic properties of lead-chalcogenides including the less known lead polonium. The calculated ground state parameters, for the rock-salt structure show good agreement with the experimental data. The enthalpy calculations show that these materials undergo a first-order phase transition from rock-salt to CsCl structure at 19.4, 15.5, 11.5 and 7.3 GPa for PbS, PbSe, PbTe and PbPo, respectively. Present calculations successfully predicted the location of the band gap at L-point of Brillouin zone as well as the value of the band gap in every case at ambient pressure. It is observed that unlike other lead-chalcogenides, PbPo is semi-metal at ambient pressure. The pressure variation of the energy gap indicates that these materials metalized under high pressures. For this purpose, the electronic structure of these materials has also been computed in parent as well as in high pressure phase.
Amide N-oxides: an ab initio molecular orbital study
NASA Astrophysics Data System (ADS)
Greenberg, Arthur; DuBois, Thomas D.
2001-06-01
There are no known examples of amide N-oxides. The present study employs ab initio molecular orbital calculations at the 6-3G ∗ level to explore potential target molecules in this class. Bridgehead bicyclic lactams appear to be attractive targets for oxidation to form the corresponding N-oxides because they have reduced (or zero) amide resonance energy. The amide N-oxide linkage is predicted to have a ca. 9-10 kcal/mol rotational barrier due to eclipsing of nonbonded oxygen atoms in the transition state. The linkage has a nearly flat conformational ( ΦON-CO) profile in the range 120-240° and this suggests that a very sterically hindered acyclic amide N-oxide may be a practical synthetic target. The oxidation of strained amides is calculated to be highly exothermic if dimethyldioxirane is employed. This reagent is predicted to react appreciably exothermically with normal, stable amides such as N, N-dimethylacetamide, thus offering the potential for generating and studying such relatively unstable amide N-oxides at low temperatures.
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
Ab initio simulations on rutile-based titania nanowires
NASA Astrophysics Data System (ADS)
Zhukovskii, Yu F.; Evarestov, R. A.
2012-08-01
The rod symmetry groups for monoperiodic (1D) nanostructures have been applied for construction of models for bulk-like TiO2 nanowires (NWs) cut from a rutile-based 3D crystal along the chosen [001] and [110] directions of crystallographic axes. In this study, we have considered nanowires described by both the Ti-atom centered rotation axes as well as the hollow site centered axes passing through the interstitial positions between the Ti and O atoms closest to the axes. The most stable [001]-oriented TiO2 NWs with rhombic cross sections are found to display the energetically preferable {110} facets only while the nanowires with quasi-square sections across the [110] axis are formed by the alternating { 1bar 10 } and {001} facets. For simulations on rutile-based nanowires possessing different diameters for each NW type, we have performed large-scale ab initio Density Functional Theory (DFT) and hybrid DFT-Hartree Fock (DFT-HF) calculations with total geometry optimization within the Generalized Gradient Approximation (GGA) in the form of the Perdew-Becke-Ernzenhof (PBE) exchange-correlation functionals (PBE and PBE0, respectively), using the formalism of linear combination of localized atomic functions (LCAO). We have simulated both structural and electronic properties of TiO2 NWs depending both on orientation and position of symmetry axes as well as on diameter and morphology of nanowires.
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.
Ab initio study of ice catalyzation of HOCl + HCl reaction
Zhou, Y.F.; Liu, C.B.
2000-06-15
The observations by Farman et al. revealed remarkable depletions in the total atmospheric ozone content in Antarctica. The observed total ozone decreased smoothing during the spring season from about 1975. Satellite observations have proved Antarctic ozone depletions over a very extended region, in general agreement with the local ground-based data of Farman et al. It was suggested that heterogeneous reactions occurring on particles in polar stratospheric clouds (PSCs) play a central role in the depletion of stratospheric ozone. Experiments proved that the reaction of HOCl + HCl was very slow in the gas phase, but on ice surface it was rapid. In this work the ice catalysis of HOCl + HCl reaction was investigated by using ab initio molecular orbital theory. The authors applied the Hartree-Fock self-consistent field and the second-order Moeller-Plesset perturbation theory with the basis sets of 6-31G* to the model system. The complexes and transition state were obtained along the reaction with and without the presence of ice surface. By comparing the results, a possible catalyzation mechanism of ice on the reaction is proposed.
Ab initio Raman spectroscopy of water under extreme conditions
NASA Astrophysics Data System (ADS)
Rozsa, Viktor; Pan, Ding; Wan, Quan; Galli, Giulia
Water exhibits one of the most complex phase diagrams of any binary compound. Despite extensive studies, the melting lines of high-pressure ice phases remain very controversial, with reports differing by hundreds of Kelvin. The boundary between ice VII and liquid phase is particularly disputed, with recent work exploring plasticity and amorphization mediating the transition. Raman measurements are often used to fingerprint melting, yet their interpretation is difficult without atomistic modeling. Here, we report a study of high P/T water where we computed Raman spectra using a method combining ab initio molecular dynamics and density functional perturbation theory, as implemented in the Qbox code. Spectra were computed for the liquid at 10 and 20 GPa, both at 1000 K, and for solid ice VII (20 GPa, 500 K). Decomposing the spectra into inter and intra molecular contributions provided insight into the dynamics of the hydrogen-bonded network at extreme conditions. The relevance of our simulation results for models of water in Earth, Uranus, and Neptune will be discussed, and an interpretation of existing experiments at high pressure will be presented.
Ab initio modeling of the motional Stark effect on MAST
De Bock, M. F. M.; Conway, N. J.; Walsh, M. J.; Carolan, P. G.; Hawkes, N. C.
2008-10-15
A multichord motional Stark effect (MSE) system has recently been built on the MAST tokamak. In MAST the {pi} and {sigma} lines of the MSE spectrum overlap due to the low magnetic field typical for present day spherical tokamaks. Also, the field curvature results in a large change in the pitch angle over the observation volume. The measured polarization angle does not relate to one local pitch angle but to an integration over all pitch angles in the observation volume. The velocity distribution of the neutral beam further complicates the measurement. To take into account volume effects and velocity distribution, an ab initio code was written that simulates the MSE spectrum on MAST. The code is modular and can easily be adjusted for other tokamaks. The code returns the intensity, polarized fraction, and polarization angle as a function of wavelength. Results of the code are presented, showing the effect on depolarization and wavelength dependence of the polarization angle. The code is used to optimize the design and calibration of the MSE diagnostic.
Curchod, Basile F. E.; Rauer, Clemens; Marquetand, Philipp; Gonzalez, Leticia; Martinez, Todd J.
2016-03-11
Full Multiple Spawning is a formally exact method to describe the excited-state dynamics of molecular systems beyond the Born-Oppenheimer approximation. However, it has been limited until now to the description of radiationless transitions taking place between electronic states with the same spin multiplicity. This Communication presents a generalization of the full and ab initio Multiple Spawning methods to both internal conversion (mediated by nonadiabatic coupling terms) and intersystem crossing events (triggered by spin-orbit coupling matrix elements) based on a spin-diabatic representation. Lastly, the results of two numerical applications, a model system and the deactivation of thioformaldehyde, validate the presented formalismmore » and its implementation.« less
Ab initio cross sections for low-energy inelastic H+Na collisions
NASA Astrophysics Data System (ADS)
Belyaev, A. K.; Grosser, J.; Hahne, J.; Menzel, T.
1999-09-01
We report ab initio results for the integral cross section of the process H+Na(3s)-->H+Na(3p) for collision energies from the threshold (2.1 eV) to 600 eV. We achieve a reasonable agreement with the experimental data, which are available for energies above 10 eV. The main contributions to the cross section come from a rotational coupling mechanism in the NaH triplet molecular system and from a curve-crossing mechanism in the singlet system. At very low energy (2.1-2.4 eV), the process is governed by a centrifugal barrier in the exit channel leading to orbital resonances. The Landau-Zener model provides a reasonable qualitative description of the radial coupling mechanism at high energies, but fails below 10 eV.
Rotational Energy Transfer of N2 Determined Using a New Ab Initio Potential Energy Surface
NASA Technical Reports Server (NTRS)
Huo, Winifred M.; Stallcop, James R.; Partridge, Harry; Langhoff, Stephen R. (Technical Monitor)
1997-01-01
A new N2-N2 rigid-rotor surface has been determined using extensive Ab Initio quantum chemistry calculations together with recent experimental data for the second virial coefficient. Rotational energy transfer is studied using the new potential energy surface (PES) employing the close coupling method below 200 cm(exp -1) and coupled state approximation above that. Comparing with a previous calculation based on the PES of van der Avoird et al.,3 it is found that the new PES generally gives larger cross sections for large (delta)J transitions, but for small (delta)J transitions the cross sections are either comparable or smaller. Correlation between the differences in the cross sections and the two PES will be attempted. The computed cross sections will also be compared with available experimental data.
Ab initio simulation of photoinduced transformation of small rings in amorphous silica
NASA Astrophysics Data System (ADS)
Donadio, Davide; Bernasconi, Marco
2005-02-01
We have studied the photoinduced transformation of small rings (three membered) in amorphous silica by Car-Parrinello simulations. The process of ring opening leading to the formation of a couple of paramagnetic centers, namely an E' and a nonbridging-oxygen hole center (NBOHC), has been proposed experimentally to occur in silica exposed to F2 laser irradiation (at 7.9eV ). By using a scheme for the simulation of rare events in ab initio molecular dynamics [M. Iannuzzi, A. Laio, and M. Parrinello, Phys. Rev. Lett. 90, 238303 (2003)], we have identified a transformation path for the opening of a three-membered ring induced by a self-trapped triplet exciton, the migration of NBOHC and formation of a couple of stable E' and NBOHC paramagnetic defects.
Ground state analytical ab initio intermolecular potential for the Cl{sub 2}-water system
Hormain, Laureline; Monnerville, Maurice Toubin, Céline; Duflot, Denis; Pouilly, Brigitte; Briquez, Stéphane; Bernal-Uruchurtu, Margarita I.; Hernández-Lamoneda, Ramón
2015-04-14
The chlorine/water interface is of crucial importance in the context of atmospheric chemistry. Modeling the structure and dynamics at this interface requires an accurate description of the interaction potential energy surfaces. We propose here an analytical intermolecular potential that reproduces the interaction between the Cl{sub 2} molecule and a water molecule. Our functional form is fitted to a set of high level ab initio data using the coupled-cluster single double (triple)/aug-cc-p-VTZ level of electronic structure theory for the Cl{sub 2} − H{sub 2}O complex. The potential fitted to reproduce the three minima structures of 1:1 complex is validated by the comparison of ab initio results of Cl{sub 2} interacting with an increasing number of water molecules. Finally, the model potential is used to study the physisorption of Cl{sub 2} on a perfectly ordered hexagonal ice slab. The calculated adsorption energy, in the range 0.27 eV, shows a good agreement with previous experimental results.
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 chemistry in parallel-portable tools and applications
Harrison, R.J.; Shepard, R. ); Kendall, R.A. )
1991-01-01
In common with many of the computational sciences, ab initio chemistry faces computational constraints to which a partial solution is offered by the prospect of highly parallel computers. Ab initio codes are large and complex (O(10{sup 5}) lines of FORTRAN), representing a significant investment of communal effort. The often conflicting requirements of portability and efficiency have been successfully resolved on vector computers by reliance on matrix oriented kernels. This proves inadequate even upon closely-coupled shared-memory parallel machines. We examine the algorithms employed during a typical sequence of calculations. Then we investigate how efficient portable parallel implementations may be derived, including the complex multi-reference singles and doubles configuration interaction algorithm. A portable toolkit, modeled after the Intel iPSC and the ANL-ACRF PARMACS, is developed, using shared memory and TCP/IP sockets. The toolkit is used as an initial platform for programs portable between LANS, Crays and true distributed-memory MIMD machines. Timings are presented. 53 refs., 4 tabs.
High-level ab initio potential energy surfaces and vibrational energies of H2CS
NASA Astrophysics Data System (ADS)
Yachmenev, Andrey; Yurchenko, Sergei N.; Ribeyre, Tristan; Thiel, Walter
2011-08-01
Six-dimensional (6D) potential energy surfaces (PESs) of H2CS have been generated ab initio using the recently proposed explicitly correlated (F12) singles and doubles coupled cluster method including a perturbational estimate of connected triple excitations, CCSD(T)-F12b [T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys. 127, 221106 (2007)] in conjunction with F12-optimized correlation consistent basis sets. Core-electron correlation, high-order correlation, scalar relativistic, and diagonal Born-Oppenheimer terms were included as additive high-level (HL) corrections. The resulting 6D PESs were represented by analytical functions which were used in variational calculations of the vibrational term values below 5000 cm-1. The best PESs obtained with and without the HL corrections, VQZ-F12* HL and VQZ-F12*, reproduce the fundamental vibrational wavenumbers with mean absolute deviations of 1.13 and 1.22 cm-1, respectively. A detailed analysis of the effects of the HL corrections shows how the VQZ-F12 results benefit from error cancellation. The present purely ab initio PESs will be useful as starting points for empirical refinements towards an accurate "spectroscopic" PES of H2CS.
Can an ab initio three-body virial equation describe the mercury gas phase?
Wiebke, J; Wormit, M; Hellmann, R; Pahl, E; Schwerdtfeger, P
2014-03-27
We report a sixth-order ab initio virial equation of state (EOS) for mercury. The virial coefficients were determined in the temperature range from 500 to 7750 K using a three-body approximation to the N-body interaction potential. The underlying two-body and three-body potentials were fitted to highly accurate Coupled-Cluster interaction energies of Hg2 (Pahl, E.; Figgen, D.; Thierfelder, C.; Peterson, K. A.; Calvo, F.; Schwerdtfeger, P. J. Chem. Phys. 2010, 132, 114301-1) and equilateral-triangular configurations of Hg3. We find the virial coefficients of order four and higher to be negative and to have large absolute values over the entire temperature range considered. The validity of our three-body, sixth-order EOS seems to be limited to small densities of about 1.5 g cm(-3) and somewhat higher densities at higher temperatures. Termwise analysis and comparison to experimental gas-phase data suggest a small convergence radius of the virial EOS itself as well as a failure of the three-body interaction model (i.e., poor convergence of the many-body expansion for mercury). We conjecture that the nth-order term of the virial EOS is to be evaluated from the full n-body interaction potential for a quantitative picture. Consequently, an ab initio three-body virial equation cannot describe the mercury gas phase. PMID:24547987
A new ab initio potential energy surface for the Ne-H 2 interaction
NASA Astrophysics Data System (ADS)
Lique, François
2009-03-01
A new accurate three-dimensional potential energy surface for the Ne-H 2 system, which explicitly takes into account the r-dependence of the H 2 vibration, was determined from ab initio calculations. It was obtained with the single and double excitation coupled-cluster method with noniterative perturbational treatment of triple excitation [CCSD(T)]. Calculations was been performed using the augmented correlation-consistent polarized quintuple zeta basis set (aug-cc-pV5Z) for the three atoms. We checked the accuracy of the present ab initio calculations. We have determined, using the new Ne-H 2 potential energy surface, differential cross-sections for the rotational excitation of the H 2 and D 2 molecules in collision with Ne and we have compared them with experimental results of Faubel et al. [M. Faubel, F.A. Gianturco, F. Ragnetti, L.Y. Rusin, F. Sondermann, U. Tappe, J.P. Toennies, J. Chem. Phys. 101 (1994) 8800]. The overall agreement confirms that the new potential energy surface can be used for the simulation of molecular collisions and/or molecular spectroscopy of the van der Waals complex Ne-H 2.
An Ab Initio Approach Towards Engineering Fischer-Tropsch Surface Chemistry
Matthew Neurock
2005-06-13
As petroleum prices continue to rise and the United States seeks to reduce its dependency on foreign oil, there is a renewed interest in the research and development of more efficient and alternative energy sources, such as fuel cells. One approach is to utilize processes that can produce long-chain hydrocarbons from other sources. One such reaction is Fischer-Tropsch synthesis. Fischer-Tropsch synthesis is a process by which syngas (CO and H{sub 2}) is converted to higher molecular weight hydrocarbons. The reaction involves a complex set of bond-breaking and bond-making reactions, such as CO and H{sub 2} activation, hydrocarbon hydrogenation reactions, and hydrocarbon coupling reactions. This report details our initial construction of an ab initio based kinetic Monte Carlo code that can be used to begin to simulate Fischer-Tropsch synthesis over model Co(0001) surfaces. The code is based on a stochastic kinetic formalism that allows us to explicitly track the transformation of all reactants, intermediates and products. The intrinsic kinetics for the simulations were derived from the ab initio results that we reported in previous year summaries.
Gimondi, Ilaria; Cavallotti, Carlo; Vanuzzo, Gianmarco; Balucani, Nadia; Casavecchia, Piergiorgio
2016-07-14
The mechanism of the O((3)P) + CH3CCH reaction was investigated using a combined experimental/theoretical approach. Experimentally the reaction dynamics was studied using crossed molecular beams (CMB) with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy. Theoretically master equation (ME) simulations were performed on a potential energy surface (PES) determined using high-level ab initio electronic structure calculations. In this paper (II) the theoretical results are described and compared with experiments, while in paper (I) are reported and discussed the results of the experimental study. The PES was investigated by determining structures and vibrational frequencies of wells and transition states at the CASPT2/aug-cc-pVTZ level using a minimal active space. Energies were then determined at the CASPT2 level increasing systematically the active space and at the CCSD(T) level extrapolating to the complete basis set limit. Two separate portions of the triplet PES were investigated, as O((3)P) can add either on the terminal or the central carbon of the unsaturated propyne bond. Minimum energy crossing points (MECPs) between the triplet and singlet PESs were searched at the CASPT2 level. The calculated spin-orbit coupling constants between the T1 and S0 electronic surfaces were ∼25 cm(-1) for both PESs. The portions of the singlet PES that can be accessed from the MECPs were investigated at the same level of theory. The system reactivity was predicted integrating stochastically the one-dimensional ME using Rice-Ramsperger-Kassel-Marcus theory to determine rate constants on the full T1/S0 PESs, accounting explicitly for intersystem crossing (ISC) using the Landau-Zener model. The computational results are compared both with the branching ratios (BRs) determined experimentally in the companion paper (I) and with those estimated in a recent kinetic study at 298 K. The ME results allow to interpret the main system reactivity: CH
Ab initio quantum transport calculations using plane waves
NASA Astrophysics Data System (ADS)
Garcia-Lekue, A.; Vergniory, M. G.; Jiang, X. W.; Wang, L. W.
2015-08-01
We present an ab initio method to calculate elastic quantum transport at the nanoscale. The method is based on a combination of density functional theory using plane wave nonlocal pseudopotentials and the use of auxiliary periodic boundary conditions to obtain the scattering states. The method can be applied to any applied bias voltage and the charge density and potential profile can either be calculated self-consistently, or using an approximated self-consistent field (SCF) approach. Based on the scattering states one can straightforwardly calculate the transmission coefficients and the corresponding electronic current. The overall scheme allows us to obtain accurate and numerically stable solutions for the elastic transport, with a computational time similar to that of a ground state calculation. This method is particularly suitable for calculations of tunneling currents through vacuum, that some of the nonequilibrium Greens function (NEGF) approaches based on atomic basis sets might have difficulty to deal with. Several examples are provided using this method from electron tunneling, to molecular electronics, to electronic devices: (i) On a Au nanojunction, the tunneling current dependence on the electrode-electrode distance is investigated. (ii) The tunneling through field emission resonances (FERs) is studied via an accurate description of the surface vacuum states. (iii) Based on quantum transport calculations, we have designed a molecular conformational switch, which can turn on and off a molecular junction by applying a perpendicular electric field. (iv) Finally, we have used the method to simulate tunnel field-effect transistors (TFETs) based on two-dimensional transition-metal dichalcogenides (TMDCs), where we have studied the performance and scaling limits of such nanodevices and proposed atomic doping to enhance the transistor performance.
Ab initio cluster study of crystalline NaF
Temple, D.K.
1992-01-01
A highly-accurate ab initio cluster model of crystalline NaF has been constructed to explore the limits of cluster methods in the treatment of ionic solids. The focus of this model was the characterization of the lattice environment and its influence on the easily-polarizable fluorine anion. The model consisted of a central all-electron fluorine anion coordinated by pseudopotentials, to represent the nearest-neighbor sodium cations, and a finite array of point charges chosen to generate the correct crystal field from the surrounding infinite ionic lattice. The wavefunction and properties of the anion were calculated using the restricted Hartree-Fock and configuration interaction techniques from quantum chemistry. An extensive analysis of basis set incompleteness errors in the anion wavefunction was performed. Important features were identified in the embedded anion, such as its distortion under the influence of the lattice compressions, its stabilization from the Madelung potential, and its changes in size due to electron correlations. Bulk properties of the rocksalt-structure (B1) NaF crystal were derived from the total mode energies, calculated as a function of the crystal volume. The properties included the zero-pressure lattice constant, cohesive energy, and bulk modulus, and the pressure-volume equation-of-state. A series of test calculations explored the relationships, and their underlying physical mechanisms, between the features of the embedded anion and the bulk properties of the crystal. These features often produced opposing changes in the properties, demonstrating the importance of a thorough and systematic treatment of the embedded anion. The most thorough test calculation gave bulk properties that were within 1% of experiment. Using an embedded anion model for the high-pressure cesium-chloride (B2) phase of NaF, the B1-to-B2 structural transition was correctly predicted at 25 GPa, in excellent agreement with the experimental values of 23 to 27 GPa.
Cosmic-Ray Modulation: an Ab Initio Approach
NASA Astrophysics Data System (ADS)
Engelbrecht, N. E.; Burger, R. A.
2014-10-01
A better understanding of cosmic-ray modulation in the heliosphere can only be gained through a proper understanding of the effects of turbulence on the diffusion and drift of cosmic rays. We present an ab initio model for cosmic-ray modulation, incorporating for the first time the results yielded by a two-component turbulence transport model. This model is solved for periods of minimum solar activity, utilizing boundary values chosen so that model results are in fair to good agreement with spacecraft observations of turbulence quantities, not only in the solar ecliptic plane but also along the out-of-ecliptic trajectory of the Ulysses spacecraft. These results are employed as inputs for modelled slab and 2D turbulence energy spectra. The latter spectrum is chosen based on physical considerations, with a drop-off at the very lowest wavenumbers commencing at the 2D outerscale. There currently exist no models or observations for this quantity, and it is the only free parameter in this study. The modelled turbulence spectra are used as inputs for parallel mean free path expressions based on those derived from quasi-linear theory and perpendicular mean free paths from extended nonlinear guiding center theory. Furthermore, the effects of turbulence on cosmic-ray drifts are modelled in a self-consistent way, employing a recently developed model for drift along the wavy current sheet. The resulting diffusion coefficients and drift expressions are applied to the study of galactic cosmic-ray protons and antiprotons using a three-dimensional, steady-state cosmic-ray modulation code, and sample solutions in fair agreement with multiple spacecraft observations are presented.
Efficient conformational space exploration in ab initio protein folding simulation.
Ullah, Ahammed; Ahmed, Nasif; Pappu, Subrata Dey; Shatabda, Swakkhar; Ullah, A Z M Dayem; Rahman, M Sohel
2015-08-01
Ab initio protein folding simulation largely depends on knowledge-based energy functions that are derived from known protein structures using statistical methods. These knowledge-based energy functions provide us with a good approximation of real protein energetics. However, these energy functions are not very informative for search algorithms and fail to distinguish the types of amino acid interactions that contribute largely to the energy function from those that do not. As a result, search algorithms frequently get trapped into the local minima. On the other hand, the hydrophobic-polar (HP) model considers hydrophobic interactions only. The simplified nature of HP energy function makes it limited only to a low-resolution model. In this paper, we present a strategy to derive a non-uniform scaled version of the real 20×20 pairwise energy function. The non-uniform scaling helps tackle the difficulty faced by a real energy function, whereas the integration of 20×20 pairwise information overcomes the limitations faced by the HP energy function. Here, we have applied a derived energy function with a genetic algorithm on discrete lattices. On a standard set of benchmark protein sequences, our approach significantly outperforms the state-of-the-art methods for similar models. Our approach has been able to explore regions of the conformational space which all the previous methods have failed to explore. Effectiveness of the derived energy function is presented by showing qualitative differences and similarities of the sampled structures to the native structures. Number of objective function evaluation in a single run of the algorithm is used as a comparison metric to demonstrate efficiency. PMID:26361554
AN AB INITIO MODEL FOR COSMIC-RAY MODULATION
Engelbrecht, N. E.; Burger, R. A.
2013-07-20
A proper understanding of the effects of turbulence on the diffusion and drift of cosmic rays (CRs) is of vital importance for a better understanding of CR modulation in the heliosphere. This study presents an ab initio model for CR modulation, incorporating for the first time the results yielded by a two-component turbulence transport model. This model is solved for solar minimum heliospheric conditions, utilizing boundary values chosen so that model results are in reasonable agreement with spacecraft observations of turbulence quantities in the solar ecliptic plane and along the out-of-ecliptic trajectory of the Ulysses spacecraft. These results are employed as inputs for modeled slab and two-dimensional (2D) turbulence energy spectra. The modeled 2D spectrum is chosen based on physical considerations, with a drop-off at the very lowest wavenumbers. There currently exist no models or observations for the wavenumber where this drop-off occurs, and it is considered to be the only free parameter in this study. The modeled spectra are used as inputs for parallel mean free path expressions based on those derived from quasi-linear theory and perpendicular mean free paths from extended nonlinear guiding center theory. Furthermore, the effects of turbulence on CR drifts are modeled in a self-consistent way, also employing a recently developed model for wavy current sheet drift. The resulting diffusion and drift coefficients are applied to the study of galactic CR protons and antiprotons using a 3D, steady-state CR modulation code, and sample solutions in fair to good agreement with multiple spacecraft observations are presented.
Efficient conformational space exploration in ab initio protein folding simulation
Ullah, Ahammed; Ahmed, Nasif; Pappu, Subrata Dey; Shatabda, Swakkhar; Ullah, A. Z. M. Dayem; Rahman, M. Sohel
2015-01-01
Ab initio protein folding simulation largely depends on knowledge-based energy functions that are derived from known protein structures using statistical methods. These knowledge-based energy functions provide us with a good approximation of real protein energetics. However, these energy functions are not very informative for search algorithms and fail to distinguish the types of amino acid interactions that contribute largely to the energy function from those that do not. As a result, search algorithms frequently get trapped into the local minima. On the other hand, the hydrophobic–polar (HP) model considers hydrophobic interactions only. The simplified nature of HP energy function makes it limited only to a low-resolution model. In this paper, we present a strategy to derive a non-uniform scaled version of the real 20×20 pairwise energy function. The non-uniform scaling helps tackle the difficulty faced by a real energy function, whereas the integration of 20×20 pairwise information overcomes the limitations faced by the HP energy function. Here, we have applied a derived energy function with a genetic algorithm on discrete lattices. On a standard set of benchmark protein sequences, our approach significantly outperforms the state-of-the-art methods for similar models. Our approach has been able to explore regions of the conformational space which all the previous methods have failed to explore. Effectiveness of the derived energy function is presented by showing qualitative differences and similarities of the sampled structures to the native structures. Number of objective function evaluation in a single run of the algorithm is used as a comparison metric to demonstrate efficiency. PMID:26361554
Ab initio no-core shell model with continuum
NASA Astrophysics Data System (ADS)
Navratil, Petr
2008-04-01
The ab initio no-core shell model (NCSM) is a many-body approach to nuclear structure of light nuclei. The NCSM adopts an effective interaction theory to transform fundamental inter-nucleon interactions into effective interactions for a specified nucleus in a selected harmonic oscillator basis space [1]. The method is capable of predicting nuclear structure from inter-nucleon forces derived from quantum chromodynamics by means of chiral effective field theory [2]. NCSM extensions to the microscopic description of nuclear reactions are now under development. In my talk, I will first discuss our recent calculations of the ^4He total photo-absorption cross section using two- and three-nucleon interactions from chiral effective field theory [3]. I will then outline our effort to augment the NCSM by the resonating group method (RGM) technique to develop a new method capable of describing simultaneously both bound states and nuclear reactions on light nuclei [4]. This approach, which preserves translational symmetry and the Pauli principle, will allow us to calculate cross sections of reactions important for astrophysics and describe weakly-bound systems from first principles. I will present our first phase shift results for neutron scattering off ^3H, ^4He and ^7Li and proton scattering off ^3He, ^4He and ^7Be using realistic nucleon-nucleon potentials. 3mm [1] P. Navr'atil, J. P. Vary and B. R. Barrett, Phys. Rev. C 62, 054311 (2000). [2] P. Navr'atil and V. G. Gueorguiev and J. P. Vary, W. E. Ormand and A. Nogga, Phys. Rev. Lett. 99, 042501 (2007). [3] S. Quaglioni and P. Navr'atil, Phys. Lett. B 652, 370 (2007). [4] S. Quaglioni and P. Navr'atil, arXiv:0712.0855.
Campetella, Marco; Bodo, Enrico; Montagna, Maria; De Santis, Serena; Gontrani, Lorenzo
2016-03-14
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. PMID:26979694
An ab initio potential energy surface and dynamics of the Ar+H2+ → ArH + + H reaction
NASA Astrophysics Data System (ADS)
Liu, Xinguo; Liu, Huirong; Zhang, Qinggang
2011-04-01
An ab initio potential energy surface (PES) for the ground state (1 2A') of the chemical reaction Ar+H2+ → ArH + + H has been constructed from a set of accurate ab initio data, which we have computed using the coupled-cluster theory including all single and double excitations plus perturbative corrections for the triples UCCSD(T) with a large orbital basis set of aug-cc-pV5Z. The new PES has a root-mean-square (rms) error of 0.5341 kcal/mol. The total integral reaction cross-sections have been calculated at three collision energies by means of the quasi-classical trajectory (QCT) calculation based on the new PES and compared with previous TSH results.
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.
High-Level Ab-Initio Calculation on the NiO₂ System
Song, Jie; Apra, Edoardo; Khait, Yuri; Hoffmann, Mark R.; Kowalski, Karol
2006-09-20
Several high-level ab-initio methods were employed in studies of the narrow singlet-triplet separation of the cyclic dorm of the nickel dioxide (NiO?). It is shown that the complete visions of the locally renormalized couples cluster method with singles, doubles, and noniterative triples (LR-CCSD(T)) approach, in contrast to the standard CCSD(T) method, provides results in concert with predictions of the density functional theory (DFT) and internally contracted multi-reference configuration interaction method (IIC-MCRCI), which favor the triplet state to be the lowest one. Relevant discussion of several aspects related to underlying CCSD calculations, indicate that the dominate roll of singly excited amplitudes violates the paradigm about the leading role to two-body effect in description of the correlation energy. These results are compared with the results of the second order Generalized Van Vleck Perturbation Theory (GVVPPT2) approach which supports the CCSD(T) predictions.
Ab initio determination of the instability growth rate of warm dense beryllium-deuterium interface
NASA Astrophysics Data System (ADS)
Wang, Cong; Li, Zi; Li, DaFang; Zhang, Ping
2015-10-01
Accurate knowledge about the interfacial unstable growth is of great importance in inertial confinement fusion. During implosions, the deuterium-tritium capsule is driven by laser beams or X-rays to access the strongly coupled and partially degenerated warm dense matter regime. At this stage, the effects of dissipative processes, such as diffusion and viscosity, have significant impact on the instability growth rates. Here, we present ab initio molecular dynamics simulations to determine the equations of state and the transport coefficients. Several models are used to estimate the reduction in the growth rate dispersion curves of Rayleigh-Taylor and Richtmyer-Meshkov instabilities with considering the presence of these dissipative effects. We show that these instability growth rates are effectively reduced when considering diffusion. The findings provide significant insights into the microscopic mechanism of the instability growth at the ablator-fuel interface and will refine the models used in the laser-driven hydrodynamic instability experiments.
Ab initio determination of the instability growth rate of warm dense beryllium-deuterium interface
Wang, Cong; Zhang, Ping; Li, Zi; Li, DaFang
2015-10-15
Accurate knowledge about the interfacial unstable growth is of great importance in inertial confinement fusion. During implosions, the deuterium-tritium capsule is driven by laser beams or X-rays to access the strongly coupled and partially degenerated warm dense matter regime. At this stage, the effects of dissipative processes, such as diffusion and viscosity, have significant impact on the instability growth rates. Here, we present ab initio molecular dynamics simulations to determine the equations of state and the transport coefficients. Several models are used to estimate the reduction in the growth rate dispersion curves of Rayleigh-Taylor and Richtmyer-Meshkov instabilities with considering the presence of these dissipative effects. We show that these instability growth rates are effectively reduced when considering diffusion. The findings provide significant insights into the microscopic mechanism of the instability growth at the ablator-fuel interface and will refine the models used in the laser-driven hydrodynamic instability experiments.
Ab initio spectroscopic characterization of borane, BH, in its X1Σ+ electronic state.
Koput, Jacek
2015-11-15
The accurate potential energy and electric dipole moment functions of borane, BH, in its X1Σ+ electronic state have been determined from ab initio calculations using the multireference averaged coupled-pair functional method in conjunction with the correlation-consistent core-valence basis sets up to septuple-zeta quality. The higher-order electron correlation, scalar relativistic, adiabatic, and nonadiabatic effects were discussed. Vibration-rotation energy levels of the (11)BH, (11)BD, (10)BH, and (10)BD isotopologues were predicted to near "spectroscopic" accuracy. For the main isotopologue (11)BH, the adiabatic dissociation energy D0 and the effective equilibrium internuclear distance r(e) were predicted to be 28,469 ± 10 cm(-1) and 1.23214 ± 0.0001 Å, respectively. PMID:26444679
Ab-initio study on crystal structure of α-RuCl3
NASA Astrophysics Data System (ADS)
Kee, Hae-Young; Kim, Heung-Sik
α -RuCl3 was recently proposed as a candidate system for materialization of Kitaev model, but precise structural information of the compound has remained elusive. For the clarification of the full three-dimensional crystal structure of α-RuCl3, we performed ab-initio electronic structure calculations including effects of spin-orbit coupling (SOC) and electron correlations. We found that SOC prevents dimerization between Ru atoms, and keeps the system close to honeycomb lattice. The ground state crystal structure has monoclinic C 2 / m -type layer stacking, but trigonal P31 12 -and orthorhombic Cmc21 -type stacking orders are comparable to the C 2 / m structure in energy, so that stacking faults can be easily introduced. The electronic structure and the jeff=1/2 pseudospin exchange interactions and possible magnetic states in α-RuCl3 will be presented.
Ab initio molecular treatment for charge transfer by P{sup 3+} ions on hydrogen and helium
Moussa, A.; Zaidi, A.; Lahmar, S.; Bacchus-Montabonel, M.-C.
2010-02-15
A theoretical treatment of charge-transfer processes induced by collision of phosphorus P{sup 3+}(3s{sup 2}){sup 1}S ions on atomic hydrogen and helium has been carried out using ab initio potential-energy curves and couplings at the multireference configuration interaction level of theory. The cross sections calculated by means of semiclassical collision methods show the existence of a significant charge transfer in the 0.1-700-keV laboratory energy range. Radial and rotational coupling interactions were analyzed for both collision systems.
Predictive Nuclear Many-Body Theory with Ab Initio Methods: A Brief Survey and A Look Ahead
NASA Astrophysics Data System (ADS)
Hergert, Heiko
2015-10-01
The reach of ab initio many-body techniques has increased tremendously in recent years, owing to new developments in many-body theory as well as advances in their numerical implementation. Coupled Cluster, Self-Consistent Green's Function, and In-Medium Similarity Renormalization Group (IM-SRG) calculations are routinely performed for isotopes in the A ~ 100 region. Moreover, these techniques have been extended to tackle open-shell nuclei, either directly or through the auxiliary step of deriving valence-space interactions for use with existing Shell Model technology. One of the most powerful aspects of ab initio methods is their capability to provide results for energies and other observables with systematic uncertainties. Together with new accurate nuclear forces (and operators) derived from Chiral Effective Field Theory, they provide a consistent framework--and a road map--for a predictive description of nuclei. This will have a critical impact on the search for the limits of nuclear existence, tests of fundamental symmetries (e.g., the search for neutrinoless double beta decay), our understanding of quenching and effective charges in phenomenological Shell Model calculations etc. Using the Multi-Reference IM-SRG as a representative example, I will survey the current capabilities of ab initio methods with an emphasis on uncertainty quantification, highlight successes in the description of ground-state properties and spectra, and preview upcoming developments like the construction of consistent transition operators.
NASA Astrophysics Data System (ADS)
Dieterich, Johannes M.; Werner, Hans-Joachim; Mata, Ricardo A.; Metz, Sebastian; Thiel, Walter
2010-01-01
Energy and free energy barriers for acetaldehyde conversion in aldehyde oxidoreductase are determined for three reaction pathways using quantum mechanical/molecular mechanical (QM/MM) calculations on the solvated enzyme. Ab initio single-point QM/MM energies are obtained at the stationary points optimized at the DFT(B3LYP)/MM level. These ab initio calculations employ local correlation treatments [LMP2 and LCCSD(T0)] in combination with augmented triple- and quadruple-zeta basis sets, and the final coupled cluster results include MP2-based corrections for basis set incompleteness and for the domain approximation. Free energy perturbation (FEP) theory is used to generate free energy profiles at the DFT(B3LYP)/MM level for the most important reaction steps by sampling along the corresponding reaction paths using molecular dynamics. The ab initio and FEP QM/MM results are combined to derive improved estimates of the free energy barriers, which differ from the corresponding DFT(B3LYP)/MM energy barriers by about 3 kcal mol-1. The present results confirm the qualitative mechanistic conclusions from a previous DFT(B3LYP)/MM study. Most favorable is a three-step Lewis base catalyzed mechanism with an initial proton transfer from the cofactor to the Glu869 residue, a subsequent nucleophilic attack that yields a tetrahedral intermediate (IM2), and a final rate-limiting hydride transfer. The competing metal center activated pathway has the same final step but needs to overcome a higher barrier in the initial step on the route to IM2. The concerted mechanism has the highest free energy barrier and can be ruled out. While confirming the qualitative mechanistic scenario proposed previously on the basis of DFT(B3LYP)/MM energy profiles, the present ab initio and FEP QM/MM calculations provide corrections to the barriers that are important when aiming at high accuracy.
Ab Initio and Phenomenological Modeling of the Phonon Spectrum of Superhard cp-BC2N
NASA Astrophysics Data System (ADS)
Basalaev, Yu. M.; Kopytov, A. V.; Pavlova, T. Yu.; Poplavnoi, A. S.
2015-11-01
The phonon spectrum of hypothetical superhard cp-BC2N is calculated based on ab initio method of density functional in the center of the Brillouin zone and interpolated over the entire Brillouin zone using the Keating phenomenological model. The interaction parameters are determined by optimization of the IR- and Ramanactive frequencies for a phenomenological model by their comparison with the results of ab initio calculations. Numerical values of short-range interaction constants and charges are in agreement with the characteristics of the chemical bond calculated ab initio. These parameters have transparent physical meaning and chemical nature and can further be used for both qualitative estimations of any physical and physico-chemical quantities and quantitative calculations of the phonon spectra of a number of isostructural compounds. The Keating phenomenological model is used to study the genesis of the phonon spectrum from the spectra of sublattices.
Electron Transport through Polyene Junctions in between Carbon Nanotubes: an Ab Initio Realization
NASA Astrophysics Data System (ADS)
Chen, Yiing-Rei; Chen, Kai-Yu; Dou, Kun-Peng; Tai, Jung-Shen; Lee, Hsin-Han; Kaun, Chao-Cheng
With both ab initio and tight-binding model calculations, we study a system of polyene bridged armchair carbon nanotube electrodes, considering one-polyene and two-polyene cases, to address aspects of quantum transport through junctions with multiple conjugated molecules. The ab initio results of the two-polyene cases not only show the interference effect in transmission, but also the sensitive dependence of such effect on the combination of relative contact sites, which agrees nicely with the tight-binding model. Moreover, we show that the discrepancy mainly brought by ab initio relaxation provides an insight into the influence upon transmission spectra, from the junction's geometry, bonding and effective potential. This work was supported by the Ministry of Science and Technology of the Republic of China under Grant Nos. 99-2112-M-003-012-MY2 and 103-2622-E-002-031, and the National Center for Theoretical Sciences of Taiwan.
Keegan, Ronan M.; Bibby, Jaclyn; Thomas, Jens; Xu, Dong; Zhang, Yang; Mayans, Olga; Winn, Martyn D.; Rigden, Daniel J.
2015-02-01
Two ab initio modelling programs solve complementary sets of targets, enhancing the success of AMPLE with small proteins. AMPLE clusters and truncates ab initio protein structure predictions, producing search models for molecular replacement. Here, an interesting degree of complementarity is shown between targets solved using the different ab initio modelling programs QUARK and ROSETTA. Search models derived from either program collectively solve almost all of the all-helical targets in the test set. Initial solutions produced by Phaser after only 5 min perform surprisingly well, improving the prospects for in situ structure solution by AMPLE during synchrotron visits. Taken together, the results show the potential for AMPLE to run more quickly and successfully solve more targets than previously suspected.
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.
An Efficient Time-Stepping Scheme for Ab Initio Molecular Dynamics Simulations
NASA Astrophysics Data System (ADS)
Tsuchida, Eiji
2016-08-01
In ab initio molecular dynamics simulations of real-world problems, the simple Verlet method is still widely used for integrating the equations of motion, while more efficient algorithms are routinely used in classical molecular dynamics. We show that if the Verlet method is used in conjunction with pre- and postprocessing, the accuracy of the time integration is significantly improved with only a small computational overhead. We also propose several extensions of the algorithm required for use in ab initio molecular dynamics. The validity of the processed Verlet method is demonstrated in several examples including ab initio molecular dynamics simulations of liquid water. The structural properties obtained from the processed Verlet method are found to be sufficiently accurate even for large time steps close to the stability limit. This approach results in a 2× performance gain over the standard Verlet method for a given accuracy. We also show how to generate a canonical ensemble within this approach.
Microwave and ab initio studies of rare gas-methane van der Waals complexes
NASA Astrophysics Data System (ADS)
Liu, Yaqian; Jäger, Wolfgang
2004-05-01
Rotational spectra of the weakly bound Kr-methane van der Waals complex were recorded using a pulsed molecular beam Fourier transform microwave spectrometer in the range from 3.5 to 18 GHz. Spectra of 25 isotopomers of Kr-methane were assigned and analyzed. For isotopomers containing CH4, 13CH4, and CD4, two sets of transitions with K=0 and one with K=1 were recorded, correlating to the j=0, 1, and 2 rotational levels of free methane, respectively (j is the rotational angular momentum quantum number of the methane monomer). For isotopomers containing CH3D and CHD3, two K=0 components were recorded, correlating to the jk=00 and 11 rotational levels of free methane (k corresponds to the projection of j onto the C3 axis of CH3D and CHD3). The obtained spectroscopic results were used to derive van der Waals bond distance R, van der Waals stretching frequency νs, and the corresponding stretching force constant ks. Nuclear spin statistical weights of individual states were obtained from molecular symmetry group analyses and were compared with the observed relative transition intensities. The tentatively assigned j=2 transitions were more intense than predicted from symmetry considerations. This is attributed to a relatively large effective dipole moment of this state, supported by ab initio dipole moment calculations. Ab initio potential energy calculations of Kr-CH4 and Ar-CH4 were done at the coupled cluster level of theory, with single and double excitations and perturbative inclusion of triple excitations, using the aug-cc-pVTZ basis set supplemented with bond functions. The theoretical results show that the angular dynamics of the dimer does not change significantly when the binding partner of methane changes from Ar to Kr. The dipole moment of Ar-CH4 was calculated at various configurations, providing a qualitative explanation for the unsuccessful spectral searches for rotational transitions of Ar-CH4.
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.
Comparison of DFT and ab initio QM/MM methods for modelling reaction in chorismate synthase
NASA Astrophysics Data System (ADS)
Lawan, Narin; Ranaghan, Kara E.; Manby, Frederick R.; Mulholland, Adrian J.
2014-07-01
Quantum mechanics/molecular mechanics (QM/MM) methods are a popular tool in the investigation of enzyme reactions. Here, we compare B3LYP density functional theory (DFT) and ab initio QM/MM methods for modelling the conversion of 5-enolpyruvylshikimate-3-phosphate to chorismate in chorismate synthase. Good agreement with experimental data is only obtained at the SCS-MP2/CHARMM27 level for a reaction mechanism in which phosphate elimination precedes proton transfer. B3LYP predicts reaction energetics that are qualitatively wrong, stressing the need for ab initio QM/MM methods, and caution in interpretation of DFT results for this enzyme.
Sakane, Shinichi; Yezdimer, Eric M.; Liu, Wenbin; Barriocanal, Jose A.; Doren, Douglas J.; Wood, Robert H.
2000-08-15
The ab initio/classical free energy perturbation (ABC-FEP) method proposed previously by Wood et al. [J. Chem. Phys. 110, 1329 (1999)] uses classical simulations to calculate solvation free energies within an empirical potential model, then applies free energy perturbation theory to determine the effect of changing the empirical solute-solvent interactions to corresponding interactions calculated from ab initio methods. This approach allows accurate calculation of solvation free energies using an atomistic description of the solvent and solute, with interactions calculated from first principles. Results can be obtained at a feasible computational cost without making use of approximations such as a continuum solvent or an empirical cavity formation energy. As such, the method can be used far from ambient conditions, where the empirical parameters needed for approximate theories of solvation may not be available. The sources of error in the ABC-FEP method are the approximations in the ab initio method, the finite sample of configurations, and the classical solvent model. This article explores the accuracy of various approximations used in the ABC-FEP method by comparing to the experimentally well-known free energy of hydration of water at two state points (ambient conditions, and 973.15 K and 600 kg/m3). The TIP4P-FQ model [J. Chem. Phys. 101, 6141 (1994)] is found to be a reliable solvent model for use with this method, even at supercritical conditions. Results depend strongly on the ab initio method used: a gradient-corrected density functional theory is not adequate, but a localized MP2 method yields excellent agreement with experiment. Computational costs are reduced by using a cluster approximation, in which ab initio pair interaction energies are calculated between the solute and up to 60 solvent molecules, while multi-body interactions are calculated with only a small cluster (5 to 12 solvent molecules). Sampling errors for the ab initio contribution to
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.
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.
Development of a spinor ab initio pseudopotential method and its application to spintronic materials
NASA Astrophysics Data System (ADS)
Theurich, Gerhard Jakob
2001-12-01
This thesis describes the extension of the density functional ab initio pseudopotential method to include the relativistic effect of spin-orbit coupling and non-collinear spin-polarizations within a generalized local density approximation. In contrast to conventional implementations, that neglect spin-orbit coupling and non-collinearity, the wavefunctions no longer separate into spatial and spin components, and are treated as general spinors. The implementation is applied to non-magnetic and magnetic systems with an emphasis on their spin properties. For GaAs and ZnSe the effects of relativistic corrections are studied focusing on the spin splittings of electronic bands, which are of great importance for spin relaxation in zincblende semiconductors. In the magnetic compounds MnSe and MnAs the reduced symmetry of the fully relativistic problem leads to small non-collinear arrangements of the self-consistent spin moments. Finally, the influence of spin-orbit coupling on the conduction and valence band exchange constants in dilute magnetic semiconductors is investigated, using the fully relativistic pseudopotential method. The results support the use of the scalar-relativistic approximation in such calculations.
Hu, Hao; Yang, Weitao
2013-01-01
Determining the free energies and mechanisms of chemical reactions in solution and enzymes is a major challenge. For such complex reaction processes, combined quantum mechanics/molecular mechanics (QM/MM) method is the most effective simulation method to provide an accurate and efficient theoretical description of the molecular system. The computational costs of ab initio QM methods, however, have limited the application of ab initio QM/MM methods. Recent advances in ab initio QM/MM methods allowed the accurate simulation of the free energies for reactions in solution and in enzymes and thus paved the way for broader application of the ab initio QM/MM methods. We review here the theoretical developments and applications of the ab initio QM/MM methods, focusing on the determination of reaction path and the free energies of the reaction processes in solution and enzymes. PMID:24146439
NASA Astrophysics Data System (ADS)
Ansari, R.; Faghihnasiri, M.; Malakpour, S.; Sahmani, S.
2015-07-01
In the current investigation, ab initio calculations are performed to explore the influence of electric field on the mechanical properties of bilayer boron nitride with AB stacking order (AB-2LBN). To accomplish this, density functional theory (DFT) within the framework of generalized gradient approximation (GGA) is implemented. It is demonstrated that the electric field has significant effects on Young's modulus and Poisson's ratio of AB-2LBN when its magnitude is small. With increasing the magnitude of electric field, these effects diminish so that the mechanical properties with and without considering the electric field become approximately identical. Also, it is shown that the equilibrium strain energy decreases linearly by increasing the magnitude of applied electric field.
Periodic Trends in Lanthanide Compounds through the Eyes of Multireference ab Initio Theory.
Aravena, Daniel; Atanasov, Mihail; Neese, Frank
2016-05-01
Regularities among electronic configurations for common oxidation states in lanthanide complexes and the low involvement of f orbitals in bonding result in the appearance of several periodic trends along the lanthanide series. These trends can be observed on relatively different properties, such as bonding distances or ionization potentials. Well-known concepts like the lanthanide contraction, the double-double (tetrad) effect, and the similar chemistry along the lanthanide series stem from these regularities. Periodic trends on structural and spectroscopic properties are examined through complete active space self-consistent field (CASSCF) followed by second-order N-electron valence perturbation theory (NEVPT2) including both scalar relativistic and spin-orbit coupling effects. Energies and wave functions from electronic structure calculations are further analyzed in terms of ab initio ligand field theory (AILFT), which allows one to rigorously extract angular overlap model ligand field, Racah, and spin-orbit coupling parameters directly from high-level ab initio calculations. We investigated the elpasolite Cs2NaLn(III)Cl6 (Ln(III) = Ce-Nd, Sm-Eu, Tb-Yb) crystals because these compounds have been synthesized for most Ln(III) ions. Cs2NaLn(III)Cl6 elpasolites have been also thoroughly characterized with respect to their spectroscopic properties, providing an exceptionally vast and systematic experimental database allowing one to analyze the periodic trends across the lanthanide series. Particular attention was devoted to the apparent discrepancy in metal-ligand covalency trends between theory and spectroscopy described in the literature. Consistent with earlier studies, natural population analysis indicates an increase in covalency along the series, while a decrease in both the nephelauxetic (Racah) and relativistic nephelauxetic (spin-orbit coupling) reduction with increasing atomic number is calculated. These apparently conflicting results are discussed on the
Ab initio electronic structure study for TTF-TCNQ under uniaxial compression
NASA Astrophysics Data System (ADS)
Ishibashi, Shoji; Hashimoto, Tamotsu; Kohyama, Masanori; Terakura, Kiyoyuki
2004-04-01
We have investigated the electronic structure of TTF-TCNQ under uniaxial compression with ab initio plane-wave pseudopotential calculations within the local-density approximation and generalized gradient approximation. Depending on the compression direction, the constituent molecules are deformed in different ways. Along with these structural deformations, quasi-one-dimensional Fermi surfaces show dramatic changes in their shapes and sizes.
Dispersion Interactions between Rare Gas Atoms: Testing the London Equation Using ab Initio Methods
ERIC Educational Resources Information Center
Halpern, Arthur M.
2011-01-01
A computational chemistry experiment is described in which students can use advanced ab initio quantum mechanical methods to test the ability of the London equation to account quantitatively for the attractive (dispersion) interactions between rare gas atoms. Using readily available electronic structure applications, students can calculate the…
NASA Technical Reports Server (NTRS)
Lee, Timothy J.; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
The ability of modern state-of-the art ab initio quantum chemical techniques to characterize reliably the gas-phase molecular structure, vibrational spectrum, electronic spectrum, and thermal stability of chlorine oxide and nitrogen oxide species will be demonstrated by presentation of some example studies. In particular the geometrical structures, vibrational spectra, and heats of formation Of ClNO2, CisClONO, and trans-ClONO are shown to be in excellent agreement with the available experimental data, and where the experimental data are either not known or are inconclusive, the ab initio results are shown to fill in the gaps and to resolve the experimental controversy. In addition, ab initio studies in which the electronic spectra and the characterization of excited electronic states of ClONO2, HONO2, ClOOC17 ClOOH, and HOOH will also be presented. Again where available, the ab initio results are compared to experimental observations, and are used to aid in the interpretation of the experimental studies.
NASA Astrophysics Data System (ADS)
Chan, Garnet Kin-Lic; Keselman, Anna; Nakatani, Naoki; Li, Zhendong; White, Steven R.
2016-07-01
Current descriptions of the ab initio density matrix renormalization group (DMRG) algorithm use two superficially different languages: an older language of the renormalization group and renormalized operators, and a more recent language of matrix product states and matrix product operators. The same algorithm can appear dramatically different when written in the two different vocabularies. In this work, we carefully describe the translation between the two languages in several contexts. First, we describe how to efficiently implement the ab initio DMRG sweep using a matrix product operator based code, and the equivalence to the original renormalized operator implementation. Next we describe how to implement the general matrix product operator/matrix product state algebra within a pure renormalized operator-based DMRG code. Finally, we discuss two improvements of the ab initio DMRG sweep algorithm motivated by matrix product operator language: Hamiltonian compression, and a sum over operators representation that allows for perfect computational parallelism. The connections and correspondences described here serve to link the future developments with the past and are important in the efficient implementation of continuing advances in ab initio DMRG and related algorithms.
Chan, Garnet Kin-Lic; Keselman, Anna; Nakatani, Naoki; Li, Zhendong; White, Steven R
2016-07-01
Current descriptions of the ab initio density matrix renormalization group (DMRG) algorithm use two superficially different languages: an older language of the renormalization group and renormalized operators, and a more recent language of matrix product states and matrix product operators. The same algorithm can appear dramatically different when written in the two different vocabularies. In this work, we carefully describe the translation between the two languages in several contexts. First, we describe how to efficiently implement the ab initio DMRG sweep using a matrix product operator based code, and the equivalence to the original renormalized operator implementation. Next we describe how to implement the general matrix product operator/matrix product state algebra within a pure renormalized operator-based DMRG code. Finally, we discuss two improvements of the ab initio DMRG sweep algorithm motivated by matrix product operator language: Hamiltonian compression, and a sum over operators representation that allows for perfect computational parallelism. The connections and correspondences described here serve to link the future developments with the past and are important in the efficient implementation of continuing advances in ab initio DMRG and related algorithms. PMID:27394094
Predicting materials for solar energy conversion: ab-initio spectroscopy of heterogeneous interfaces
NASA Astrophysics Data System (ADS)
Galli, Giulia
We will discuss some progress in predicting materials for solar energy conversion using ab initio calculations, in particular we will focus on heterogeneous interfaces between photo-electrodes and water and between nanocomposites. We will also address the problem of building much needed tighter connections between computational and laboratory experiments.
CVRQD ab initio ground-state adiabatic potential energy surfaces for the water molecule.
Barletta, Paolo; Shirin, Sergei V; Zobov, Nikolai F; Polyansky, Oleg L; Tennyson, Jonathan; Valeev, Edward F; Császár, Attila G
2006-11-28
The high accuracy ab initio adiabatic potential energy surfaces (PESs) of the ground electronic state of the water molecule, determined originally by Polyansky et al. [Science 299, 539 (2003)] and called CVRQD, are extended and carefully characterized and analyzed. The CVRQD potential energy surfaces are obtained from extrapolation to the complete basis set of nearly full configuration interaction valence-only electronic structure computations, augmented by core, relativistic, quantum electrodynamics, and diagonal Born-Oppenheimer corrections. We also report ab initio calculations of several quantities characterizing the CVRQD PESs, including equilibrium and vibrationally averaged (0 K) structures, harmonic and anharmonic force fields, harmonic vibrational frequencies, vibrational fundamentals, and zero-point energies. They can be considered as the best ab initio estimates of these quantities available today. Results of first-principles computations on the rovibrational energy levels of several isotopologues of the water molecule are also presented, based on the CVRQD PESs and the use of variational nuclear motion calculations employing an exact kinetic energy operator given in orthogonal internal coordinates. The variational nuclear motion calculations also include a simplified treatment of nonadiabatic effects. This sophisticated procedure to compute rovibrational energy levels reproduces all the known rovibrational levels of the water isotopologues considered, H(2) (16)O, H(2) (17)O, H(2) (18)O, and D(2) (16)O, to better than 1 cm(-1) on average. Finally, prospects for further improvement of the ground-state adiabatic ab initio PESs of water are discussed. PMID:17144700
Ab initio charge-carrier mobility model for amorphous molecular semiconductors
NASA Astrophysics Data System (ADS)
Massé, Andrea; Friederich, Pascal; Symalla, Franz; Liu, Feilong; Nitsche, Robert; Coehoorn, Reinder; Wenzel, Wolfgang; Bobbert, Peter A.
2016-05-01
Accurate charge-carrier mobility models of amorphous organic molecular semiconductors are essential to describe the electrical properties of devices based on these materials. The disordered nature of these semiconductors leads to percolative charge transport with a large characteristic length scale, posing a challenge to the development of such models from ab initio simulations. Here, we develop an ab initio mobility model using a four-step procedure. First, the amorphous morphology together with its energy disorder and intermolecular charge-transfer integrals are obtained from ab initio simulations in a small box. Next, the ab initio information is used to set up a stochastic model for the morphology and transfer integrals. This stochastic model is then employed to generate a large simulation box with modeled morphology and transfer integrals, which can fully capture the percolative charge transport. Finally, the charge-carrier mobility in this simulation box is calculated by solving a master equation, yielding a mobility function depending on temperature, carrier concentration, and electric field. We demonstrate the procedure for hole transport in two important molecular semiconductors, α -NPD and TCTA. In contrast to a previous study, we conclude that spatial correlations in the energy disorder are unimportant for α -NPD. We apply our mobility model to two types of hole-only α -NPD devices and find that the experimental temperature-dependent current density-voltage characteristics of all devices can be well described by only slightly decreasing the simulated energy disorder strength.
NASA Astrophysics Data System (ADS)
Smith, Grant D.; Jaffe, Richard L.; Yoon, Do. Y.
1998-06-01
High-level ab initio quantum chemistry calculations are shown to predict conformer populations of 1,2-dimethoxypropane and 5-methoxy-1,3-dioxane that are consistent with gas-phase NMR vicinal coupling constant measurements. The conformational energies of the cyclic ether 5-methoxy-1,3-dioxane are found to be consistent with those predicted by a rotational isomeric state (RIS) model based upon the acyclic analog 1,2-dimethoxypropane. The quantum chemistry and RIS calculations indicate the presence of strong attractive 1,5 C(H 3)⋯O electrostatic interactions in these molecules, similar to those found in 1,2-dimethoxyethane.
Kalemos, Apostolos; Prosmiti, Rita
2014-09-14
We present for the first time a coherent ab initio study of 39 states of valence, Rydberg, and ion-pair character of the diatomic interhalogen ICl species through large scale multireference variational methods including spin-orbit effects coupled with quantitative basis sets. Various avoided crossings are responsible for a non-adiabatic behaviour creating a wonderful vista for its theoretical description. Our molecular constants are compared with all available experimental data with the aim to assist experimentalists especially in the high energy regime of up to ∼95 000 cm{sup −1}.
Ab initio calculations of the vibrational and dielectric properties of PbSnTe alloys
NASA Astrophysics Data System (ADS)
Scolfaro, Luisa; Rezende Neto, A. R.; Leite Alves, H. W.; Petersen, J. E.; Myers, T. H.; Borges, P. D.
Thermoelectric devices have promise in dealing with the challenges of the growing demand for alternative clean energy and Te-based materials well-known candidates for them. Recently, we have shown that the high values for the dielectric constant, together with anharmonic LA-TO coupling, reduces the lattice thermal conductivity and enhances the electronic conductivity in PbTe. Also, it was shown that by alloying this material with Se, the electronic conductivity of the alloys is also enhanced. But, it is not clear if the same occurs when alloying with Sn. We show, in this work, our ab initio results for the vibrational and dielectric properties of Pb1-xSnxTe alloys. The calculations were carried out by using the Density Functional Theory, and the alloys were described by both the Virtual Crystal Approximation and Cluster Expansion Method. Our results show that the anharmonic LA-TO coupling enhances and reach its maximum for Sn concentration values of 0.75, corresponding to the maximum value for the dielectric constant, which is higher than that obtained for PbTe
Carbon and proton Overhauser DNP from MD simulations and ab initio calculations: TEMPOL in acetone.
Küçük, Sami Emre; Biktagirov, Timur; Sezer, Deniz
2015-10-14
A computational analysis of the Overhauser effect is reported for the proton, methyl carbon, and carbonyl carbon nuclei of liquid acetone doped with the nitroxide radical TEMPOL. A practical methodology for calculating the dynamic nuclear polarization (DNP) coupling factors by accounting for both dipole-dipole and Fermi-contact interactions is presented. The contribution to the dipolar spectral density function of nuclear spins that are not too far from TEMPOL is computed through classical molecular dynamics (MD) simulations, whereas the contribution of distant spins is included analytically. Fermi contacts are obtained by subjecting a few molecules from every MD snapshot to ab initio quantum mechanical calculations. Scalar interaction is found to be an essential part of the (13)C Overhauser DNP. While mostly detrimental to the carbonyl carbon of acetone it is predicted to result in large enhancements of the methyl carbon signal at magnetic fields of 9 T and beyond. In contrast, scalar coupling is shown to be negligible for the protons of acetone. The additional influence of proton polarization on the carbon DNP (three-spin effect) is also analyzed computationally. Its effect, however, is concluded to be practically insignificant for liquid acetone. PMID:26343351
PSI3: an open-source Ab Initio electronic structure package.
Crawford, T Daniel; Sherrill, C David; Valeev, Edward F; Fermann, Justin T; King, Rollin A; Leininger, Matthew L; Brown, Shawn T; Janssen, Curtis L; Seidl, Edward T; Kenny, Joseph P; Allen, Wesley D
2007-07-15
PSI3 is a program system and development platform for ab initio molecular electronic structure computations. The package includes mature programming interfaces for parsing user input, accessing commonly used data such as basis-set information or molecular orbital coefficients, and retrieving and storing binary data (with no software limitations on file sizes or file-system-sizes), especially multi-index quantities such as electron repulsion integrals. This platform is useful for the rapid implementation of both standard quantum chemical methods, as well as the development of new models. Features that have already been implemented include Hartree-Fock, multiconfigurational self-consistent-field, second-order Møller-Plesset perturbation theory, coupled cluster, and configuration interaction wave functions. Distinctive capabilities include the ability to employ Gaussian basis functions with arbitrary angular momentum levels; linear R12 second-order perturbation theory; coupled cluster frequency-dependent response properties, including dipole polarizabilities and optical rotation; and diagonal Born-Oppenheimer corrections with correlated wave functions. This article describes the programming infrastructure and main features of the package. PSI3 is available free of charge through the open-source, GNU General Public License. PMID:17420978
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)
Timko, Jeff; Kuyucak, Serdar
2012-11-01
Polarization is an important component of molecular interactions and is expected to play a particularly significant role in inhomogeneous environments such as pores and interfaces. Here we investigate the effects of polarization in the gramicidin A ion channel by performing quantum mechanics/molecular mechanics molecular dynamics (MD) simulations and comparing the results with those obtained from classical MD simulations with non-polarizable force fields. We consider the dipole moments of backbone carbonyl groups and channel water molecules as well as a number of structural quantities of interest. The ab initio results show that the dipole moments of the carbonyl groups and water molecules are highly sensitive to the hydrogen bonds (H-bonds) they participate in. In the absence of a K+ ion, water molecules in the channel are quite mobile, making the H-bond network highly dynamic. A central K+ ion acts as an anchor for the channel waters, stabilizing the H-bond network and thereby increasing their average dipole moments. In contrast, the K+ ion has little effect on the dipole moments of the neighboring carbonyl groups. The weakness of the ion-peptide interactions helps to explain the near diffusion-rate conductance of K+ ions through the channel. We also address the sampling issue in relatively short ab initio MD simulations. Results obtained from a continuous 20 ps ab initio MD simulation are compared with those generated by sampling ten windows from a much longer classical MD simulation and running each window for 2 ps with ab initio MD. Both methods yield similar results for a number of quantities of interest, indicating that fluctuations are fast enough to justify the short ab initio MD simulations.
NASA Astrophysics Data System (ADS)
Xavier, F. George D.; Kumar, Sanjay
2010-10-01
Ab initio global adiabatic and quasidiabatic potential energy surfaces of lowest four electronic (1-4 A3″) states of the H++O2 system have been computed in the Jacobi coordinates (R,r,γ) using Dunning's cc-pVTZ basis set at the internally contracted multireference (single and double) configuration interaction level of accuracy, which are relevant to the dynamics studies of inelastic vibrational and charge transfer processes observed in the scattering experiments. The computed equilibrium geometry parameters of the bound [HO2]+ ion in the ground electronic state and other parameters for the transition state for the isomerization process, HOO+⇌OOH+ are in good quantitative agreement with those available from the high level ab initio calculations, thus lending credence to the accuracy of the potential energy surfaces. The nonadiabatic couplings between the electronic states have been analyzed in both the adiabatic and quasidiabatic frameworks by computing the nonadiabatic coupling matrix elements and the coupling potentials, respectively. It is inferred that the dynamics of energy transfer processes in the scattering experiments carried out in the range of 9.5-23 eV would involve all the four electronic states.
Giansiracusa, Marcus J; Vonci, Michele; Van den Heuvel, Willem; Gable, Robert W; Moubaraki, Boujemaa; Murray, Keith S; Yu, Dehong; Mole, Richard A; Soncini, Alessandro; Boskovic, Colette
2016-06-01
comprised of contributions from large MJ, giving rise to slow magnetic relaxation. Although no direct evidence for intramolecular RE···RE magnetic coupling is observed in either magnetic or INS studies, on the basis of the ab initio calculations, we find noncollinear magnetic axes in 1-Er that are coplanar with the erbium triangle and radially arranged with respect to the triangle's centroid; thus, we argue that the absence of magnetic coupling in this system arises from dipolar and antiferromagnetic superexchange interactions that cancel each other out. PMID:27203849
Cuny, Jérôme; Xie, Yu; Pickard, Chris J; Hassanali, Ali A
2016-02-01
Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful experimental tools to probe the local atomic order of a wide range of solid-state compounds. However, due to the complexity of the related spectra, in particular for amorphous materials, their interpretation in terms of structural information is often challenging. These difficulties can be overcome by combining molecular dynamics simulations to generate realistic structural models with an ab initio evaluation of the corresponding chemical shift and quadrupolar coupling tensors. However, due to computational constraints, this approach is limited to relatively small system sizes which, for amorphous materials, prevents an adequate statistical sampling of the distribution of the local environments that is required to quantitatively describe the system. In this work, we present an approach to efficiently and accurately predict the NMR parameters of very large systems. This is achieved by using a high-dimensional neural-network representation of NMR parameters that are calculated using an ab initio formalism. To illustrate the potential of this approach, we applied this neural-network NMR (NN-NMR) method on the (17)O and (29)Si quadrupolar coupling and chemical shift parameters of various crystalline silica polymorphs and silica glasses. This approach is, in principal, general and has the potential to be applied to predict the NMR properties of various materials. PMID:26730889
Ab initio kinetics studies of hydrogen atom abstraction from methyl propanoate.
Tan, Ting; Yang, Xueliang; Ju, Yiguang; Carter, Emily A
2016-02-14
The kinetics of hydrogen abstraction by five radicals (H, CH3, O((3)P), OH, and HO2) from a biodiesel surrogate, methyl propanoate (MP), is theoretically investigated. We employ high-level ab initio quantum chemistry methods, coupled-cluster singles and doubles with perturbative triples correction (CCSD(T)) and multi-reference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and obtain chemically accurate reaction energetics. Overall, MRSDCI + DS predicts comparable energetics to CCSD(T) for MP + H/CH3/O/OH. The rate constants are computed using transition state theory (TST-Rice-Ramsperger-Kassel-Marcus theory) in conjunction with the separable-hindered-rotor approximation, variable reaction coordinate TST, and the multi-structure all-structure (MS-AS) approach. A simplified method, semi-multi-structure, is also employed for MP + OH/HO2, and the rate coefficients with this less expensive method are in good agreement with the results obtained with the MS-AS method. The fitted modified Arrhenius expressions are provided over a temperature range of 250 to 2000 K. The predicted rate coefficients for MP + OH agree remarkably well with experimental data over a wide temperature range. Branching ratio analysis of all the studied reactions shows that abstractions of the secondary H atoms within MP are expected to dominate the consumption of fuel at low temperatures, and the contributions of abstractions from the two methyl groups increase with temperature for all abstracting radicals. PMID:26796249
Optimized energy landscape exploration using the ab initio based activation-relaxation technique.
Machado-Charry, Eduardo; Béland, Laurent Karim; Caliste, Damien; Genovese, Luigi; Deutsch, Thierry; Mousseau, Normand; Pochet, Pascal
2011-07-21
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 C(20) clusters, vacancy diffusion in bulk silicon, and reconstruction of the 4H-SiC surface. PMID:21786982
Ab initio ro-vibronic spectroscopy of SiCCl (X{sup ~2}Π)
Brites, Vincent; Mitrushchenkov, Alexander O.; Léonard, Céline; Peterson, Kirk A.
2014-07-21
The full dimensional potential energy surfaces of the {sup 2}A{sup ′} and {sup 2}A{sup ′′} electronic components of X{sup ~2}Π SiCCl have been computed using the explicitly correlated coupled cluster method, UCCSD(T)-F12b, combined with a composite approach taking into account basis set incompleteness, core-valence correlation, scalar relativity, and higher order excitations. The spin-orbit and dipole moment surfaces have also been computed ab initio. The ro-vibronic energy levels and absorption spectrum at 5 K have been determined from variational calculations. The influence of each correction on the fundamental frequencies is discussed. An assignment is proposed for bands observed in the LIF experiment of Smith et al. [J. Chem. Phys. 117, 6446 (2002)]. The overall agreement between the experimental and calculated ro-vibronic levels is better than 7 cm{sup −1} which is comparable with the 10–20 cm{sup −1} resolution of the emission spectrum.
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
The equilibrium geometry of the SC3H radical: an ab initio study
NASA Astrophysics Data System (ADS)
Flores, J. R.; Gómez, F. J.
The SC3 H radical is known by experiment to have a linear equilibrium structure, but even rather high-level ab initio computations give a bent equilibrium geometry. A theoretical study of the SCCCH radical has been carried out in order to analyse the influence of several factors in the computed equilibrium structure. Quadratic configuration interaction QCISD(T) and restricted coupled cluster RCCSD(T) computations have been performed in combination with large basis sets. Spin-orbit effects have been taken into account through the Breit-Pauli Hamiltonian using multi-configuration SCF and configuration interaction wavefunctions. Our final results indicate that the equilibrium structure must be linear, in agreement with the experimental studies [MCCARTHY, M. C., VRTILEK, J. M., GOTTLIEB, C. A., WANG, W., and THADDEUS, P., 1994, Astrophys. J., 431, L127; HIRAHARA, Y., OHSHIMA, Y., and ENDO, Y., 1994, J. chem. Phys., 101, 7342]. Both spin-orbit and electron correlation effects appear to be of comparable importance, but an adequate computation of the correlation energy has been much more difficult and has ultimately required bas.is set extrapolations.
Ab initio approach to effective single-particle energies in doubly closed shell nuclei
NASA Astrophysics Data System (ADS)
Duguet, T.; Hagen, G.
2012-03-01
The present work discusses, from an ab initio standpoint, the definition, the meaning, and the usefulness of effective single-particle energies (ESPEs) in doubly closed shell nuclei. We perform coupled-cluster calculations to quantify to what extent selected closed-shell nuclei in the oxygen and calcium isotopic chains can effectively be mapped onto an effective independent-particle picture. To do so, we revisit in detail the notion of ESPEs in the context of strongly correlated many-nucleon systems and illustrate the necessity of extracting ESPEs through the diagonalization of the centroid matrix, as originally argued by Baranger. For the purpose of illustration, we analyze the impact of correlations on observable one-nucleon separation energies and nonobservable ESPEs in selected closed-shell oxygen and calcium isotopes. We then state and illustrate the nonobservability of ESPEs. Similarly to spectroscopic factors, ESPEs can indeed be modified by a redefinition of inaccessible quantities while leaving actual observables unchanged. This leads to the absolute necessity of employing consistent structure and reaction models based on the same nuclear Hamiltonian to extract the shell structure in a meaningful fashion from experimental data.
A Simple ab Initio Model for the Hydrated Electron That Matches Experiment.
Kumar, Anil; Walker, Jonathan A; Bartels, David M; Sevilla, Michael D
2015-08-27
Since its discovery over 50 years ago, the "structure" and properties of the hydrated electron have been a subject for wonderment and also fierce debate. In the present work we seriously explore a minimal model for the aqueous electron, consisting of a small water anion cluster embedded in a polarized continuum, using several levels of ab initio calculation and basis set. The minimum energy "zero Kelvin" structure found for any 4-water (or larger) anion cluster, at any post-Hartree–Fock theory level, is very similar to a recently reported embedded-DFT-in-classical-water-MD simulation (Uhlig, Marsalek, and Jungwirth, J. Phys. Chem. Lett. 2012, 3, 3071−3075), with four OH bonds oriented toward the maximum charge density in a small central "void". The minimum calculation with just four water molecules does a remarkably good job of reproducing the resonance Raman properties, the radius of gyration derived from the optical spectrum, the vertical detachment energy, and the hydration free energy. For the first time we also successfully calculate the EPR g-factor and (low temperature ice) hyperfine couplings. The simple tetrahedral anion cluster model conforms very well to experiment, suggesting it does in fact represent the dominant structural motif of the hydrated electron. PMID:26275103
Ab initio study for the hydrogen abstraction reactions on toluene and tetralin.
Beste, Ariana; Harrison, Robert J; Britt, Phillip F; Buchanan III, A C
2006-01-01
Hydrogen abstraction reactions play a key role in many thermal and catalytic processes involved in the production of fuels and chemicals. In this paper, the hydrogen abstraction reactions on toluene and tetralin by the benzyl radical are investigated by ab initio methods. These reactions are representatives of similar reactions occurring in the thermolysis of lignin model compounds containing the phenethyl phenyl ether (PPE) structural moiety. The title reactions serve to calibrate the theoretical methods to be used in the study of PPE pyrolysis through comparison of the reaction barriers with reliable experimental values. We used two different hybrid density functionals (BHandHLYP, B3LYP) and second-order perturbation theory to obtain equilibrium and transition state geometries. We recomputed selected energy barriers at the B3LYP geometries with the coupled cluster singles and doubles (CCSD) method. Multiple transition states were found for both reactions. BHandHLYP underestimates and second-order perturbation theory overestimates the reaction barriers; B3LYP energy barriers agree well with experiment and the corresponding CCSD energy barriers. The flat potential energy surface around the saddle points causes numerical inaccuracies. We observe the break down of the harmonic approximation in the calculation of low frequencies.
Parametrization and Validation of Coarse Grained Force-Fields Derived from ab Initio Calculations.
Prampolini, Giacomo
2006-05-01
A novel multisite interaction potential, suitable for computer simulations of complex materials as liquid crystals or polymers, is proposed and parametrized. Its validation is achieved through Monte Carlo numerical experiments at constant temperature and pressure, performed on the p-n-phenyls series and a typical mesogenic molecule (5CB). The model is constructed by connecting an array of anisotropic Gay-Berne sites and a collection of isotropic Lennard-Jones sites. The former mimics the rigid planar six-membered rings of the molecule, while the latter represents the flexible chain, if present. Such intermolecular potential, coupled with an intramolecular part to account for molecular flexibility, is parametrized from ab initio information only, obtained through the recently proposed Fragmentation-Reconstruction Method (FRM). Computer simulations are performed on all systems by exploring phase behavior at several temperatures and by comparing the resulting thermodynamic and structural properties with the relevant experimental data. Despite the simplicity of the present models, the good agreement with the experimental measures suggests the possibility of adopting such hybrid potentials for those systems with a large number of atoms, where high computational cost does not allow the use of more accurate atomistic potentials. PMID:26626663
Tunneling of electrons via rotor-stator molecular interfaces: Combined ab initio and model study
NASA Astrophysics Data System (ADS)
Petreska, Irina; Ohanesjan, Vladimir; Pejov, Ljupčo; Kocarev, Ljupčo
2016-07-01
Tunneling of electrons through rotor-stator anthracene aldehyde molecular interfaces is studied with a combined ab initio and model approach. Molecular electronic structure calculated from first principles is utilized to model different shapes of tunneling barriers. Together with a rectangular barrier, we also consider a sinusoidal shape that captures the effects of the molecular internal structure more realistically. Quasiclassical approach with the Simmons' formula for current density is implemented. Special attention is paid on conformational dependence of the tunneling current. Our results confirm that the presence of the side aldehyde group enhances the interesting electronic properties of the pure anthracene molecule, making it a bistable system with geometry dependent transport properties. We also investigate the transition voltage and we show that conformation-dependent field emission could be observed in these molecular interfaces at realistically low voltages. The present study accompanies our previous work where we investigated the coherent transport via strongly coupled delocalized orbital by application of Non-equilibrium Green's Function Formalism.
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)
Molina Sanchez, Alejandro; Wirtz, Ludger
2014-03-01
The interaction of graphene with substrates can alter its electronic and vibrational properties and is relevant for the practical use of graphene. In this work, we describe the graphene-substrate interaction through the theoretical study of the vibrational properties. We focus on three paradigmatic cases where the interaction strength changes gradually: graphene@BN, graphene@Ir(111), and graphene@SiC. We use ab initio methods to obtain the phonon band structure, the density of states, and the strength of the electron-phonon coupling. Graphene on boron nitride exhibits a weak interaction but a non-negligible shift of the 2D Raman band. We explain this observation by connecting the increase of the dielectric screening with the softening of the electron-phonon interaction. Graphene on iridium, also displays weak interaction but the substrate is a metal. In this case the electron-electron interaction in graphene is screened by a metal electron gas. In the last case, we study the buffer layer of graphene on silicon carbide. The strong hybridization of graphene with silicon carbide changes substantially the electronic structure of graphene. All the calculations are compared to experimental data of Raman spectroscopy and angle-resolved inelastic electron scattering. University of Luxembourg.
Yoo, Su-Hyun; Lee, Ji-Hwan; Delley, Bernard; Soon, Aloysius
2014-09-14
A first-principles description and prediction of brominated nanocrystals of Pd is presented. In particular, we conducted an extensive study of the adsorption behaviour of Br on various Pd surfaces (including both low and high Miller-index surfaces) as a function of its surface coverage. By coupling our calculated surface energies with ab initio (electrochemical) thermodynamics and the Gibbs-Wulff shape model, we find that the relative stability of the Pd surfaces is strongly modified by Br, allowing high Miller-index surfaces of Pd (namely the (210) surface) to become competitively favourable at moderate concentrations of Br. We also show that Pd nanoparticles assume a cube-like crystal shape at high concentrations of Br, exposing mainly the (100) facets with a Br surface coverage of 0.5 ML. This not only confirms and explains recent solution synthesis results, but also provides a quantitative atomic picture of the exposed surface facets, which is crucial in understanding the local surface chemistry of shape-controlled nanoparticles for better nanocatalyst design. PMID:25075669
Microwave and ab initio studies of the Xe-CH4 van der Waals complex
NASA Astrophysics Data System (ADS)
Wen, Qing; Jäger, Wolfgang
2006-01-01
An ab initio potential-energy surface of the Xe-CH4 van der Waals complex was constructed at the coupled cluster level of theory with single, double, and perturbatively included triple excitations. The recently developed small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta basis set was used for the xenon atom and Dunning's augmented correlation-consistent polarized valence triple-zeta basis set for the other atoms. The basis sets were supplemented with bond functions. Dipole moments were also calculated at various configurations. Rotational spectra of the Xe-CH4 van der Waals complex were recorded using a pulsed-nozzle Fourier transform microwave spectrometer. The isotopomers studied include those of CH4,CH134,CD4,CH3D, and CHD3 with the five most abundant Xe isotopes. Transitions within three internal rotor states, namely, the j =0,K=0;j=1,K=0; and j =2,K=1 states, were observed and assigned. Nuclear quadrupole hyperfine structures due to the presence of Xe131(I=3/2) were detected and analyzed. It was found that the j =1,K=0 state is perturbed by a Coriolis interaction with a nearby j =1,K=1 state. For isotopomers containing CH3D and CHD3, the j =2 states are no longer metastable and could not be observed. The spectroscopic results were used to derive structural and dynamical information of the Xe-CH4 complex.
NASA Astrophysics Data System (ADS)
Neukirch, Amanda; Nei, Wanyi; Pedesseau, Laurent; Even, Jacky; Katan, Claudine; Mohite, Aditya; Tretiak, Segrei
2015-03-01
The need for an inexpensive, clean, and plentiful source of energy has generated large amounts of research in an assortment of solution processed organic and hybrid organic-inorganic solar cells. A relative newcomer to the field of solution processed photovoltaics is the lead halide perovskite solar cell. In the past 5 years, the efficiencies of devices made from this material have increased from 3.5% to nearly 20%. Despite the rapid development of organic-inorganic perovskite solar cells, a thorough understanding of the fundamental photophysical processes driving the high performance of these devices is not well understood. I am using state-of-the-art ab initio computational techniques in order to characterize the properties at the interface of perovskite devices in order to aide in materials design and device engineering. I will present an in-depth analysis of the electronic and optical properties of bulk and surface states of pure and mixed halide systems. The high-level static quantum mechanical calculations, including spin-orbit-coupling and the many body GW approach, identify the key electronic states involved in photoinduced dynamics. This knowledge provides important information on how the optical properties change with variations to the system. Supported by the DOE, the LANL LDRD program XW11, and CNLS.
Redox Switches for Single-Molecule Magnet Activity: An Ab Initio Insight.
Vieru, Veacheslav; Chibotaru, Liviu F
2016-04-01
A dinuclear Co(II) complex (1) featuring unprecedented anodic and cathodic switches for single-molecule magnet (SMM) activity has been recently investigated (J. Am. Chem. Soc. 2013, 135, 14670). The presence of sandwiched radicals in different oxidation states of this compound mediates magnetic coupling between the high-spin (S=3/2) cobalt ions, which gives rise to SMM activity in both the oxidized ([1(OEt2 )](+) ) and reduced ([1](-) ) states. This feature represents the first example of a SMM exhibiting fully reversible, dual ON/OFF switchability. Here we apply ab initio and broken-symmetry DFT calculations to elucidate the mechanisms responsible for magnetic properties and magnetization blocking in these compounds. It is found that due to the strong delocalization of the magnetic molecular orbital, there is a strong antiferromagnetic interaction between the radical and cobalt ions. The lack of high axiality of the cobalt centres explains why these compounds possess slow relaxation of magnetization only in an applied dc magnetic field. PMID:26918833
Ottonello, G; Zuccolini, M Vetuschi; Belmonte, D
2010-09-14
We present the results of a computational investigation with ab initio procedures of the structure-energy and vibrational properties of silica clusters in a dielectric continuum with dielectric constant ε=3.8, through density functional theory/B3LYP gas phase calculations coupled with a polarized continuum model approach [integral equation formalism applied to a polarized continuum (IEFPCM)] and those of the periodical structure D(6h) which leads to the α-cristobalite polymorph of silica when subjected to symmetry operations with the same functional within the linear combination of atomic orbitals (LCAO) approximation and in the framework of Bloch's theorem. Based on the computed energies and vibrational features, an aggregate of the D(6h) network and the monomer locally ordered in the short-medium range and both present in the glass in a mutual arrangement lacking of spatial continuity reproduces satisfactorily the experimentally observed low T heat capacity and the deviation from the Debye T(3) law. Above T(g), the experimental heat capacity of the liquid is perfectly reproduced summing to the internal modes the translational and rotational contributions to the bulk heat capacity and subtracting the (acoustic) terms arising from coherent motion (no longer existent). PMID:20849179
Melting of sodium under high pressure. An ab-initio study
González, D. J.; González, L. E.
2015-08-17
We report ab-initio molecular dynamics simulations of dense liquid/solid sodium for a pressure range from 0 to 100 GPa. The simulations have been performed with the orbital free ab-initio molecular dynamics method which, by using the electron density as the basic variable, allows to perform simulations with large samples and for long runs. The calculated melting curve shows a maximum at a pressure ≈ 30 GPa and it is followed by a long, steep decrease. These features are in good agreement with the experimental data. For various pressures along the melting curve, we have calculated several liquid static properties (pair distribution functions, static structure factors and short-range order parameters) in order to analyze the structural effects of pressure.
Polysiloxanes: ab initio force field and structural, conformational and thermophysical properties
NASA Astrophysics Data System (ADS)
Sun, Huai; Rigby, David
1997-07-01
Various levels of ab initio calculation have been performed to determine the optimum strategy for parameterization of the valence parameters of a CFF-type force field for siloxanes and polysiloxanes. Electrostatic nonbond parameters have been determined using scaled electrostatic potential (ESP) charges adjusted for known systematic differences between ab initio and experimental data, while van der Waals nonbond parameters have been determined using a classical approach involving fitting to experimental liquid density and cohesive energy density data measured at atmospheric pressure and a single temperature for a set of four small molecules. Simulations have been performed on molecular crystals, liquids and isolated molecules, yielding results which agree favorably with available experimental data. Properties calculated include unit cell parameters and crystal densities, liquid densities from 303-473 K and 0-1800 bar, dependence of oligomer density and solubility parameters on chain length and temperature, gas-phase geometries and vibrational frequencies, and gas and liquid-phase conformational behavior.
Yadav, J.S.; Hermsmeier, M.; Gund, T. )
1989-01-01
We have calculated the net atomic charges and molecular electrostatic potentials of two potent nicotinic acetylcholine receptor agonists, isoarecolone and acetylpiperazine, by three different methods to see how well they correlate and if the simplest method gives the same predictive results. The calculational methods involved calculating net atomic charges by semiempirical (MNDO from MOPAC) and ab initio (Mulliken) and ab initio (potential derived) at STO-3G basis set level. Some deviations were observed when comparisons were made atom by atom, but when group comparisons were made, good correlations were observed. When these partial charges were used to calculate the respective molecular electrostatic potentials on the van der Waals surface, very good correlations were obtained. This study shows that for routine electrostatic calculations, semiempirical MNDO Calculations give similar results and thus lead to similar predictions.
Ab initio molecular dynamics simulation of liquid water by quantum Monte Carlo
Zen, Andrea; Luo, Ye Mazzola, Guglielmo Sorella, Sandro; Guidoni, Leonardo
2015-04-14
Although liquid water is ubiquitous in chemical reactions at roots of life and climate on the earth, the prediction of its properties by high-level ab initio molecular dynamics simulations still represents a formidable task for quantum chemistry. In this article, we present a room temperature simulation of liquid water based on the potential energy surface obtained by a many-body wave function through quantum Monte Carlo (QMC) methods. The simulated properties are in good agreement with recent neutron scattering and X-ray experiments, particularly concerning the position of the oxygen-oxygen peak in the radial distribution function, at variance of previous density functional theory attempts. Given the excellent performances of QMC on large scale supercomputers, this work opens new perspectives for predictive and reliable ab initio simulations of complex chemical systems.
B28: the smallest all-boron cage from an ab initio global search
NASA Astrophysics Data System (ADS)
Zhao, Jijun; Huang, Xiaoming; Shi, Ruili; Liu, Hongsheng; Su, Yan; King, R. Bruce
2015-09-01
Our ab initio global searches reveal the lowest-energy cage for B28, which is built from two B12 units and prevails over the competing structural isomers such as planar, bowl, and tube. This smallest boron cage extends the scope of all-boron fullerene and provides a new structural motif of boron clusters and nanostructures.Our ab initio global searches reveal the lowest-energy cage for B28, which is built from two B12 units and prevails over the competing structural isomers such as planar, bowl, and tube. This smallest boron cage extends the scope of all-boron fullerene and provides a new structural motif of boron clusters and nanostructures. Electronic supplementary information (ESI) available: Planar isomer structures of B28 and spatial distributions of front molecular orbitals. See DOI: 10.1039/c5nr04034e
Ab initio investigation of light-induced relativistic spin-flip effects in magneto-optics
NASA Astrophysics Data System (ADS)
Mondal, Ritwik; Berritta, Marco; Carva, Karel; Oppeneer, Peter M.
2015-05-01
Excitation of a metallic ferromagnet such as Ni with an intensive femtosecond laser pulse causes an ultrafast demagnetization within approximately 300 fs. It was proposed that the ultrafast demagnetization measured in femtosecond magneto-optical experiments could be due to relativistic light-induced processes. We perform an ab initio investigation of the influence of relativistic effects on the magneto-optical response of Ni. To this end, first, we develop a response theory formulation of the additional appearing ultrarelativistic terms in the Foldy-Wouthuysen transformed Dirac Hamiltonian due to the electromagnetic field, and second, we compute the influence of relativistic light-induced spin-flip transitions on the magneto-optics. Our ab initio calculations of relativistic spin-flip optical excitations predict that these can give only a very small contribution (≤0.1 %) to the laser-induced magnetization change in Ni.
DFT and ab initio quantum chemical studies on p-cyanobenzoic acid
NASA Astrophysics Data System (ADS)
Arjunan, V.; Rani, T.; Varalakshmy, L.; Mohan, S.; Tedlamelekot, F.
2011-05-01
The Fourier transform infrared (FTIR) and FT-Raman spectra of p-cyanobenzoic acid (CBA) have been recorded in the range 4000-400 and 4000-100 cm -1, respectively. The complete vibrational assignment and analysis of the fundamental modes of the compound were carried out using the observed FTIR and FT-Raman data. The vibrational frequencies determined experimentally were compared with theoretical wavenumbers obtained from ab initio HF and DFT-B3LYP gradient calculations employing 6-31G**, 6-311++G** and cc-pVTZ basis sets for the optimised geometry of the compound. The geometry and normal modes of vibration obtained from the HF and DFT methods are in good agreement with the experimental data. The normal coordinate analysis was also carried out with ab initio force fields utilising Wilson's FG matrix method. The interactions of cyano and carboxylic acid groups with the skeletal vibrational modes were investigated.
Study of atomic structure of liquid Hg-In alloys using ab-initio molecular dynamics
Sharma, Nalini; Ahluwalia, P. K.; Thakur, Anil
2015-05-15
Ab-initio molecular dynamics simulations are performed to study the structural properties of liquid Hg-In alloys. The interatomic interactions are described by ab-initio pseudopotentials given by Troullier and Martins. Five liquid Hg-In mixtures (Hg{sub 10}In{sub 90}, Hg{sub 30}In{sub 70}, Hg{sub 50}In{sub 50}, Hg{sub 70}In{sub 30} and Hg{sub 90}In{sub 10}) at 299K are considered. The radial distribution function g(r) and structure factor S(q) of considered alloys are compared with respective experimental results for liquid Hg (l-Hg) and (l-In). The radial distribution function g(r) shows the presence of short range order in the systems considered. Smooth curves of Bhatia-Thornton partial structure factors factor shows the presence of liquid state in the considered alloys.
Ab initio NMR Confirmed Evolutionary Structure Prediction for Organic Molecular Crystals
NASA Astrophysics Data System (ADS)
Pham, Cong-Huy; Kucukbenli, Emine; de Gironcoli, Stefano
2015-03-01
Ab initio crystal structure prediction of even small organic compounds is extremely challenging due to polymorphism, molecular flexibility and difficulties in addressing the dispersion interaction from first principles. We recently implemented vdW-aware density functionals and demonstrated their success in energy ordering of aminoacid crystals. In this work we combine this development with the evolutionary structure prediction method to study cholesterol polymorphs. Cholesterol crystals have paramount importance in various diseases, from cancer to atherosclerosis. The structure of some polymorphs (e.g. ChM, ChAl, ChAh) have already been resolved while some others, which display distinct NMR spectra and are involved in disease formation, are yet to be determined. Here we thoroughly assess the applicability of evolutionary structure prediction to address such real world problems. We validate the newly predicted structures with ab initio NMR chemical shift data using secondary referencing for an improved comparison with experiments.
Knockout reactions from p-shell nuclei : tests of ab initio structure models.
Grinyer, G. F.; Bazin, D.; Gade, A.; Tostevin, J. A.; Adrich, P.; Bowen, M. D.; Brown, B. A.; Campbell, C. M.; Cook, J. M.; Glasmacher, T.; McDaniel, S.; Navratil, P.; Obertelli, A.; Quaglioni, S.; Siwek, K.; Terry, J. R.; Weisshaar, D.; Wiringa, R. B.
2011-04-22
Absolute cross sections have been determined following single neutron knockout reactions from {sup 10}Be and {sup 10}C at intermediate energy. Nucleon density distributions and bound-state wave function overlaps obtained from both variational Monte Carlo (VMC) and no core shell model (NCSM) ab initio calculations have been incorporated into the theoretical description of knockout reactions. Comparison to experimental cross sections demonstrates that the VMC approach, with the inclusion of 3-body forces, provides the best overall agreement while the NCSM and conventional shell-model calculations both overpredict the cross sections by 20% to 30% for {sup 10}Be and by 40% to 50% for {sup 10}C, respectively. This study gains new insight into the importance of 3-body forces and continuum effects in light nuclei and provides a sensitive technique to assess the accuracy of ab initio calculations for describing these effects.
Knockout Reactions from p-Shell Nuclei: Tests of Ab Initio Structure Models
Grinyer, G. F.; Bazin, D.; Adrich, P.; Obertelli, A.; Weisshaar, D.; Gade, A.; Bowen, M. D.; Brown, B. A.; Campbell, C. M.; Cook, J. M.; Glasmacher, T.; McDaniel, S.; Siwek, K.; Terry, J. R.; Tostevin, J. A.; Navratil, P.; Quaglioni, S.; Wiringa, R. B.
2011-04-22
Absolute cross sections have been determined following single neutron knockout reactions from {sup 10}Be and {sup 10}C at intermediate energy. Nucleon density distributions and bound-state wave function overlaps obtained from both variational Monte Carlo (VMC) and no core shell model (NCSM) ab initio calculations have been incorporated into the theoretical description of knockout reactions. Comparison to experimental cross sections demonstrates that the VMC approach, with the inclusion of 3-body forces, provides the best overall agreement while the NCSM and conventional shell-model calculations both overpredict the cross sections by 20% to 30% for {sup 10}Be and by 40% to 50% for {sup 10}C, respectively. This study gains new insight into the importance of 3-body forces and continuum effects in light nuclei and provides a sensitive technique to assess the accuracy of ab initio calculations for describing these effects.
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 studies of equations of state and chemical reactions of reactive structural materials
NASA Astrophysics Data System (ADS)
Zaharieva, Roussislava
subject of studies of the shock or thermally induced chemical reactions of the two solids comprising these reactive materials, from first principles, is a relatively new field of study. The published literature on ab initio techniques or quantum mechanics based approaches consists of the ab initio or ab initio-molecular dynamics studies in related fields that contain a solid and a gas. One such study in the literature involves a gas and a solid. This is an investigation of the adsorption of gasses such as carbon monoxide (CO) on Tungsten. The motivation for these studies is to synthesize alternate or synthetic fuel technology by Fischer-Tropsch process. In this thesis these studies are first to establish the procedure for solid-solid reaction and then to extend that to consider the effects of mechanical strain and temperature on the binding energy and chemisorptions of CO on tungsten. Then in this thesis, similar studies are also conducted on the effect of mechanical strain and temperature on the binding energies of Titanium and hydrogen. The motivations are again to understand the method and extend the method to such solid-solid reactions. A second motivation is to seek strained conditions that favor hydrogen storage and strain conditions that release hydrogen easily when needed. Following the establishment of ab initio and ab initio studies of chemical reactions between a solid and a gas, the next step of research is to study thermally induced chemical reaction between two solids (Ni+Al). Thus, specific new studies of the thesis are as follows: (1) Ab initio Studies of Binding energies associated with chemisorption of (a) CO on W surfaces (111, and 100) at elevated temperatures and strains and (b) adsorption of hydrogen in titanium base. (2) Equations of state of mixtures of reactive material structures from ab initio methods. (3) Ab initio studies of the reaction initiation, transition states and reaction products of intermetallic mixtures of (Ni+Al) at elevated
Large-scale ab initio configuration interaction calculations for light nuclei
NASA Astrophysics Data System (ADS)
Maris, Pieter; Metin Aktulga, H.; Caprio, Mark A.; Çatalyürek, Ümit V.; Ng, Esmond G.; Oryspayev, Dossay; Potter, Hugh; Saule, Erik; Sosonkina, Masha; Vary, James P.; Yang, Chao; Zhou, Zheng
2012-12-01
In ab-initio Configuration Interaction calculations, the nuclear wavefunction is expanded in Slater determinants of single-nucleon wavefunctions and the many-body Schrodinger equation becomes a large sparse matrix problem. The challenge is to reach numerical convergence to within quantified numerical uncertainties for physical observables using finite truncations of the infinite-dimensional basis space. We discuss strategies for constructing and solving the resulting large sparse matrix eigenvalue problems on current multicore computer architectures. Several of these strategies have been implemented in the code MFDn, a hybrid MPI/OpenMP Fortran code for ab-initio nuclear structure calculations that can scale to 100,000 cores and more. Finally, we will conclude with some recent results for 12C including emerging collective phenomena such as rotational band structures using SRG evolved chiral N3LO interactions.
Song, Lingchun; Han, Jaebeom; Lin, Yen-lin; Xie, Wangshen; Gao, Jiali
2010-01-01
The explicit polarization (X-Pol) method has been examined using ab initio molecular orbital theory and density functional theory. The X-Pol potential was designed to provide a novel theoretical framework for developing next-generation force fields for biomolecular simulations. Importantly, the X-Pol potential is a general method, which can be employed with any level of electronic structure theory. The present study illustrates the implementation of the X-Pol method using ab initio Hartree—Fock theory and hybrid density functional theory. The computational results are illustrated by considering a set of bimolecular complexes of small organic molecules and ions with water. The computed interaction energies and hydrogen bond geometries are in good accord with CCSD(T) calculations and B3LYP/aug-cc-pVDZ optimizations. PMID:19618944
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
2013-01-01
: 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 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.
Liquid Be, Ca and Ba. An orbital-free ab-initio molecular dynamics study
Rio, B. G. del; González, L. E.
2015-08-17
Several static and dynamic properties of liquid beryllium (l-Be), liquid calcium (l-Ca) and liquid barium (l-Ba) near their triple point have been evaluated by the orbital-free ab initio molecular dynamics method (OF-AIMD), where the interaction between valence electrons and ions is described by means of local pseudopotentials. These local pseudopotentials used were constructed through a force-matching process with those obtained from a Kohn-Sham ab initio molecular dynamics study (KS-AIMD) of a reduced system with non-local pseudopotentials. The calculated static structures show good agreement with the available experimental data, including an asymmetric second peak in the structure factor which has been linked to the existence of a marked icosahedral short-range order in the liquid. As for the dynamic properties, we obtain collective density excitations whose associated dispersion relations exhibit a positive dispersion.
Electronic properties of liquid Hg-In alloys : Ab-initio molecular dynamics study
NASA Astrophysics Data System (ADS)
Sharma, Nalini; Thakur, Anil; Ahluwalia, P. K.
2016-05-01
Ab-initio molecular dynamics simulations are performed to study the structural properties of liquid Hg-In alloys. The interatomic interactions are described by ab-initio pseudopotentials given by Troullier and Martins. Three liquid Hg-In alloys (Hg10In90, Hg30In70,. Hg50In50, Hg70In30, and Hg90Pb10) at 299 K are considered. The calculated results for liquid Hg (l-Hg) and lead (l-In) are also drawn. Along with the calculated results of considered five liquid alloys of Hg-In alloy. The results obtained from electronic properties namely total density of state and partial density of states help to find the local arrangement of Hg and In atoms and the presence of liquid state in the considered five alloys.
NASA Astrophysics Data System (ADS)
Knoop, S.; Żuchowski, P. S.; KÈ©dziera, D.; Mentel, Ł.; Puchalski, M.; Mishra, H. P.; Flores, A. S.; Vassen, W.
2014-08-01
We have investigated the ultracold interspecies scattering properties of metastable triplet He and Rb. We performed state-of-the-art ab initio calculations of the relevant interaction potential, and measured the interspecies elastic cross section for an ultracold mixture of metastable triplet He4 and Rb87 in a quadrupole magnetic trap at a temperature of 0.5 mK. Our combined theoretical and experimental study gives an interspecies scattering length a4+87=+17-4+1a0, which prior to this work was unknown. More general, our work shows the possibility of obtaining accurate scattering lengths using ab initio calculations for a system containing a heavy, many-electron atom, such as Rb.
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
Implementation of a vector potential method in an ab initio Hartree-Fock code
NASA Astrophysics Data System (ADS)
Tevekeliyska, Violina; Springborg, Michael; Champagne, Benoît; Kirtman, Bernard
2012-12-01
For extended systems exposed to an external, electrostatic field, the presence of the field leads to an extra term (E⃗. P⃗) to the Hamiltonian, where E⃗ is the field vector and P⃗ is the polarization of the system of interest. In order to find out how a polymer chain responds to an external electric perturbation, a field with a charge and a current term for the polarization is added to an ab initio Hartree-Fock Hamiltonian. The polarization expression is taken from an efficient vector potential approach (VPA) [1] for calculating electronic and nuclear responses of infinite periodic systems to finite electric fields and is implemented in the ab initio LCAO-SCF algorithm [3], which computes band structure of regular or helical polymers, taking into account the one-dimensional translational symmetry. A smoothing procedure for numerical differentiation of the orbital coefficients is used in order to calculate self-consistently the charge flow contribution to the polarization.
Matsushita, Y. Murakawa, T. Shimamura, K. Oishi, M. Ohyama, T. Kurita, N.
2015-02-27
The catabolite activator protein (CAP) is one of the regulatory proteins controlling the transcription mechanism of gene. Biochemical experiments elucidated that the complex of CAP with cyclic AMP (cAMP) is indispensable for controlling the mechanism, while previous molecular simulations for the monomer of CAP+cAMP complex revealed the specific interactions between CAP and cAMP. However, the effect of cAMP-binding to CAP on the specific interactions between CAP and DNA is not elucidated at atomic and electronic levels. We here considered the ternary complex of CAP, cAMP and DNA in solvating water molecules and investigated the specific interactions between them at atomic and electronic levels using ab initio molecular simulations based on classical molecular dynamics and ab initio fragment molecular orbital methods. The results highlight the important amino acid residues of CAP for the interactions between CAP and cAMP and between CAP and DNA.
NASA Astrophysics Data System (ADS)
Shibuta, Yasushi; Shimamura, Kohei; Oguri, Tomoya; Arifin, Rizal; Shimojo, Fuyuki; Yamaguchi, Shu
2015-03-01
The growth mechanism of carbon nanotubes (CNT) has been widely discussed both from experimental and computational studies. Regarding the computational studies, most of the studies focuses on the aggregation of isolate carbon atoms on the catalytic metal nanoparticle, whereas the initial dissociation of carbon source molecules should affect the yield and quality of the products. On the other hand, we have studied the dissociation process of carbon source molecules on the metal surface by the ab initio molecular dynamics simulation. In the study, we investigate the ethanol dissociation on Pt and Ni clusters by ab initio MD simulations to discuss the initial stage of CNT growth by alcohol CVD technique. Part of this research is supported by the Grant-in-Aid for Young Scientists (a) (No. 24686026) from MEXT, Japan.
Hydrogen adsorption in ZIF-7: A DFT and ab-initio molecular dynamics study
NASA Astrophysics Data System (ADS)
Dixit, Mudit; Major, Dan Thomas; Pal, Sourav
2016-05-01
Primary H2 adsorption sites in a zeolitic imidazolate framework, ZIF-7, are identified using ab-initio density functional theory (DFT) based molecular dynamics annealing simulations. The simulations suggest several low energy adsorption sites. The effect of light transition metal decoration on hydrogen storage properties was studied. Our ab-intio DFT calculations illustrate that decorating the ZIF with Sc increases both the number of H2 molecules, as well as the H2 binding energy. The binding energy (∼25 kJ/mol per H2) at 8H2 loading in the pore, suggests that Sc-ZIFs can be potential candidates for hydrogen storage.
Relating Ab Initio Mechanical Behavior of Intergranular Glassy Films in Γ-Si3N4 to Continuum Scales
NASA Astrophysics Data System (ADS)
Ouyang, L.; Chen, J.; Ching, W.; Misra, A.
2006-05-01
Nanometer thin intergranular glassy films (IGFs) form in polycrystalline ceramics during sintering at high temperatures. The structure and properties of these IGFs are significantly changed by doping with rare earth elements. We have performed highly accurate large-scale ab initio calculations of the mechanical properties of both undoped and Yittria doped (Y-IGF) model by theoretical uniaxial tensile experiments. Uniaxial strain was applied by incrementally stretching the super cell in one direction, while the other two dimensions were kept constant. At each strain, all atoms in the model were fully relaxed using Vienna Ab initio Simulation Package VASP. The relaxed model at a given strain serves as the starting position for the next increment of strain. This process is carried on until the total energy (TE) and stress data show that the "sample" is fully fractured. Interesting differences are seen between the stress-strain response of undoped and Y-doped models. For the undoped model, the stress-strain behavior indicates that the initial atomic structure of the IGF is such that there is negligible coupling between the x- and the y-z directions. However, once the behavior becomes non- linear the lateral stresses increase, indicating that the atomic structure evolves with loading [1]. To relate the ab initio calculations to the continuum scales we analyze the atomic-scale deformation field under this uniaxial loading [1]. The applied strain in the x-direction is mostly accommodated by the IGF part of the model and the crystalline part experiences almost negligible strain. As the overall strain on the sample is incrementally increased, the local strain field evolves such that locations proximal to the softer spots attract higher strains. As the load progresses, the strain concentration spots coalesce and eventually form persistent strain localization zone across the IGF. The deformation pattern obtained through ab initio calculations indicates that it is possible to
Plotnikov, Nikolay; Kamerlin, Shina Caroline Lynn; Warshel, Arieh
2011-01-01
Recent years have seen tremendous effort in the development of approaches with which to obtain quantum mechanics/molecular mechanics (QM/MM) free energies for reactions in the condensed phase. Nevertheless, there remain significant challenges to address, particularly the high computational cost involved in performing proper configurational sampling and in particular in obtaining ab initio QM/MM (QM(ai)/MM) free energy surfaces. One increasingly popular approach that seems to offer an ideal way to progress in this direction is the elegant metadynamics (MTD) approach. However, in the current work we point out the subtle efficiency problems associated with this approach, and illustrate that we have at hand what is arguably a more powerful approach. More specifically, we demonstrate the effectiveness of an updated version of our original idea of using a classical reference potential for QM(ai)/MM calculations [J. Phys. Chem. B. 102 (1998), 2293)], which we refer to as “paradynamics” (PD). This approach is based on the use of an empirical valence bond (EVB) reference potential, which is already similar to the real ab initio potential. The reference potential is fitted to the ab initio potential by an iterative and, to a great degree, automated refinement procedure. The corresponding free energy profile is then constructed using the refined EVB potential, and the linear response approximation (LRA) is used to evaluate the QM(ai)/MM activation free energy barrier. The automated refinement of the EVB surface (and thus the reduction of the difference between the reference and ab initio potentials) is a key factor in accelerating the convergence of the LRA approach. We apply our PD approach to a test reaction, namely the SN2 reaction between chloride ion and methyl chloride, and demonstrate that, at present, this approach is far more powerful and cost effective than the metadynamics approach (at least in its current implementation). We also discuss the general features
Transport coefficients of helium-argon mixture based on ab initio potential
NASA Astrophysics Data System (ADS)
Sharipov, Felix; Benites, Victor J.
2015-10-01
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.
Optical and other material properties of SiO2 from ab initio studies
NASA Astrophysics Data System (ADS)
Warmbier, Robert; Mohammed, Faris; Quandt, Alexander
2014-07-01
The optical properties of photonic devices largely depend on the dielectric properties of the underlying materials. We apply modern ab initio methods to study crystalline SiO2 phases, which serve as toy models for amorphous glass. We discuss the dielectric response from the infrared to the VIS/UV, which is crucial for glass based photonic applications. Low density silica, like cristobalite, may provide a good basis for high transmission optical devices.
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.
NASA Astrophysics Data System (ADS)
Yurchenko, Sergei N.; Carvajal, Miguel; Thiel, Walter; Jensen, Per
2006-09-01
We report a six-dimensional CCSD(T)/aug-cc-pVTZ dipole moment surface for the electronic ground state of PH 3 computed ab initio on a large grid of 10 080 molecular geometries. Parameterized, analytical functions are fitted through the ab initio data, and the resulting dipole moment functions are used, together with a potential energy function determined by refining an existing ab initio surface in fittings to experimental wavenumber data, for simulating absorption spectra of the first three polyads of PH 3, i.e., ( ν2, ν4), ( ν1, ν3, 2 ν2, 2 ν4, ν2 + ν4), and ( ν1 + ν2, ν3 + ν2, ν1 + ν4, ν3 + ν4, 2 ν2 + ν4, ν2 + 2 ν4, 3 ν2, 3 ν4). The resulting theoretical transition moments show excellent agreement with experiment. A line-by-line comparison of the simulated intensities of the ν2/ ν4 band system with 955 experimental intensity values reported by Brown et al. [L.R. Brown, R.L. Sams, I. Kleiner, C. Cottaz, L. Sagui, J. Mol. Spectrosc. 215 (2002) 178-203] gives an average absolute percentage deviation of 8.7% (and a root-mean-square deviation of 0.94 cm -1 for the transition wavenumbers). This is very remarkable since the calculations rely entirely on ab initio dipole moment surfaces and do not involve any adjustment of these surfaces to reproduce the experimental intensities. Finally, we predict the line strengths for transitions between so-called cluster levels (near-degenerate levels formed at high rotational excitation) for J up to 60.
An ab initio MIA study of TIBO derivatives R79882 and R82913
NASA Astrophysics Data System (ADS)
Peeters, Anik; Van Alsenoy, C.
1995-04-01
The gas phase structure of two TIBO compounds (R79882 and R82913), potent inhibitors of the reverse transcriptase of HIV1, was studied with ab initio Hartree-Fock methods using the MIA approach. For compound R82913 the geometry of a dimer and of the respective monomers was fully optimized and compared with experiment. For compound R79882 a complete geometry optimization of 15 different conformers was performed.
Ab initio study of the ν(CO 2) mode in EDA complexes
NASA Astrophysics Data System (ADS)
Jamróz, M. H.; Dobrowolski, J. Cz.; Bajdor, K.; Borowiak, M. A.
1995-04-01
Stabilization energy, geometry and ν2 mode of CO 2 molecule in EDA complexes with organic electron donors are ab initio modeled using SPARTAN program. We prove that the splitting of ν2 mode, observed previously in IR spectra, is an effect of removing the double degeneracy of this mode in the complex resulted from the deformation of CO 2 moiety. The dependence of the deformation on complex stabilization energy is discussed.
Ab initio calculation of positron distribution, ACAR and lifetime in TTF-TCNQ
NASA Astrophysics Data System (ADS)
Ishibashi, Shoji; Kohyama, Masanori
2000-06-01
We have performed ab initio calculations of positron distribution, ACAR and lifetime in the quasi-one-dimensional organic conductor TTF-TCNQ. The electronic structure is obtained within the LDA, while the positron state is calculated either with the LDA or with the GGA. Except the positron lifetime, differences between the LDA and GGA results are rather small. The obtained results are compared with our previous experiments and calculations.
Methylchloride adsorbed on Si(0 0 1): an ab initio study
NASA Astrophysics Data System (ADS)
Preuss, M.; Schmidt, W. G.; Seino, K.; Bechstedt, F.
2004-07-01
We present ab initio calculations of the adsorption of methylchloride (CH 3Cl) on Si(0 0 1). Among multiple plausible adsorption geometries, we find five thermodynamically favorable configurations. These lead to strong geometrical changes in the Si surface structure as well as to significant charge transfer processes. The stability of the adsorption structures is discussed in terms of electrostatics. The results are compared to recent experimental and theoretical findings.
Ab initio molecular dynamics study of liquid sodium and cesium up to critical point
Yuryev, Anatoly A.; Gelchinski, Boris R.
2015-08-17
Ab initio modeling of liquid metals Na and K is carried out using the program SIESTA. We have determined the parameters of the model (the optimal step, the number of particles, the initial state etc) and calculated a wide range of properties: the total energy, pair correlation function, coefficient of self-diffusion, heat capacity, statistics of Voronoi polyhedra, the density of electronic states up to the critical temperature.
Manson, S.T.; Miller, J.H.
1987-01-01
Ab initio calculations of single and double differential cross sections for ionization by fast, charged particles within the framework of the Born approximation are presented. In addition, a semi-empirical method based on the asymptotic Bethe-Born expansion is also discussed. Both are applied to ionization of helium by electrons and protons in an effort to assess their accuracy and validity. Agreement with experiment is quite good. The implications for other targets is discussed.
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.
Yamaji, Youhei
2015-12-31
Recently, condensed-matter ab initio approaches to strongly correlated electrons confined in crystalline solids have been developed and applied to transition-metal oxides and molecular conductors. In this paper, an ab initio scheme based on constrained random phase approximations and localized Wannier orbitals is applied to a spin liquid candidate Na{sub 2}IrO{sub 3} and is shown to reproduce experimentally observed specific heat.
Ab initio based thermal property predictions at a low cost: An error analysis
NASA Astrophysics Data System (ADS)
Lejaeghere, Kurt; Jaeken, Jan; Van Speybroeck, Veronique; Cottenier, Stefaan
2014-01-01
Ab initio calculations often do not straightforwardly yield the thermal properties of a material yet. It requires considerable computational efforts, for example, to predict the volumetric thermal expansion coefficient αV or the melting temperature Tm from first principles. An alternative is to use semiempirical approaches. They relate the experimental values to first-principles predictors via fits or approximative models. Before applying such methods, however, it is of paramount importance to be aware of the expected errors. We therefore quantify these errors at the density-functional theory level using the Perdew-Burke-Ernzerhof functional for several semiempirical approximations of αV and Tm, and compare them to the errors from fully ab initio methods, which are computationally more intensive. We base our conclusions on a benchmark set of 71 ground-state elemental crystals. For the thermal expansion coefficient, it appears that simple quasiharmonic theory, in combination with different approximations to the Grüneisen parameter, provides a similar overall accuracy as exhaustive first-principles phonon calculations. For the melting temperature, expensive ab initio molecular-dynamics simulations still outperform semiempirical methods.
NASA Astrophysics Data System (ADS)
Geng, Hua Y.
2015-02-01
A multilevel approach to sample the potential energy surface in a path integral formalism is proposed. The purpose is to reduce the required number of ab initio evaluations of energy and forces in ab initio path integral molecular dynamics (AI-PIMD) simulation, without compromising the overall accuracy. To validate the method, the internal energy and free energy of an Einstein crystal are calculated and compared with the analytical solutions. As a preliminary application, we assess the performance of the method in a realistic model-the FCC phase of dense atomic hydrogen, in which the calculated result shows that the acceleration rate is about 3 to 4-fold for a two-level implementation, and can be increased up to 10 times if extrapolation is used. With only 16 beads used for the ab initio potential sampling, this method gives a well converged internal energy. The residual error in pressure is just about 3 GPa, whereas it is about 20 GPa for a plain AI-PIMD calculation with the same number of beads. The vibrational free energy of the FCC phase of dense hydrogen at 300 K is also calculated with an AI-PIMD thermodynamic integration method, which gives a result of about 0.51 eV/proton at a density of rs = 0.912.
Dominant Modes in Light Nuclei - Ab Initio View of Emergent Symmetries
NASA Astrophysics Data System (ADS)
Draayer, J. P.; Dytrych, T.; Launey, K. D.; Dreyfuss, A. C.; Langr, D.
2015-01-01
An innovative symmetry-guided concept is discussed with a focus on emergent symmetry patterns in complex nuclei. In particular, the ab initio symmetry-adapted no-core shell model (SA-NCSM), which capitalizes on exact as well as partial symmetries that underpin the structure of nuclei, provides remarkable insight into how simple symmetry patterns emerge in the many-body nuclear dynamics from first principles. This ab initio view is complemented by a fully microscopic no-core symplectic shell-model framework (NCSpM), which, in turn, informs key features of the primary physics responsible for the emergent phenomena of large deformation and alpha-cluster substructures in studies of the challenging Hoyle state in Carbon-12 and enhanced collectivity in intermediate-mass nuclei. Furthermore, by recognizing that deformed configurations often dominate the low-energy regime, the SA-NCSM provides a strategy for determining the nature of bound states of nuclei in terms of a relatively small subspace of the symmetry-reorganized complete model space, which opens new domains of nuclei for ab initio investigations, namely, the intermediate-mass region, including isotopes of Ne, Mg, and Si.
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.
Geng, Hua Y.
2015-02-15
A multilevel approach to sample the potential energy surface in a path integral formalism is proposed. The purpose is to reduce the required number of ab initio evaluations of energy and forces in ab initio path integral molecular dynamics (AI-PIMD) simulation, without compromising the overall accuracy. To validate the method, the internal energy and free energy of an Einstein crystal are calculated and compared with the analytical solutions. As a preliminary application, we assess the performance of the method in a realistic model—the FCC phase of dense atomic hydrogen, in which the calculated result shows that the acceleration rate is about 3 to 4-fold for a two-level implementation, and can be increased up to 10 times if extrapolation is used. With only 16 beads used for the ab initio potential sampling, this method gives a well converged internal energy. The residual error in pressure is just about 3 GPa, whereas it is about 20 GPa for a plain AI-PIMD calculation with the same number of beads. The vibrational free energy of the FCC phase of dense hydrogen at 300 K is also calculated with an AI-PIMD thermodynamic integration method, which gives a result of about 0.51 eV/proton at a density of r{sub s}=0.912.
Crystal structure and magnetism in α -RuCl3 : An ab initio study
NASA Astrophysics Data System (ADS)
Kim, Heung-Sik; Kee, Hae-Young
2016-04-01
α -RuCl3 has been proposed recently as an excellent playground for exploring Kitaev physics on a two-dimensional (2D) honeycomb lattice. However, structural clarification of the compound has not been completed, which is crucial in understanding the physics of this system. Here, using ab initio electronic structure calculations, we study a full three-dimensional (3D) structure of α -RuCl3 , including the effects of spin-orbit coupling (SOC) and electronic correlations. The three major results are as follows: (i) SOC suppresses dimerization of Ru atoms, which exists in other Ru compounds such as isostructural Li2RuO3 , and makes the honeycomb closer to an ideal one. (ii) The nearest-neighbor Kitaev exchange interaction between the jeff=1 /2 pseudospin strongly depends on the Ru-Ru distance and the Cl position, originating from the nature of the edge-sharing geometry. (iii) The optimized 3D structure without electronic correlations has P 3 ¯1 m space-group symmetry independent of SOC, but including electronic correlation changes the optimized 3D structure to either C 2 /m or C m c 21 within 0.1 meV per formula unit (f.u.) energy difference. The reported P 3112 structure is also close in energy. The interlayer spin-exchange coupling is a few percent of the in-plane spin-exchange terms, confirming that α -RuCl3 is close to a 2D system. We further suggest how to increase the Kitaev term via tensile strain, which sheds light in realizing the Kitaev spin-liquid phase in this system.
Origin of the Hadži ABC structure: An ab initio study
NASA Astrophysics Data System (ADS)
Van Hoozen, Brian L.; Petersen, Poul B.
2015-11-01
Medium and strong hydrogen bonds are well known to give rise to broad features in the vibrational spectrum often spanning several hundred wavenumbers. In some cases, these features can span over 1000 cm-1 and even contain multiple broad peaks. One class of strongly hydrogen-bonded dimers that includes many different phosphinic, phosphoric, sulfinic, and selenic acid homodimers exhibits a three-peaked structure over 1500 cm-1 broad. This unusual feature is often referred to as the Hadži ABC structure. The origin of this feature has been debated since its discovery in the 1950s. Only a couple of theoretical studies have attempted to interpret the origin of this feature; however, no previous study has been able to reproduce this feature from first principles. Here, we present the first ab initio calculation of the Hadži ABC structure. Using a reduced dimensionality calculation that includes four vibrational modes, we are able to reproduce the three-peak structure and much of the broadness of the feature. Our results indicate that Fermi resonances of the in-plane bend, out-of-plane bend, and combination of these bends play significant roles in explaining this feature. Much of the broadness of the feature and the ability of the OH stretch mode to couple with many overtone bending modes are captured by including an adiabatically separated dimer stretch mode in the model. This mode modulates the distance between the monomer units and accordingly the strength of the hydrogen-bonds causing the OH stretch frequency to shift from 2000 to 3000 cm-1. Using this model, we were also able to reproduce the vibrational spectrum of the deuterated isotopologue which consists of a single 500 cm-1 broad feature. Whereas previous empirical studies have asserted that Fermi resonances contribute very little to this feature, our study indicates that while not appearing as a separate peak, a Fermi resonance of the in-plane bend contributes substantially to the feature.
Ab initio characterization of the conical intersections involved in the photochemistry of phenol
Vieuxmaire, Olivier P. J.; Domcke, Wolfgang; Lan, Zhenggang; Sobolewski, Andrzej L.
2008-12-14
The nature of the vibronic interactions between the {sup 1}{pi}{pi}* (A{sup '}), the {sup 1}{pi}{sigma}* (A{sup ''}), and the S{sub 0} (A{sup '}) states at the CI{sub {pi}}{sub {pi}}{sub */{pi}}{sub {sigma}}{sub *} and CI{sub {pi}}{sub {sigma}}{sub */{pi}}{sub {pi}} conical intersections has been investigated by accurate ab initio calculations. Potential energy surfaces have been constructed at the complete-active-space self-consistent-field and multireference configuration-interaction (MRCI) levels of theory along each of the ten normal coordinates of A{sup ''} symmetry that potentially can be coupling modes at these conical intersections. The OH torsion was found to be by far the strongest coupling mode in each case. As for benzene, a 'channel three' radiationless decay mechanism associated with a prefulvenic conical intersection, CI{sub pref}, was found to exist in phenol. The reaction path connecting the prefulvenic form of phenol with the minimum-energy structure of the S{sub 1} state was computed at different levels of theory. The barrier to be overcome for the opening of the prefulvenic decay channel is estimated as 6370 cm{sup -1} at the MRCI level, that is, about 2300 cm{sup -1} above the energy of CI{sub {pi}}{sub {pi}}{sub */{pi}}{sub {sigma}}{sub *}. With sufficient excess energy in the S{sub 1} state, the prefulvenic decay mechanism can be in competition with the hydrogen detachment process.
NASA Astrophysics Data System (ADS)
Kim, Heung-Sik; Kin-Ho Lee, Eric; Kim, Yong Baek
2015-12-01
The recently discovered three-dimensional hyperhoneycomb iridate, β-Li2IrO3, has raised hopes for the realization of the dominant Kitaev interaction between spin-orbit entangled local moments due to its near-ideal lattice structure. If true, this material may lie close to the sought-after quantum spin-liquid phase in three dimensions. Utilizing ab initio electronic structure calculations, we first show that the spin-orbit entangled basis, j\\text{eff} = 1/2 , correctly captures the low-energy electronic structure. The effective spin model derived in the strong-coupling limit supplemented by the ab initio results is shown to be dominated by the Kitaev interaction. We demonstrated that the possible range of parameters is consistent with a non-coplanar spiral magnetic order found in a recent experiment. All of these analyses suggest that β-Li2IrO3 may be the closest among known materials to the Kitaev spin-liquid regime.
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.
Ab Initio Studies of Shock-Induced Chemical Reactions of Inter-Metallics
NASA Astrophysics Data System (ADS)
Zaharieva, Roussislava; Hanagud, Sathya
2009-06-01
Shock-induced and shock assisted chemical reactions of intermetallic mixtures are studied by many researchers, using both experimental and theoretical techniques. The theoretical studies are primarily at continuum scales. The model frameworks include mixture theories and meso-scale models of grains of porous mixtures. The reaction models vary from equilibrium thermodynamic model to several non-equilibrium thermodynamic models. The shock-effects are primarily studied using appropriate conservation equations and numerical techniques to integrate the equations. All these models require material constants from experiments and estimates of transition states. Thus, the objective of this paper is to present studies based on ab initio techniques. The ab inito studies, to date, use ab inito molecular dynamics. This paper presents a study that uses shock pressures, and associated temperatures as starting variables. Then intermetallic mixtures are modeled as slabs. The required shock stresses are created by straining the lattice. Then, ab initio binding energy calculations are used to examine the stability of the reactions. Binding energies are obtained for different strain components super imposed on uniform compression and finite temperatures. Then, vibrational frequencies and nudge elastic band techniques are used to study reactivity and transition states. Examples include Ni and Al.
Periodic arrays of intercalated atoms in twisted bilayer graphene: An ab initio investigation
NASA Astrophysics Data System (ADS)
Miwa, R. H.; Venezuela, P.; Morell, Eric Suárez
2015-09-01
We have performed an ab initio investigation of transition metals (TMs =Mo ,Ru ,Co ,andPt ) embedded in twisted bilayer graphene (tBG) layers. Our total energy results reveal that, triggered by the misalignment between the graphene layers, Mo and Ru atoms may form a quasiperiodic (triangular) array of intercalated atoms. In contrast, the formation of those structures is not expected for the other TMs, the Co and Pt atoms. The net magnetic moment (m ) of Mo and Ru atoms may be quenched upon intercalation, depending on the stacking region (AA or AB). For instance, we find a magnetic moment of 0.3 μB(1.8 μB) for Ru atoms intercalated between the AA (AB) regions of the stacked twisted layers. Through simulated scanning tunneling microscopy (STM) images, we verify that the presence of intercalated TMs can be identified by the formation of bright (hexagonal) spots lying on the graphene surface.
Ab initio study of piezomagnetic effect in Mn-antiperovskites
NASA Astrophysics Data System (ADS)
Sandeman, Karl; Gercsi, Zsolt; Zemen, Jan
2015-03-01
The magnetocaloric and magnetoelastic coupling that often occur together in magnetically frustrated materials have great potential for practical applications ranging from magnetic refrigeration to data processing and storage. Here we focus on the manganese antiperovskite family. Negative thermal expansion (NTE) close to the Néel temperature and a large magnetovolume effect have been measured recently in Mn3NiN using temperature dependent neutron powder diffraction. A large piezomagnetic effect has been predicted in Mn3GaN from Density Functional Theory (DFT) calculations. We perform a computational study of the piezomagnetic effect in three metallic Mn3XN antiperovskites, based our recent DFT model of anisotropic thermal expansions and large magnetoelastic coupling. We confirm the existence of a fully compensated non-collinear antiferromagnetic ground state and predict canting and the change of magnitude of the Mn local magnetic moments as a function of applied biaxial strain, finding that the induced net magnetization reaches values of 0.1 μB per Mn per 1% of strain. The research leading to these results has received funding from the European Community's 7th Framework Programme under Grant agreement 310748 ``DRREAM.''
An experimental and ab initio study on the abiotic reduction of uranyl by ferrous iron
NASA Astrophysics Data System (ADS)
Taylor, S. D.; Marcano, M. C.; Rosso, K. M.; Becker, U.
2015-05-01
It is important to understand the mechanisms controlling the removal of uranyl from solution from an environmental standpoint, particularly whether soluble Fe(II) is capable of reducing soluble U(VI) to insoluble U(IV). Experiments were performed to shed light into discrepancies of recent studies about precipitation of U-containing solids without changing oxidation states versus precipitation/reduction reactions, especially with respect to the kinetics of these reactions. To understand the atomistic mechanisms, thermodynamics, and kinetics of these redox processes, ab initio electron transfer (ET) calculations, using Marcus theory, were applied to study the reduction of U(VI)aq to U(V)aq by Fe(II)aq (the first rate-limiting ET-step). Outer-sphere (OS) and inner-sphere (IS) Fe-U complexes were modeled to represent simple species within a homogeneous environment through which ET could occur. Experiments on the chemical reduction were performed by reacting 1 mM Fe(II)aq at pH 7.2 with high (i.e., 0.16 mM) and lower (i.e., 0.02 mM) concentrations of U(VI)aq. At higher U concentration, a rapid decrease in U(VI)aq was observed within the first hour of reaction. XRD and XPS analyses of the precipitates confirmed the presence of (meta)schoepite phases, where up to ∼25% of the original U was reduced to U4+ and/or U5+-containing phases. In contrast, at 0.02 mM U, the U(VI)aq concentration remained fairly constant for the first 3 h of reaction and only then began to decrease due to slower precipitation kinetics. XPS spectra confirm the partial chemical reduction U associated with the precipitate (up to ∼30%). Thermodynamic calculations support that the reduction of U(VI)aq to U(IV)aq by Fe(II)aq is energetically unfavorable. The batch experiments in this study show U(VI) is removed from solution by precipitation and that transitioning to a heterogeneous system in turn enables the solid U phase to be partially reduced. Ab initio ET calculations revealed that OS ET is
Ab initio multimode linewidth theory for arbitrary inhomogeneous laser cavities
NASA Astrophysics Data System (ADS)
Pick, A.; Cerjan, A.; Liu, D.; Rodriguez, A. W.; Stone, A. D.; Chong, Y. D.; Johnson, S. G.
2015-06-01
We present a multimode laser-linewidth theory for arbitrary cavity structures and geometries that contains nearly all previously known effects and also finds new nonlinear and multimode corrections, e.g., a correction to the α factor due to openness of the cavity and a multimode Schawlow-Townes relation (each linewidth is proportional to a sum of inverse powers of all lasing modes). Our theory produces a quantitatively accurate formula for the linewidth, with no free parameters, including the full spatial degrees of freedom of the system. Starting with the Maxwell-Bloch equations, we handle quantum and thermal noise by introducing random currents whose correlations are given by the fluctuation-dissipation theorem. We derive coupled-mode equations for the lasing-mode amplitudes and obtain a formula for the linewidths in terms of simple integrals over the steady-state lasing modes.
Carrier multiplication in silicon nanocrystals: ab initio results
Ossicini, Stefano
2015-01-01
Summary One of the most important goals in the field of renewable energy is the development of original solar cell schemes employing new materials to overcome the performance limitations of traditional solar cell devices. Among such innovative materials, nanostructures have emerged as an important class of materials that can be used to realize efficient photovoltaic devices. When these systems are implemented into solar cells, new effects can be exploited to maximize the harvest of solar radiation and to minimize the loss factors. In this context, carrier multiplication seems one promising way to minimize the effects induced by thermalization loss processes thereby significantly increasing the solar cell power conversion. In this work we analyze and quantify different types of carrier multiplication decay dynamics by analyzing systems of isolated and coupled silicon nanocrystals. The effects on carrier multiplication dynamics by energy and charge transfer processes are also discussed. PMID:25821673
Quarti, Claudio; Mosconi, Edoardo; De Angelis, Filippo
2015-04-14
The last two years have seen the unprecedentedly rapid emergence of a new class of solar cells, based on hybrid organic-inorganic halide perovskites. The success of this class of materials is due to their outstanding photoelectrochemical properties coupled to their low cost, mainly solution-based, fabrication techniques. Solution processed materials are however often characterized by an inherent flexible structure, which is hardly mapped into a single local minimum energy structure. In this perspective, we report on the interplay between structural and electronic properties of hybrid lead iodide perovskites investigated using ab initio molecular dynamics (AIMD) simulations, which allow the dynamical simulation of disordered systems at finite temperature. We compare the prototypical MAPbI3 (MA = methylammonium) perovskite in its cubic and tetragonal structure with the trigonal phase of FAPbI3 (FA = formamidinium), investigating different starting arrangements of the organic cations. Despite the relatively short time scale amenable to AIMD, typically a few tens of ps, this analysis demonstrates the sizable structural flexibility of this class of materials, showing that the instantaneous structure could significantly differ from the time and thermal averaged structure. We also highlight the importance of the organic-inorganic interactions in determining the fluxional properties of this class of materials. A peculiar spatial localization of the valence and conduction band edges is also found, with a dynamics in the range of 0.1 ps, which is associated with the positional dynamics of the organic cations within the cubo-octahedral perovskite cage. This asymmetry in the spatial localization of the band edges is expected to ease exciton dissociation and assist the initial stages of charge separation, possibly constituting one of the key factors for the impressive photovoltaic performances of hybrid lead-iodide perovskites. PMID:25766785
Ab initio studies on the spin-forbidden cooling transitions of the LiRb molecule.
You, Yang; Yang, Chuan-Lu; Zhang, Qing-Qing; Wang, Mei-Shan; Ma, Xiao-Guang; Liu, Wen-Wang
2016-07-20
The spin-forbidden cooling of the LiRb molecule is investigated based on ab initio quantum chemistry calculations. The multireference configuration interaction method is used to generate the potential energy curves (PECs) of the ground state X(1)Σ(+) and the low-lying excited states a(3)Σ(+), B(1)Π, and b(3)Π. The spin-orbit coupling effects for the PECs and the transition dipole moments (TDMs) between the X(1)Σ(+), b(3)Π and a(3)Σ(+) states are also calculated. The analytical functions for the PECs are deduced. The rovibrational energy levels, the spectroscopic parameters and the Franck-Condon factors (FCF) are determined by solving the Schrödinger equation of nuclear movement with the obtained analytical functions. The b(3)Π0 ↔ X(1)Σ(+) and b(3)Π1 ↔ X(1)Σ(+) transitions have highly diagonal distributed FCFs and non-zero TDMs, demonstrating that the LiRb molecule could be a very promising candidate for laser cooling. Therefore, a three-cycle laser cooling scheme for the molecule has been proposed based on these two spin-forbidden transitions. Using the radiative lifetime and linewidth calculated from the obtained TDM functions, we present further analysis of the cooling of LiRb and the corresponding KRb molecule. The transition b(3)Π0 ↔ X(1)Σ(+) is found to be a practical transition to cool the LiRb molecule, and a sub-microkelvin cool temperature could be reached for the KRb molecule using a similar laser cooling scheme. PMID:27388722
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.
The In-Medium Similarity Renormalization Group: A novel ab initio method for nuclei
NASA Astrophysics Data System (ADS)
Hergert, H.; Bogner, S. K.; Morris, T. D.; Schwenk, A.; Tsukiyama, K.
2016-03-01
We present a comprehensive review of the In-Medium Similarity Renormalization Group (IM-SRG), a novel ab initio method for nuclei. The IM-SRG employs a continuous unitary transformation of the many-body Hamiltonian to decouple the ground state from all excitations, thereby solving the many-body problem. Starting from a pedagogical introduction of the underlying concepts, the IM-SRG flow equations are developed for systems with and without explicit spherical symmetry. We study different IM-SRG generators that achieve the desired decoupling, and how they affect the details of the IM-SRG flow. Based on calculations of closed-shell nuclei, we assess possible truncations for closing the system of flow equations in practical applications, as well as choices of the reference state. We discuss the issue of center-of-mass factorization and demonstrate that the IM-SRG ground-state wave function exhibits an approximate decoupling of intrinsic and center-of-mass degrees of freedom, similar to Coupled Cluster (CC) wave functions. To put the IM-SRG in context with other many-body methods, in particular many-body perturbation theory and non-perturbative approaches like CC, a detailed perturbative analysis of the IM-SRG flow equations is carried out. We conclude with a discussion of ongoing developments, including IM-SRG calculations with three-nucleon forces, the multi-reference IM-SRG for open-shell nuclei, first non-perturbative derivations of shell-model interactions, and the consistent evolution of operators in the IM-SRG. We dedicate this review to the memory of Gerry Brown, one of the pioneers of many-body calculations of nuclei.
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.
An Ab Initio Approach Towards Engineering Fischer-Tropsch Surface Chemistry
Matthew Neurock; Siddharth Chopra
2003-09-11
As the US seeks to develop an energy strategy that reduces the reliance on foreign oil, there is a renewed interest in the research and development of the Fischer Tropsch synthesis for converting syngas into long chain hydrocarbon products. This report investigates some of the basic elementary steps for Fischer-Tropsch synthesis over ideal Pt, Ru and carbon-covered Pt and Ru metal surfaces by using ab initio density functional theoretical calculations. We examine in detail the adsorption sites as well as the binding energies for C, CH, CH{sub 2}, CH3 and CH4 on Pt(111), Ru(0001), 2x2-C-Pt(111) and 2x2-C-Ru(0001). The results indicate that the binding energies increase with decreasing the hydrogen in the fragment molecule, i.e. CH{sub 4} < CH{sub 3} < CH{sub 2} < CH < C. More specifically the work analyzes the elementary steps involved in the activation of methane. This is simply the reverse set of steps necessary for the hydrogenation of C to CH{sub 4}. The results indicate that these hydrocarbon intermediates bind more strongly to Ru than Pt. The introduction of co-adsorbed carbon atoms onto both Ru(0001) as well as Pt(111) significantly increased the overall energies as well as the activation barriers for C-H bond activation. The results suggest that Ru may be so active that it initially can initially activate CH4 into CH or C but ultimately it dies because the CH and C intermediates poison the surface and thus kill its activity. Methane can dissociate on Pt but subsequent hydrocarbon coupling reactions act to remove the surface carbon.
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
Ab initio quantum Monte Carlo simulations of the uniform electron gas without fixed nodes
NASA Astrophysics Data System (ADS)
Groth, S.; Schoof, T.; Dornheim, T.; Bonitz, M.
2016-02-01
The uniform electron gas (UEG) at finite temperature is of key relevance for many applications in the warm dense matter regime, e.g., dense plasmas and laser excited solids. Also, the quality of density functional theory calculations crucially relies on the availability of accurate data for the exchange-correlation energy. Recently, results for N =33 spin-polarized electrons at high density, rs=r ¯/aB≲4 , and low temperature have been obtained with the configuration path integral Monte Carlo (CPIMC) method [T. Schoof et al., Phys. Rev. Lett. 115, 130402 (2015), 10.1103/PhysRevLett.115.130402]. To achieve these results, the original CPIMC algorithm [T. Schoof et al., Contrib. Plasma Phys. 51, 687 (2011), 10.1002/ctpp.201100012] had to be further optimized to cope with the fermion sign problem (FSP). It is the purpose of this paper to give detailed information on the manifestation of the FSP in CPIMC simulations of the UEG and to demonstrate how it can be turned into a controllable convergence problem. In addition, we present new thermodynamic results for higher temperatures. Finally, to overcome the limitations of CPIMC towards strong coupling, we invoke an independent method—the recently developed permutation blocking path integral Monte Carlo approach [T. Dornheim et al., J. Chem. Phys. 143, 204101 (2015), 10.1063/1.4936145]. The combination of both approaches is able to yield ab initio data for the UEG over the entire density range, above a temperature of about one half of the Fermi temperature. Comparison with restricted path integral Monte Carlo data [E. W. Brown et al., Phys. Rev. Lett. 110, 146405 (2013), 10.1103/PhysRevLett.110.146405] allows us to quantify the systematic error arising from the free particle nodes.
Accurate ab initio Quartic Force Fields of Cyclic and Bent HC2N Isomers
NASA Technical Reports Server (NTRS)
Inostroza, Natalia; Huang, Xinchuan; Lee, Timothy J.
2012-01-01
Highly correlated ab initio quartic force field (QFFs) are used to calculate the equilibrium structures and predict the spectroscopic parameters of three HC2N isomers. Specifically, the ground state quasilinear triplet and the lowest cyclic and bent singlet isomers are included in the present study. Extensive treatment of correlation effects were included using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T). Dunning s correlation-consistent basis sets cc-pVXZ, X=3,4,5, were used, and a three-point formula for extrapolation to the one-particle basis set limit was used. Core-correlation and scalar relativistic corrections were also included to yield highly accurate QFFs. The QFFs were used together with second-order perturbation theory (with proper treatment of Fermi resonances) and variational methods to solve the nuclear Schr dinger equation. The quasilinear nature of the triplet isomer is problematic, and it is concluded that a QFF is not adequate to describe properly all of the fundamental vibrational frequencies and spectroscopic constants (though some constants not dependent on the bending motion are well reproduced by perturbation theory). On the other hand, this procedure (a QFF together with either perturbation theory or variational methods) leads to highly accurate fundamental vibrational frequencies and spectroscopic constants for the cyclic and bent singlet isomers of HC2N. All three isomers possess significant dipole moments, 3.05D, 3.06D, and 1.71D, for the quasilinear triplet, the cyclic singlet, and the bent singlet isomers, respectively. It is concluded that the spectroscopic constants determined for the cyclic and bent singlet isomers are the most accurate available, and it is hoped that these will be useful in the interpretation of high-resolution astronomical observations or laboratory experiments.
Ab initio modeling of 2D layered organohalide lead perovskites.
Fraccarollo, Alberto; Cantatore, Valentina; Boschetto, Gabriele; Marchese, Leonardo; Cossi, Maurizio
2016-04-28
A number of 2D layered perovskites A2PbI4 and BPbI4, with A and B mono- and divalent ammonium and imidazolium cations, have been modeled with different theoretical methods. The periodic structures have been optimized (both in monoclinic and in triclinic systems, corresponding to eclipsed and staggered arrangements of the inorganic layers) at the DFT level, with hybrid functionals, Gaussian-type orbitals and dispersion energy corrections. With the same methods, the various contributions to the solid stabilization energy have been discussed, separating electrostatic and dispersion energies, organic-organic intralayer interactions and H-bonding effects, when applicable. Then the electronic band gaps have been computed with plane waves, at the DFT level with scalar and full relativistic potentials, and including the correlation energy through the GW approximation. Spin orbit coupling and GW effects have been combined in an additive scheme, validated by comparing the computed gap with well known experimental and theoretical results for a model system. Finally, various contributions to the computed band gaps have been discussed on some of the studied systems, by varying some geometrical parameters and by substituting one cation in another's place. PMID:27131557
Ab initio modeling of 2D layered organohalide lead perovskites
NASA Astrophysics Data System (ADS)
Fraccarollo, Alberto; Cantatore, Valentina; Boschetto, Gabriele; Marchese, Leonardo; Cossi, Maurizio
2016-04-01
A number of 2D layered perovskites A2PbI4 and BPbI4, with A and B mono- and divalent ammonium and imidazolium cations, have been modeled with different theoretical methods. The periodic structures have been optimized (both in monoclinic and in triclinic systems, corresponding to eclipsed and staggered arrangements of the inorganic layers) at the DFT level, with hybrid functionals, Gaussian-type orbitals and dispersion energy corrections. With the same methods, the various contributions to the solid stabilization energy have been discussed, separating electrostatic and dispersion energies, organic-organic intralayer interactions and H-bonding effects, when applicable. Then the electronic band gaps have been computed with plane waves, at the DFT level with scalar and full relativistic potentials, and including the correlation energy through the GW approximation. Spin orbit coupling and GW effects have been combined in an additive scheme, validated by comparing the computed gap with well known experimental and theoretical results for a model system. Finally, various contributions to the computed band gaps have been discussed on some of the studied systems, by varying some geometrical parameters and by substituting one cation in another's place.
NASA Astrophysics Data System (ADS)
Cremer, Dieter; Dorofeeva, Olga V.; Mastryukov, Vladimir S.
1981-09-01
Restricted Hartree—Fock calculations on 21 planar and puckered conformers of azetidine have been done employing a split valence basis augmented by d functions. Complete geometry optimizations have been performed for eight conformers. In this way the puckering potential of azetidine is explored over the range -40° < ø (puckering angle) < 40°, for both sp3 and sp2 hybridization of the nitrogen atom. In its equatorial form, azetidine is slightly more puckered than cyclobutane. This is because of a decrease of van der Waals' repulsion between H atoms. Charge effects lead to destabilization of the axial forms. There is only moderate coupling between puckering and methylene group rocking. Previously published electron diffraction (ED) data are reinvestigated using vibrational corrections and information from the ab initio calculations. On the basis of this MO constrained ED (MOCED) analysis a puckering angle φ = 35.1(1.8)° is found. Observed rg and re bond distances are compared with ab initio values.
NASA Astrophysics Data System (ADS)
Jakubek, Z. J.; Bunker, P. R.; Zachwieja, M.; Nakhate, S. G.; Simard, B.; Yurchenko, S. N.; Thiel, W.; Jensen, Per
2006-03-01
In this work, the X˜B12 and ÃA12 electronic states of the phosphino (PH2) free radical have been studied by dispersed fluorescence and ab initio methods. PH2 molecules were produced in a molecular free-jet apparatus by laser vaporizing a silicon rod in the presence of phosphine (PH3) gas diluted in helium. The laser-induced fluorescence, from the excited ÃA12 electronic state down to the ground electronic state, was dispersed and analyzed. Ten (υ1υ2υ3) vibrationally excited levels of the ground electronic state, with υ1⩽2, υ2⩽6, and υ3=0, have been observed. Ab initio potential-energy surfaces for the X˜B12 and ÃA12 electronic states have been calculated at 210 points. These two states correlate with a Πu2 state at linearity and they interact by the Renner-Teller coupling and spin-orbit coupling. Using the ab initio potential-energy surfaces with our RENNER computer program system, the vibronic structure and relative intensities of the ÃA12→X˜B12 emission band system have been calculated in order to corroborate the experimental assignments.
Multifunctional properties of zinc oxide: An ab-initio study
NASA Astrophysics Data System (ADS)
Gopal, Priya
In this dissertation, we present our results of first-principles calculations of the electronic and magnetic properties of a range of ZnO-based systems. ZnO is a II-VI wide band gap semiconductor, is a strong piezoelectric, is transparent and can be easily fabricated in a variety of microstructurures from bulk single crystals to thin films to nanostructures. Recent reports also suggest the possibility of magnetic ZnO as a potential spintronic material. Due to its range of properties, ZnO has numerous applications including varistors, piezoelectric transducers, sonars, acoustic devices, solar cells, UV blocking, chemical catalysts and transparent conducting oxides. In this work, we explore the following "multifunctional aspects" of ZnO-based systems. First we calculate the structural, polarization and piezoelectric properties of wurtzite-structure ZnO, MgO and CdO (used in band gap engineering of ZnO) to explore the relationship between optical and piezoelectric functionalities. Second, we investigate wurtzite-structure MnO to understand coexisting magnetic and piezoelectric functionalities. Finally, we study the nature of magnetic interactions in ZnO doped with 3d transition metals to explore the simultaneous magnetic, semiconducting and piezoelectric properties. Our main findings are: (i) Polarization differences between MgO and ZnO, and CdO and ZnO are large suggesting that strong polarization gradients will accompany the changes in band gap induced by Cd or Mg doping. (ii) wurtzite-structure MnO is antiferromagnetic, with a strong magnetostructural coupling and piezoelectric response; this indicates the possibility of coexistence of magnetism and piezoelectricity. (iii) In transition-metal doped ZnO we do not observe a room-temperature ferromagnetic interaction. We find that the magnetic ground state is highly sensitive to details of the calculations such as k-point mesh, details of the geometry optimizations and the choice of exchange correlation functional.
Organic/inorganic hybrid materials: challenges for ab initio methodology.
Draxl, Claudia; Nabok, Dmitrii; Hannewald, Karsten
2014-11-18
terms of interfacial electronic structure. We face the problem of a so far hidden variable, namely, electron-vibrational coupling, regarding level alignment at interfaces between organic and inorganic semiconductors. Poly(para-phenylene) adsorbed on graphene and encapsulated in carbon nanotubes represent case studies to demonstrate the impact of polarization effects and exciton delocalization in optoelectronic excitations, respectively. Polaron-induced band narrowing and its consequences for charge transport in organic crystals is exemplified for the HOMO bandwidth in naphthalene crystals. On the basis of these prototypical systems, we discuss what is missing to reach predictive power on a quantitative level for organic/inorganic hybrid materials and, thus, open a perspective toward the computational discovery of new materials for optoelectronic applications. PMID:25171272
NASA Astrophysics Data System (ADS)
Subotnik, Joseph
In this talk, I will give a broad overview of our work in nonadiabatic dynamics, i.e. the dynamics of strongly coupled nuclear-electronic motion whereby the relaxation of a photo-excited electron leads to the heating up of phonons. I will briefly discuss how to model such nuclear motion beyond mean field theory. Armed with the proper framework, I will then focus on how to calculate one flavor of electron-phonon couplings, known as derivative couplings in the chemical literature. Derivative couplings are the matrix elements that couple adiabatic electronic states within the Born-Oppenheimer treatment, and I will show that these matrix elements show spurious poles using formal (frequency-independent) time-dependent density functional theory. To correct this TD-DFT failure, a simple approximation will be proposed and evaluated. Finally, time permitting, I will show some ab initio calculations whereby one can use TD-DFT derivative couplings to study electronic relaxation through a conical intersection.
Ab initio pressure-dependent reaction kinetics of methyl propanoate radicals.
Tan, Ting; Yang, Xueliang; Ju, Yiguang; Carter, Emily A
2015-12-14
The unimolecular dissociation and isomerization kinetics of the three methyl propanoate (MP) radicals, CH3CH2C(=O)OĊH2 (MP-m), CH3ĊHC(=O)OCH3 (MP-α), and ĊH2CH2C(=O)OCH3 (MP-β), are theoretically investigated using high-level ab initio methods and the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) theory. Stationary-point energies are obtained using the coupled cluster singles and doubles with perturbative triples correction (CCSD(T)), multi-reference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and multi-reference averaged coupled pair functional (MRACPF2) theories. The isomerization barriers between the three radicals are predicted to be generally lower than the corresponding bond dissociation channels, leading to a strongly coupled reaction system in subsequent kinetics studies. The phenomenological temperature- and pressure-dependent rate coefficients are computed using the RRKM/ME theory over a temperature range of 500 to 2000 K and at a pressure range of 0.01 atm to the high-pressure limit, which are then fitted to modified Arrhenius expressions. The β-scission rate coefficients of MP-α to CH3CHC(=O) and CH3O[combining dot above] are predicted to be the smallest because of its highest activation energy among all studied unimolecular reactions channels. Analysis of branching fractions shows that both MP-m and MP-α radicals mainly decompose to the bimolecular products CH3CH2Ċ(=O) and H2CO, whereas the MP-β radical primarily decomposes via cleavage of a C-C bond to form C2H4 and CH3OĊ(=O). The isomerization channels dominate at low temperatures, the branching fractions of which decrease with increasing temperature and become very minor at about 2000 K. Our accurate rate coefficients and branching fractions help to illuminate the unique combustion properties of MP. PMID:26536041
NASA Technical Reports Server (NTRS)
Lee, Timothy J.; Rice, Julia E.
1992-01-01
The equilibrium structures, harmonic vibrational frequencies, IR intensities, and relative energetics of HNO3 and its protonated form H2NO3+ were investigated using double-zeta plus polarization and triple-zeta plus polarization basis sets in conjunction with high-level ab initio methods. The latter include second-order Moller-Plesset perturbation theory, the single and double excitation coupled cluster (CCSD) methods, a perturbational estimate of the effects of connected triple excitations (CCSD(T)), and the self-consistent field. To determine accurate energy differences CCSD(T) energies were computed using large atomic natural orbital basis sets. Four different isomers of H2NO3+ were considered. The lowest energy form of protonated nitric acid was found to correspond to a complex between H2O and NO2+, which is consistent with earlier theoretical and experimental studies.
NASA Astrophysics Data System (ADS)
Fiuza, S. M.; Van Besien, E.; Milhazes, N.; Borges, F.; Marques, M. P. M.
2004-05-01
A conformational analysis of 3-(3,4,5-trihydroxyphenyl)-2-propenoic acid (3,4,5-trihydroxycinnamic acid, THPPE), a trihydroxylated cinnamic acid analogous to caffeic acid (a natural compound often present in diet), was carried out by Raman spectroscopy coupled to Ab initio MO calculations. Apart from the optimised geometrical parameters for the most stable conformers of this compound, and for one of its dimeric species, the corresponding harmonic vibrational frequencies, as well as potential-energy profiles for rotation around several bonds within the molecule, were obtained. Twenty one distinct conformers were found for THPPE, the lowest energy ones—THPPE 1 and THPPE 2—displaying a completely planar geometry. The conformational preferences of this system were thus found to be mainly ruled by the stabilising effect of π-electron delocalisation. At the light of these results, a complete assignment of the corresponding solid state Raman spectra was performed.
Ab initio theory of magnetic-field-induced odd-frequency two-band superconductivity in MgB2
NASA Astrophysics Data System (ADS)
Aperis, Alex; Maldonado, Pablo; Oppeneer, Peter M.
2015-08-01
We develop the anisotropic Eliashberg framework for superconductivity in the presence of an applied magnetic field. Using as input the ab initio calculated electron and phonon band structures and electron-phonon coupling, we solve self-consistently the anisotropic Eliashberg equations for the archetypal superconductor MgB2. We find two self-consistent solutions, time-even two-band superconductivity, as well as unconventional time-odd s -wave spin triplet two-band superconductivity emerging with applied field. We provide the full momentum, frequency, and spin-resolved dependence and magnetic field-temperature phase diagrams of the time-even and time-odd superconducting pair amplitudes and predict fingerprints of this novel odd-frequency state in tunneling experiments.
Ab initio study of charge-transfer dynamics in collisions of C{sup 2+} ions with hydrogen chloride
Rozsalyi, E.; Vibok, A.; Bene, E.; Halasz, G. J.; Bacchus-Montabonel, M. C.
2011-05-15
Ab initio quantum chemistry molecular calculations followed by a semiclassical dynamical treatment in the keV collision energy range have been developed for the study of the charge-transfer process in collisions of C{sup 2+} ions with hydrogen chloride. The mechanism has been investigated in detail in connection with avoided crossings between states involved in the reaction. A simple mechanism driven by a strong nonadiabatic coupling matrix element has been pointed out for this process. A comparative analysis with the halogen fluoride target corresponding to a similar electronic configuration shows a quite different charge-transfer mechanism leading to a very different behavior of the cross sections. Such behavior may be correlated to specific nonadiabatic interactions observed in these collision systems.
Waldrop, Jonathan M; Song, Bo; Patkowski, Konrad; Wang, Xiaopo
2015-05-28
A new highly accurate potential energy curve for the krypton dimer was constructed using coupled-cluster calculations up to the singles, doubles, triples, and perturbative quadruples level, including corrections for core-core and core-valence correlation and for relativistic effects. The ab initio data points were fitted to an analytic potential which was used to compute the most important transport properties of the krypton gas. The viscosity, thermal conductivity, self-diffusion coefficient, and thermal diffusion factor were calculated by the kinetic theory at low density and temperatures from 116 to 5000 K. The comparisons with literature experimental data as well as with values from other pair potentials indicate that our new potential is superior to all previous ones. The transport property values computed in this work are recommended as standard values over the complete temperature range. PMID:26026447
NASA Astrophysics Data System (ADS)
Sousa, J. B.; Calheiros, R.; Rio, V.; Borges, F.; Marques, M. P. M.
2006-02-01
A conformational analysis of ethyl 3-(3,4,5-trihydroxyphenyl)-2-propenoate (ethyl 3,4,5-trihydroxycinnamate, ETHPPE), a polyphenolic cinnamic ester which displays antiproliferative activity towards human adenocarcinoma cells, was carried out by Raman spectroscopy coupled to ab initio MO calculations. Apart from the optimised geometrical parameters for the most stable conformations of this compound (both for the trans and cis isomers), the corresponding harmonic vibrational frequencies were obtained. Eighteen distinct geometries were found, 12 for the lowest energy trans isomer and six for the cis species. The conformational preferences of this system were verified to be mainly ruled by the stabilising effect of π-electron delocalisation, a planar geometry being favoured. The orientation of the ester moiety showed to be the most determinant factor for the overall stability of the molecule. In the light of these results, a complete assignment of the corresponding Raman pattern was performed.
Koukaras, Emmanuel N; Zdetsis, Aristides D; Sigalas, Michael M
2012-09-26
On the basis of the attractive possibility of efficient hydrogen storage in light metal hydrides, we have examined a large variety of Mg(n)H(m) nanoclusters and (MgH(2))(n) nanocrystals (n = 2-216, m = 2-436) using high level coupled cluster, CCSD(T), ab initio methods, and judicially chosen density functional calculations of comparable quality and (near chemical) accuracy. Our calculated desorption energies as a function of size and percentage of hydrogen have pinpointed optimal regions of sizes and concentrations of hydrogen which are in full agreement with recent experimental findings. Furthermore, our results reproduce the experimental desorption energy of 75.5 kJ/mol for the infinite system with remarkable accuracy (76.5 ± 1.5 kJ/mol). PMID:22920498
NASA Technical Reports Server (NTRS)
Lee, Timothy J.; Rice, Julia E.
1993-01-01
Ab initio quantum mechanical methods, including coupled-cluster theory, are used to determine the equilibrium geometries, dipole moments, and harmonic vibrational frequencies of ClONO2, NO2(+), and four isomers of protonated ClONO2. It was found that, for the equilibrium structures and harmonic frequencies of ClONO2, HOCl, and NO2(+), the highest-level theoretical predictions are consistent with the available experimental information concerning the reactions of ClONO2 and HOCl with HCl on the surface of polar stratospheric clouds (PSCs). The study supports a recent hypothesis that the reaction of ClONO2 on the surface of PSCs is proton catalyzed, although the mechanism is different.
Wang, Zhongquan; Feng, Eryin; Yu, Haijun; Zhang, Chunzao; Du, Jianming
2011-01-01
A new high quality three-dimensional potential energy surface for the Ne–CO van der Waals complex is developed using the CCSD(T) method and avqz/avqz+33221 basis set. The ab initio calculation is performed in a total of 1365 configurations with supermolecule method. There is a single global minimum located in a nearly T-shaped geometry. The global minimum energy is −49.4090 cm−1 at Re =6.40a0 and θe =82.5° for V00 . Using the three-dimensional potential energy surface, we have calculated bound rovibrational energy levels up to J = 10 including the Coriolis coupling terms. Compared with the experimental transition frequencies, the theoretical results are in good agreement with the experimental results. PMID:21241112
NASA Astrophysics Data System (ADS)
Roy, Sourav; Majumder, Sonjoy
2015-07-01
Isotope shifts (ISs) of D1(n s S1/22 →n p P1/22 ) and D2(n s S1/22 →n p P3/22 ) transitions for the first three elements of the K isoelectronic sequence are calculated with all-order correlated relativistic coupled-cluster theory. To get precise ab initio results, all the orbitals of the reference states have been optimized by a linear regression technique. Some of the important IS estimations reported here were not available in literature. Interesting features of odd-even staggering and magic neutron number effects are also observed in the case of volume shift calculations. Energy-level corrections for a few levels due to a change in the nuclear model from a point nucleus to a Fermi nucleus have been studied with interesting correlation features and compared with other theoretical results available in literature.
NASA Astrophysics Data System (ADS)
Jorissen, Kevin; Story, Shauna; Rehr, John
2014-03-01
Accurate calculations of x-ray absorption spectra (XAS) often require linking several materials science codes. To reduce the complexity and support the hardware requirements of such calculations, we have virtualized XAS modeling workflows using a Cloud-based approach, with interfacing and configuration of codes handled by developers, and virtual HPC resources allocated on demand. When coupled to user-friendly GUIs this puts powerful multi-code simulations in the hands of general users. For instance, FEFF users can improve XAS interpretation and analysis using accurate ab initio Debye-Waller factors and self energy from the ABINIT DFT code, rather than semi-empirical models. Additionally, such workflows allow robust automation of large-scale calculation sets such as the Materials Project where our approach could enable a theoretical spectroscopy database of many thousands of structures for systematic study of materials. Supported by NSF-1216716.
NASA Astrophysics Data System (ADS)
Pati, Ranjit; Karna, Shashi P.
2002-01-01
The dependence of electron transfer (ET) coupling element, VAB, on the length of rigid-rod-like systems consisting of bicyclo[1.1.1]pentane (BCP), cubane (CUB), and bicyclo[2.2.2]octane (BCO) monomers, has been investigated with the use of ab initio Hartree-Fock (HF) method employing Marcus-Hush two-state (TS) model. The value of VAB decreases exponentially with increase in the number of the cage units of the σ-bonded molecules. The calculated decay constant, β, shows good agreement with previously reported data. For molecular length⩾15 Å, the value of VAB becomes negligibly small, suggesting complete suppression of the through bond direct tunneling contribution to ET process.
NASA Astrophysics Data System (ADS)
Faghaninia, Alireza; Ager, Joel W.; Lo, Cynthia S.
2015-06-01
Accurate models of carrier transport are essential for describing the electronic properties of semiconductor materials. To the best of our knowledge, the current models following the framework of the Boltzmann transport equation (BTE) either rely heavily on experimental data (i.e., semiempirical), or utilize simplifying assumptions, such as the constant relaxation time approximation (BTE-cRTA). While these models offer valuable physical insights and accurate calculations of transport properties in some cases, they often lack sufficient accuracy—particularly in capturing the correct trends with temperature and carrier concentration. We present here a transport model for calculating low-field electrical drift mobility and Seebeck coefficient of n -type semiconductors, by explicitly considering relevant physical phenomena (i.e., elastic and inelastic scattering mechanisms). We first rewrite expressions for the rates of elastic scattering mechanisms, in terms of ab initio properties, such as the band structure, density of states, and polar optical phonon frequency. We then solve the linear BTE to obtain the perturbation to the electron distribution—resulting from the dominant scattering mechanisms—and use this to calculate the overall mobility and Seebeck coefficient. Therefore, we have developed an ab initio model for calculating mobility and Seebeck coefficient using the Boltzmann transport (aMoBT) equation. Using aMoBT, we accurately calculate electrical transport properties of the compound n -type semiconductors, GaAs and InN, over various ranges of temperature and carrier concentration. aMoBT is fully predictive and provides high accuracy when compared to experimental measurements on both GaAs and InN, and vastly outperforms both semiempirical models and the BTE-cRTA. Therefore, we assert that this approach represents a first step towards a fully ab initio carrier transport model that is valid in all compound semiconductors.
Ho, T.; Rabitz, H.
1996-02-01
A general interpolation method for constructing smooth molecular potential energy surfaces (PES{close_quote}s) from {ital ab} {ital initio} data are proposed within the framework of the reproducing kernel Hilbert space and the inverse problem theory. The general expression for an {ital a} {ital posteriori} error bound of the constructed PES is derived. It is shown that the method yields globally smooth potential energy surfaces that are continuous and possess derivatives up to second order or higher. Moreover, the method is amenable to correct symmetry properties and asymptotic behavior of the molecular system. Finally, the method is generic and can be easily extended from low dimensional problems involving two and three atoms to high dimensional problems involving four or more atoms. Basic properties of the method are illustrated by the construction of a one-dimensional potential energy curve of the He{endash}He van der Waals dimer using the exact quantum Monte Carlo calculations of Anderson {ital et} {ital al}. [J. Chem. Phys. {bold 99}, 345 (1993)], a two-dimensional potential energy surface of the HeCO van der Waals molecule using recent {ital ab} {ital initio} calculations by Tao {ital et} {ital al}. [J. Chem. Phys. {bold 101}, 8680 (1994)], and a three-dimensional potential energy surface of the H{sup +}{sub 3} molecular ion using highly accurate {ital ab} {ital initio} calculations of R{umlt o}hse {ital et} {ital al}. [J. Chem. Phys. {bold 101}, 2231 (1994)]. In the first two cases the constructed potentials clearly exhibit the correct asymptotic forms, while in the last case the constructed potential energy surface is in excellent agreement with that constructed by R{umlt o}hse {ital et} {ital al}. using a low order polynomial fitting procedure. {copyright} {ital 1996 American Institute of Physics.}
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).
Velaga, Srinath C; Anderson, Brian J
2014-01-16
Gas hydrate deposits are receiving increased attention as potential locations for CO2 sequestration, with CO2 replacing the methane that is recovered as an energy source. In this scenario, it is very important to correctly characterize the cage occupancies of CO2 to correctly assess the sequestration potential as well as the methane recoverability. In order to predict accurate cage occupancies, the guest–host interaction potential must be represented properly. Earlier, these potential parameters were obtained by fitting to experimental equilibrium data and these fitted parameters do not match with those obtained by second virial coefficient or gas viscosity data. Ab initio quantum mechanical calculations provide an independent means to directly obtain accurate intermolecular potentials. A potential energy surface (PES) between H2O and CO2 was computed at the MP2/aug-cc-pVTZ level and corrected for basis set superposition error (BSSE), an error caused due to the lower basis set, by using the half counterpoise method. Intermolecular potentials were obtained by fitting Exponential-6 and Lennard-Jones 6-12 models to the ab initio PES, correcting for many-body interactions. We denoted this model as the “VAS” model. Reference parameters for structure I carbon dioxide hydrate were calculated using the VAS model (site–site ab initio intermolecular potentials) as Δμ(w)(0) = 1206 ± 2 J/mol and ΔH(w)(0) = 1260 ± 12 J/mol. With these reference parameters and the VAS model, pure CO2 hydrate equilibrium pressure was predicted with an average absolute deviation of less than 3.2% from the experimental data. Predictions of the small cage occupancy ranged from 32 to 51%, and the large cage is more than 98% occupied. The intermolecular potentials were also tested by calculating the pure CO2 density and diffusion of CO2 in water using molecular dynamics simulations. PMID:24328234
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.
Ab initio potentials of F+Li2 accessible at ultracold temperatures
NASA Astrophysics Data System (ADS)
Wright, K. W. A.; Lane, Ian C.
2010-09-01
Ab initio calculations for the strongly exoergic Li2+F harpoon reaction are presented using density-functional theory, complete active space self-consistent field, and multireference configuration interaction methods to argue that this reaction would be an ideal candidate for investigation with ultracold molecules. The lowest six states are calculated with the aug-correlation-consistent polarized valence triple-zeta basis set and at least two can be accessed by a ground rovibronic Li2 molecule with zero collision energy at all reaction geometries. The large reactive cross section (characteristic of harpoon reactions) and chemiluminescent products are additional attractive features of these reactions.
Temperature-Dependent Diffusion Coefficients from ab initio Computations: Hydrogen in Nickel
E Wimmer; W Wolf; J Sticht; P Saxe; C Geller; R Najafabadi; G Young
2006-03-16
The temperature-dependent mass diffusion coefficient is computed using transition state theory. Ab initio supercell phonon calculations of the entire system provide the attempt frequency, the activation enthalpy, and the activation entropy as a function of temperature. Effects due to thermal lattice expansion are included and found to be significant. Numerical results for the case of hydrogen in nickel demonstrate a strong temperature dependence of the migration enthalpy and entropy. Trapping in local minima along the diffusion path has a pronounced effect especially at low temperatures. The computed diffusion coefficients with and without trapping bracket the available experimental values over the entire temperature range between 0 and 1400 K.
Converging sequences in the ab initio no-core shell model
Forssen, C.; Vary, J. P.; Caurier, E.; Navratil, P.
2008-02-15
We demonstrate the existence of multiple converging sequences in the ab initio no-core shell model. By examining the underlying theory of effective operators, we expose the physical foundations for the alternative pathways to convergence. This leads us to propose a revised strategy for evaluating effective interactions for A-body calculations in restricted model spaces. We suggest that this strategy is particularly useful for applications to nuclear processes in which states of both parities are used simultaneously, such as for transition rates. We demonstrate the utility of our strategy with large-scale calculations in light nuclei.
Ab initio prediction of protein structure with both all-atom and simplified force fields
NASA Astrophysics Data System (ADS)
Scheraga, Harold
2004-03-01
Using only a physics-based ab initio method, and both all-atom (ECEPP/3) and simplified united-residue (UNRES) force fields, global optimization of both potential functions with Monte Carlo-plus-Minimization (MCM) and Conformational Space Annealing (CSA), respectively, provides predicted structures of proteins without use of knowledge-based information. The all-atom approach has been applied to the 46-residue protein A, and the UNRES approach has been applied to larger CASP targets. The predicted structures will be described.
Debela, T. T.; Wang, X. D.; Cao, Q. P.; Zhang, D. X.; Wang, S. Y.; Wang, Cai-Zhuang; Jiang, J. Z.
2013-12-12
Atomic structure transitions of liquid niobium during solidification, at different temperatures from 3200 to 1500 K, were studied by using ab initio molecular dynamics simulations. The local atomic structure variations with temperature are investigated by using the pair-correlation function, the structure factor, the bond-angle distribution function, the Honeycutt–Anderson index, Voronoi tessellation and the cluster alignment methods. Our results clearly show that, upon quenching, the icosahedral short-range order dominates in the stable liquid and supercooled liquid states before the system transforms to crystalline body-center cubic phase at a temperature of about 1830 K.
Simple synthesis, structure and ab initio study of 1,4-benzodiazepine-2,5-diones
NASA Astrophysics Data System (ADS)
Jadidi, Khosrow; Aryan, Reza; Mehrdad, Morteza; Lügger, Thomas; Ekkehardt Hahn, F.; Ng, Seik Weng
2004-04-01
A simple procedure for the synthesis of pyrido[2,1-c][1,4] benzodiazepine-6,12-dione ( 1) and 1,4-benzodiazepine-2,5-diones ( 2a- 2d), using microwave irradiation and/or conventional heating is reported. The configuration of 1 was determined by single-crystal X-ray diffraction. A detailed ab initio B3LYP/6-31G* calculation of structural parameters and substituent effects on ring inversion barriers (Δ G#) and also free energy differences (Δ G0) for benzodiazepines are reported.
NASA Astrophysics Data System (ADS)
Kozlov, Maxim I.; Poddubnyy, Vladimir V.; Glebov, Ilya O.; Belov, Aleksandr S.; Khokhlov, Daniil V.
2016-02-01
The electronic properties of light-harvesting complexes determine the efficiency of energy transfer in photosynthetic antennae. Ab initio calculations of the electronic properties of bacteriochlorophylls (composing the LH1 complex of the purple bacteria Thermochromatium tepidum) were performed. Based on these calculations, the excitonic Hamiltonian of a native cyclic complex and the Hamiltonians of open complexes with several removed bacteriochlorophylls were constructed. Absorption spectra calculated based on these Hamiltonians agree well with the experimental data. We found that the parameters of interaction between the neighboring bacteriochlorophylls are significantly larger than the empirical parameters suggested previously.
Initial oxidation of TiAl: An ab-initio investigation
Bakulin, Alexander V. Kulkova, Svetlana E.; Hu, Qing-Miao; Yang, Rui
2014-11-14
We present ab-initio investigation of oxygen adsorption up to two monolayer coverage on the stoichiometric TiAl(100) surface to illustrate the initial oxidation stage. The formation of band gap near the Fermi level demonstrates the transformation from metal to oxide surface with increasing oxygen coverage. The oxidation of Ti rather than Al is observed from our electronic structure calculations. The energy barriers of oxygen diffusion between different sites on surface as well as in subsurface and bulk region are derived. It is shown that the diffusion of oxygen is much easier on the surface than that into the subsurface region.
A comparative ab initio and DFT study of polyaniline leucoemeraldine base and its oligomers.
Mishra, Abhishek Kumar; Tandon, Poonam
2009-11-01
Ab initio Hartree-Fock (HF) and density functional theory (DFT) calculations are being performed to investigate the geometric, vibrational, and electronic properties of the polyaniline leucoemeraldine base (PANI-LB). Vibrational spectra of PANI-LB have been analyzed using the DFT oligomer approach, and complete assignments are being reported. Lower region spectral assignments of the PANI-LB which were not being reported earlier are being done in the present work. DFT calculations with the 6-31G** basis set produce very good results of not only vibrational modes but also of energy band gap. PMID:19827802
Ab initio study on electronically excited states of lithium isocyanide, LiNC
NASA Astrophysics Data System (ADS)
Yasumatsu, Hisato; Jeung, Gwang-Hi
2014-01-01
The electronically excited states of the lithium isocyanide molecule, LiNC, were studied by means of ab initio calculations. The bonding nature of LiNC up to ˜10 eV is discussed on the basis of the potential energy surfaces according to the interaction between the ion-pair and covalent states. The ion-pair states are described by Coulomb attractive interaction in the long distance range, while the covalent ones are almost repulsive or bound with a very shallow potential dent. These two states interact each other to form adiabatic potential energy surfaces with non-monotonic change in the potential energy with the internuclear distance.
Ramsdellite-structured LiTiO 2: A new phase predicted from ab initio calculations
NASA Astrophysics Data System (ADS)
Koudriachova, M. V.
2008-06-01
A new phase of highly lithiated titania with potential application as an anode in Li-rechargeable batteries is predicted on the basis of ab initio calculations. This phase has a composition LiTiO2 and may be accessed through electrochemical lithiation of ramsdellite-structured TiO2 at the lowest potential reported for titanium dioxide based materials. The potential remains constant over a wide range of Li-concentrations. The new phase is metastable with respect to a tetragonally distorted rock salt structure, which hitherto has been the only known polymorph of LiTiO2.
Vibrational energy levels for CH4 from an ab initio potential
NASA Technical Reports Server (NTRS)
Schwenke, D. W.; Partridge, H.
2001-01-01
Many areas of astronomy and astrophysics require an accurate high temperature spectrum of methane (CH4). The goal of the present research is to determine an accurate ab initio potential energy surface (PES) for CH4. As a first step towards this goal, we have determined a PES including up to octic terms. We compare our results with experiment and to a PES based on a quartic expansion. Our octic PES gives good agreement with experiment for all levels, while the quartic PES only for the lower levels.
NASA Astrophysics Data System (ADS)
Pietrucci, Fabio; Andreoni, Wanda
2011-08-01
Social permutation invariant coordinates are introduced describing the bond network around a given atom. They originate from the largest eigenvalue and the corresponding eigenvector of the contact matrix, are invariant under permutation of identical atoms, and bear a clear signature of an order-disorder transition. Once combined with ab initio metadynamics, these coordinates are shown to be a powerful tool for the discovery of low-energy isomers of molecules and nanoclusters as well as for a blind exploration of isomerization, association, and dissociation reactions.
An ab initio molecular dynamics study of the roaming mechanism of the H2+HOC+ reaction
NASA Astrophysics Data System (ADS)
Yu, Hua-Gen
2011-08-01
We report here a direct ab initio molecular dynamics study of the p-/o-H2+HOC+ reaction on the basis of the accurate SAC-MP2 potential energy surface. The quasi-classical trajectory method was employed. This work largely focuses on the study of reaction mechanisms. A roaming mechanism was identified for this molecular ion-molecule reaction. The driving forces behind the roaming mechanism were thoroughly investigated by using a trajectory dynamics approach. In addition, the thermal rate coefficients of the H2+HOC+ reaction were calculated in the temperature range [25, 300] K and are in good agreement with experiments.
Ab initio no core calculations of light nuclei and preludes to Hamiltonian quantum field theory
Vary, J. P.; Maris, P.; Honkanen, H.; Li, J.; Shirokov, A. M.; Brodsky, S. J.; Harindranath, A.
2009-12-17
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 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.
DAMMIF, a program for rapid ab-initio shape determination in small-angle scattering
Franke, Daniel; Svergun, Dmitri I.
2009-01-01
DAMMIF, a revised implementation of the ab-initio shape-determination program DAMMIN for small-angle scattering data, is presented. The program was fully rewritten, and its algorithm was optimized for speed of execution and modified to avoid limitations due to the finite search volume. Symmetry and anisometry constraints can be imposed on the particle shape, similar to DAMMIN. In equivalent conditions, DAMMIF is 25–40 times faster than DAMMIN on a single CPU. The possibility to utilize multiple CPUs is added to DAMMIF. The application is available in binary form for major platforms.
Phosphine adsorption and dissociation on the Si(001) surface: An ab initio survey of structures
NASA Astrophysics Data System (ADS)
Warschkow, O.; Wilson, H. F.; Marks, N. A.; Schofield, S. R.; Curson, N. J.; Smith, P. V.; Radny, M. W.; McKenzie, D. R.; Simmons, M. Y.
2005-09-01
We report a comprehensive ab initio survey of possible dissociation intermediates of phosphine (PH3) on the Si(001) surface. We assign three scanning tunneling microscopy (STM) features, commonly observed in room-temperature dosing experiments, to PH2+H , PH+2H , and P+3H species, respectively, on the basis of calculated energetics and STM simulation. These assignments and a time series of STM images which shows these three STM features converting into another, allow us to outline a mechanism for the complete dissociation of phosphine on the Si(001) surface. This mechanism closes an important gap in the understanding of the doping process of semiconductor devices.
Ab initio predictions on the rotational spectra of carbon-chain carbene molecules
NASA Technical Reports Server (NTRS)
Maluendes, S. A.; McLean, A. D.; Loew, G. H. (Principal Investigator)
1992-01-01
We predict rotational constants for the carbon-chain molecules H2C=(C=)nC, n=3-8, using ab initio computations, observed values for the earlier members in the series, H2CCC and H2CCCC with n=1 and 2, and empirical geometry corrections derived from comparison of computation and experiment on related molecules. H2CCC and H2CCCC have already been observed by radioastronomy; higher members in the series, because of their large dipole moments, which we have calculated, are candidates for astronomical searches. Our predictions can guide searches and assist in both astronomical and laboratory detection.
Thorium in tungsten: construction of interatomic EAM potentials from ab initio data
NASA Astrophysics Data System (ADS)
Eberhard, Bernd; Haider, Ferdinand
2013-07-01
The interatomic interaction potential of tungsten and thorium crystals and those of hypothetical tungsten and thorium alloys within the embedded atom approach are considered. The corresponding Ansatz functions are fitted against full potential linear augmented plane wave data of real tungsten- and thorium- and hypothetical tungsten-thorium-crystals. The result is interatomic potentials, ready for use within classical molecular dynamics schemes. A cross check of the resulting force scheme derived by comparison of ab initio and classical molecular dynamics data is provided. Furthermore, we used the potentials to calculate the phonon dispersion relations, which then serve as an additional check.
Ab initio potentials of F+Li{sub 2} accessible at ultracold temperatures
Wright, K. W. A.; Lane, Ian C.
2010-09-15
Ab initio calculations for the strongly exoergic Li{sub 2}+F harpoon reaction are presented using density-functional theory, complete active space self-consistent field, and multireference configuration interaction methods to argue that this reaction would be an ideal candidate for investigation with ultracold molecules. The lowest six states are calculated with the aug-correlation-consistent polarized valence triple-zeta basis set and at least two can be accessed by a ground rovibronic Li{sub 2} molecule with zero collision energy at all reaction geometries. The large reactive cross section (characteristic of harpoon reactions) and chemiluminescent products are additional attractive features of these reactions.
Ab initio pseudopotential calculation for TTF-TCNQ and TSeF-TCNQ
NASA Astrophysics Data System (ADS)
Ishibashi, Shoji; Kohyama, Masanori
2000-09-01
We have investigated the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ [at room temperature (RT) and 100 K] and TSeF-TCNQ (RT), which have isomorphic crystal structure, by an ab initio plane-wave pseudopotential band calculation. To express the exchange and correlation energy for electrons, we used both the local density approximation and generalized gradient approximation for comparison. For each case, electronic band dispersions were calculated along several symmetric lines and tight-binding parameters were evaluated. The Fermi surface shape was also obtained. The six sets of results (for three structures and two approximations) were compared systematically.
Fully ab initio finite-size corrections for charged-defect supercell calculations.
Freysoldt, Christoph; Neugebauer, Jörg; Van de Walle, Chris G
2009-01-01
In ab initio theory, defects are routinely modeled by supercells with periodic boundary conditions. Unfortunately, the supercell approximation introduces artificial interactions between charged defects. Despite numerous attempts, a general scheme to correct for these is not yet available. We propose a new and computationally efficient method that overcomes limitations of previous schemes and is based on a rigorous analysis of electrostatics in dielectric media. Its reliability and rapid convergence with respect to cell size is demonstrated for charged vacancies in diamond and GaAs. PMID:19257218
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
Temperature dependent mechanical properties of Mo-Si-B compounds via ab initio molecular dynamics
NASA Astrophysics Data System (ADS)
Dharmawardhana, C. C.; Sakidja, R.; Aryal, S.; Ching, W. Y.
2013-07-01
A new method was proposed to obtain high temperature mechanical properties with a combination of ab initio molecular dynamics and stress-strain analyses. It was applied to compounds in the Mo-Si-B ternary system, namely, T1 (Mo5Si3) and T2 (Mo5SiB2) phases. The calculated coefficient of thermal expansion, thermal expansion anisotropy, and elastic constants agree well with those from the available experiments. The method enables us to theoretically access these properties up to 2000 K.
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.
Stoddard, Nathan; Pichler, Peter; Duscher, Gerd J M; Windl, Wolfgang
2005-01-01
In this Letter, we present ab initio results identifying a new diffusion path for the nitrogen pair complex in silicon, resulting in an effective diffusivity of 67exp(-2.38 eV/kT) cm{sup 2}/s. This nudged elastic band result is compared with other nitrogen diffusion paths and mechanisms, and is determined to have unmatched agreement with experimental results. It is also shown that careful consideration of total energy corrections and use of a fully temperature-dependent diffusion prefactor have modest but important effects on the calculation of diffusivity for paired and for interstitial nitrogen.
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.
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.
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.
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
NASA Technical Reports Server (NTRS)
Komornicki, A.; Jaffe, R. L.
1979-01-01
The infrared spectral intensities for HOCl and HO2 have been calculated using a new ab initio technique. Theoretical results for the geometries, vibrational frequencies, and the dipole moments of these species are also reported. All of the calculations were performed at the SCF level using near Hartree-Fock quality basis sets. The results for the molecular geometries and the vibrational frequencies are in good agreement with available experimental data. It is believed that the computed intensities are accurate to at least 50%. The results should be helpful in attempts to determine the stratospheric abundance of HOCl and HO2 by in situ infrared spectroscopic measurements.
The role of Metals in Amyloid Aggregation: A Test Case for ab initio Simulations
Minicozzi, V.; Rossi, G. C.; Stellato, F.; Morante, S.
2007-12-26
First principle ab initio molecular dynamics simulations of the Car-Parrinello type have proved to be of invaluable help in understanding the microscopic mechanisms of chemical bonding both in solid state physics and in structural biophysics. In this work we present as test cases the study of the Cu coordination mode in two especially important examples: Prion protein and {beta}-amyloids. Using medium size PC-clusters as well as larger parallel platforms, we are able to deal with systems comprising 300 to 500 atoms and 1000 to 1500 electrons for as long as 2-3 ps. We present structural results which confirm indications coming from NMR and XAS data.
NASA Astrophysics Data System (ADS)
Ishimura, Hiromi; Kadoya, Ryushi; Suzuki, Tomoya; Murakawa, Takeru; Shulga, Sergiy; Kurita, Noriyuki
2015-07-01
Alzheimer's disease is caused by accumulation of amyloid-β (Aβ) peptides in a brain. To suppress the production of Aβ peptides, it is effective to inhibit the cleavage of amyloid precursor protein (APP) by secretases. However, because the secretases also play important roles to produce vital proteins for human body, inhibitors for the secretases may have side effects. To propose new agents for protecting the cleavage site of APP from the attacking of the γ-secretase, we have investigated here the specific interactions between a short APP peptide and curcumin derivatives, using protein-ligand docking as well as ab initio molecular simulations.
Experimental and ab initio infrared study of chi-, kappa- and alpha-aluminas formed from gibbsite
Favaro, L.; Boumaza, A.; Roy, P.; Ledion, J.; Sattonnay, G.; Brubach, J.B.; Huntz, A.M.; Tetot, R.
2010-04-15
chi-, kappa- and alpha-alumina phases formed by dehydration of micro-grained gibbsite between 773 and 1573 K are studied using infrared spectroscopy (IR). The structural transitions evidenced by X-ray diffraction (XRD) were interpreted by comparing IR measurements with ab initio simulations (except for the chi form whose complexity does not allow a reliable simulation). For each phase, IR spectrum presents specific bands corresponding to transverse optical (TO) modes of Al-O stretching and bending under 900 cm{sup -1}. The very complex chi phase, obtained at 773 K, provides a distinctive XRD pattern in contrast with the IR absorbance appearing as a broad structure extending between 200 and 900 cm{sup -1} resembling the equivalent spectra for gamma-alumina phase. kappa-alumina is forming at 1173 K and its rich IR spectrum is in good qualitative agreement with ab initio simulations. This complexity reflects the large number of atoms in the kappa-alumina unit cell and the wide range of internuclear distances as well as the various coordinances of both Al and O atoms. Ab initio simulations suggest that this form of transition alumina demonstrates a strong departure from the simple pattern observed for other transition alumina. At 1573 K, the stable alpha-ALPHAl{sub 2}OMICRON{sub 3} develops. Its IR spectra extends in a narrower energy range as compared to transition alumina and presents characteristics features similar to model alpha-ALPHAl{sub 2}OMICRON{sub 3}. Ab initio calculations show again a very good general agreement with the observed IR spectra for this phase. In addition, for both kappa- and alpha-ALPHAl{sub 2}OMICRON{sub 3}, extra modes, measured at high energy (above 790 cm{sup -1} for kappa and above 650 cm{sup -1} for alpha), can originate from either remnant chi-alumina or from surface modes. - Graphical abstract: Infrared spectra of the sequence Gibbsite ->chi->kappa->alpha-Al{sub 2}O{sub 3} obtained from 24 h calcinations of Gibbsite at 773 K, 1173 K
NASA Astrophysics Data System (ADS)
Pham, Thi Nu; Ono, Shota; Ohno, Kaoru
2016-04-01
Doing ab initio molecular dynamics simulations, we demonstrate a possibility of hydrogenation of carbon monoxide producing methanol step by step. At first, the hydrogen atom reacts with the carbon monoxide molecule at the excited state forming the formyl radical. Formaldehyde was formed after adding one more hydrogen atom to the system. Finally, absorption of two hydrogen atoms to formaldehyde produces methanol molecule. This study is performed by using the all-electron mixed basis approach based on the time dependent density functional theory within the adiabatic local density approximation for an electronic ground-state configuration and the one-shot GW approximation for an electronic excited state configuration.
Enhancing mechanical toughness of aluminum surfaces by nano-boron implantation: An ab initio study
NASA Astrophysics Data System (ADS)
Zhu, Zhen; Kwon, Dae-Gyeon; Kwon, Young-Kyun; Tománek, David
2015-01-01
Searching for ways to enhance surface hardness of aluminum, we study the equilibrium structure, stability, elastic properties and formation dynamics of a boron-enriched surface using ab initio density functional calculations. We used molecular dynamics simulations to model the implantation of energetic boron nanoparticles in Al and identify structural arrangements that optimize the formation of strong covalent Bsbnd Al bonds. Nano-indentation simulations based on constrained optimization suggest that presence of boron nanostructures in the subsurface region enhances significantly the mechanical hardness of aluminum surfaces.
Holst, Bastian; French, Martin; Redmer, Ronald
2011-06-15
Using Kubo's linear response theory, we derive expressions for the frequency-dependent electrical conductivity (Kubo-Greenwood formula), thermopower, and thermal conductivity in a strongly correlated electron system. These are evaluated within ab initio molecular dynamics simulations in order to study the thermoelectric transport coefficients in dense liquid hydrogen, especially near the nonmetal-to-metal transition region. We also observe significant deviations from the widely used Wiedemann-Franz law, which is strictly valid only for degenerate systems, and give an estimate for its valid scope of application toward lower densities.
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.
Ab initio molecular simulations on specific interactions between amyloid beta and monosaccharides
NASA Astrophysics Data System (ADS)
Nomura, Kazuya; Okamoto, Akisumi; Yano, Atsushi; Higai, Shin'ichi; Kondo, Takashi; Kamba, Seiji; Kurita, Noriyuki
2012-09-01
Aggregation of amyloid β (Aβ) peptides, which is a key pathogenetic event in Alzheimer's disease, can be caused by cell-surface saccharides. We here investigated stable structures of the solvated complexes of Aβ with some types of monosaccharides using molecular simulations based on protein-ligand docking and classical molecular mechanics methods. Moreover, the specific interactions between Aβ and the monosaccharides were elucidated at an electronic level by ab initio fragment molecular orbital calculations. Based on the results, we proposed which type of monosaccharide prefers to have large binding affinity to Aβ and inhibit the Aβ aggregation.
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
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.
Electronic and transport properties edge functionalized graphene nanoribbons-An ab initio approach
Chauhan, Satyendra Singh; Srivastava, Pankaj; Shrivastva, A. K.
2014-04-24
With the help of ab initio approach we have investigated the electronic and transport properties of edge functionalized zigzag graphene nanoribbons using density functional theory. We have studied the energetic stability and Fermi energy of ZGNRs. We have reported that the edge functionalization of zigzag graphene nanoribbons can break the degeneracy that can be used to promote the onset of a semiconducting to metal transition or a half metal to semiconducting state. The edge functionalization also promotes a metal-semimetal transition. It has also been observed that the transmission spectrum of the edge functionalized ZGNRs are different from those of pristine.
Improving Li2O2 conductivity via polaron preemption: An ab initio study of Si doping
NASA Astrophysics Data System (ADS)
Timoshevskii, Vladimir; Feng, Zimin; Bevan, Kirk H.; Goodenough, John; Zaghib, Karim
2013-08-01
We report on ab initio electronic structure simulations of Li2O2, where 1.6% of lithium atoms are substituted by silicon. It is demonstrated that this leads to the formation of conducting impurity states in the band gap of Li2O2. We show that these states originate from the antibonding orbitals of the oxygen pairs and are remarkably stable against possible polaron formation (upon electron injection). Through this polaron preemption mechanism, the proposed compound is expected to show significantly higher electronic mobility than stoichiometric Li2O2, which could have significant applications in lithium-air batteries.
Phonon spectrum of lead oxychloride Pb3O2Cl2: Ab initio calculation and experiment
NASA Astrophysics Data System (ADS)
Zakir'yanov, D. O.; Chernyshev, V. A.; Zakir'yanova, I. D.
2016-02-01
IR and Raman spectra of Pb3O2Cl2 in the range of 50-600 cm-1 have been detected for the first time. Ab initio calculations of the crystal structure and the phonon spectrum of Pb3O2Cl2 in the framework of LCAO approach have been performed by the Hartree-Fock method and in the framework of the density functional theory with the use of hybrid functionals. The results of calculations have made it possible to interpret the experimental vibration spectra and reveal silent modes, which do not manifest themselves in these spectra but influence the optical properties of the crystal.
NASA Astrophysics Data System (ADS)
Pagliai, Marco; Muniz-Miranda, Francesco; Cardini, Gianni; Righini, Roberto; Schettino, Vincenzo
2011-05-01
In order to extract spectroscopic information from trajectories obtained by classical or ab initio molecular dynamics simulations, usually Fourier transforms are employed. In recent years wavelet transforms have been shown to be a valid alternative tool to analyze time-series, due to their capability of localizing a signal both in time and frequency. In this article wavelet transforms are applied for the analysis of Car-Parrinello molecular dynamics simulations to the purpose of time-correlating structural and spectroscopic properties of methyl acetate dissolved in water and methanol. The results demonstrate the possibility of obtaining information that may be of valuable help in the interpretation of time-resolved spectroscopic data.
Scalable numerical approach for the steady-state ab initio laser theory
NASA Astrophysics Data System (ADS)
Esterhazy, S.; Liu, D.; Liertzer, M.; Cerjan, A.; Ge, L.; Makris, K. G.; Stone, A. D.; Melenk, J. M.; Johnson, S. G.; Rotter, S.
2014-08-01
We present an efficient and flexible method for solving the non-linear lasing equations of the steady-state ab initio laser theory. Our strategy is to solve the underlying system of partial differential equations directly, without the need of setting up a parametrized basis of constant flux states. We validate this approach in one-dimensional as well as in cylindrical systems, and demonstrate its scalability to full-vector three-dimensional calculations in photonic-crystal slabs. Our method paves the way for efficient and accurate simulations of microlasers which were previously inaccessible.
NASA Astrophysics Data System (ADS)
Venâncio, Mateus F.; Rocha, Willian R.
2015-10-01
Ab initio molecular dynamics simulations were used to investigate the early chemical events involved in the dynamics of nitric oxide (NOrad), nitrosonium cation (NO+) and nitroxide anion (NO-) in aqueous solution. The NO+ ion is very reactive in aqueous solution having a lifetime of ∼4 × 10-13 s, which is shorter than the value of 3 × 10-10 s predicted experimentally. The NO+ reacts generating the nitrous acid as an intermediate and the NO2- ion as the final product. The dynamics of NOrad revealed the reversibly formation of a transient anion radical species HONOrad -.
Goodswen, Stephen J; Kennedy, Paul J; Ellis, John T
2012-01-01
Next generation sequencing technology is advancing genome sequencing at an unprecedented level. By unravelling the code within a pathogen's genome, every possible protein (prior to post-translational modifications) can theoretically be discovered, irrespective of life cycle stages and environmental stimuli. Now more than ever there is a great need for high-throughput ab initio gene finding. Ab initio gene finders use statistical models to predict genes and their exon-intron structures from the genome sequence alone. This paper evaluates whether existing ab initio gene finders can effectively predict genes to deduce proteins that have presently missed capture by laboratory techniques. An aim here is to identify possible patterns of prediction inaccuracies for gene finders as a whole irrespective of the target pathogen. All currently available ab initio gene finders are considered in the evaluation but only four fulfil high-throughput capability: AUGUSTUS, GeneMark_hmm, GlimmerHMM, and SNAP. These gene finders require training data specific to a target pathogen and consequently the evaluation results are inextricably linked to the availability and quality of the data. The pathogen, Toxoplasma gondii, is used to illustrate the evaluation methods. The results support current opinion that predicted exons by ab initio gene finders are inaccurate in the absence of experimental evidence. However, the results reveal some patterns of inaccuracy that are common to all gene finders and these inaccuracies may provide a focus area for future gene finder developers. PMID:23226328
Origin of the Hadži ABC structure: An ab initio study
Van Hoozen, Brian L.; Petersen, Poul B.
2015-11-14
Medium and strong hydrogen bonds are well known to give rise to broad features in the vibrational spectrum often spanning several hundred wavenumbers. In some cases, these features can span over 1000 cm{sup −1} and even contain multiple broad peaks. One class of strongly hydrogen-bonded dimers that includes many different phosphinic, phosphoric, sulfinic, and selenic acid homodimers exhibits a three-peaked structure over 1500 cm{sup −1} broad. This unusual feature is often referred to as the Hadži ABC structure. The origin of this feature has been debated since its discovery in the 1950s. Only a couple of theoretical studies have attempted to interpret the origin of this feature; however, no previous study has been able to reproduce this feature from first principles. Here, we present the first ab initio calculation of the Hadži ABC structure. Using a reduced dimensionality calculation that includes four vibrational modes, we are able to reproduce the three-peak structure and much of the broadness of the feature. Our results indicate that Fermi resonances of the in-plane bend, out-of-plane bend, and combination of these bends play significant roles in explaining this feature. Much of the broadness of the feature and the ability of the OH stretch mode to couple with many overtone bending modes are captured by including an adiabatically separated dimer stretch mode in the model. This mode modulates the distance between the monomer units and accordingly the strength of the hydrogen-bonds causing the OH stretch frequency to shift from 2000 to 3000 cm{sup −1}. Using this model, we were also able to reproduce the vibrational spectrum of the deuterated isotopologue which consists of a single 500 cm{sup −1} broad feature. Whereas previous empirical studies have asserted that Fermi resonances contribute very little to this feature, our study indicates that while not appearing as a separate peak, a Fermi resonance of the in-plane bend contributes substantially to
Stavretis, Shelby E.; Atanasov, Mihail; Podlesnyak, Andrey A.; Hunter, Seth C.; Neese, Frank; Xue, Zi-Ling
2015-10-02
Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe(TPP)X] (X = F, Br, I; H2TPP = tetraphenylporphyrin) are determined by inelastic neutron scattering (INS). The ZFS values are D = 4.49(9) cm–1 for tetragonal polycrystalline [Fe(TPP)F], and D = 8.8(2) cm–1, E = 0.1(2) cm–1 and D = 13.4(6) cm–1, E = 0.3(6) cm–1 for monoclinic polycrystalline [Fe(TPP)Br] and [Fe(TPP)I], respectively. Along with our recent report of the ZFS value of D = 6.33(8) cm–1 for tetragonal polycrystalline [Fe(TPP)Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe(TPP)X] (X =more » F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting D of the 6A1 ground state. D was calculated from wave functions of the electronic multiplets spanned by the d5 configuration of Fe(III) along with spin–orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of D from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the S = 3/2 excited states on ligand field theory was used to characterize the σ- and π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX63- complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of D and decrease of the π- and σ-antibonding energies eλX (λ = σ, π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the experimentally deduced trend
Stavretis, Shelby E; Atanasov, Mihail; Podlesnyak, Andrey A; Hunter, Seth C; Neese, Frank; Xue, Zi-Ling
2015-10-19
Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe(TPP)X] (X = F, Br, I; H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are D = 4.49(9) cm⁻¹ for tetragonal polycrystalline [Fe(TPP)F], and D = 8.8(2) cm⁻¹, E = 0.1(2) cm⁻¹ and D = 13.4(6) cm⁻¹, E = 0.3(6) cm⁻¹ for monoclinic polycrystalline [Fe(TPP)Br] and [Fe(TPP)I], respectively. Along with our recent report of the ZFS value of D = 6.33(8) cm⁻¹ for tetragonal polycrystalline [Fe(TPP)Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe(TPP)X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting D of the ⁶A₁ ground state. D was calculated from wave functions of the electronic multiplets spanned by the d⁵ configuration of Fe(III) along with spin–orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of D from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the S = 3/2 excited states on ligand field theory was used to characterize the σ- and π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX₆³⁻ complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of D and decrease of the π- and σ-antibonding energies e(λ)(X) (λ = σ, π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the
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}.
NASA Astrophysics Data System (ADS)
Kalugina, Yulia N.; Lokshtanov, Sergei E.; Cherepanov, Victor N.; Vigasin, Andrey A.
2016-02-01
We present new three-dimensional potential energy surface (PES) and dipole moment surfaces (DMSs) for the CH4-Ar van der Waals system. Ab initio calculations of the PES and DMS were carried out using the closed-shell single- and double-excitation coupled cluster approach with non-iterative perturbative treatment of triple excitations. The augmented correlation-consistent aug-cc-pVXZ (X = D,T,Q) basis sets were employed, and the energies obtained were then extrapolated to the complete basis set limit. The dipole moment surface was obtained using aug-cc-pVTZ basis set augmented with mid-bond functions for better description of exchange interactions. The second mixed virial coefficient was calculated and compared to available experimental data. The equilibrium constant for true dimer formation was calculated using classical partition function based on the knowledge of ab initio PES. Temperature variations of the zeroth spectral moment of the rototranslational collision-induced band as well as its true dimer constituent were traced with the use of the Boltzmann-weighted squared induced dipole properly integrated over respective phase space domains. Height profiles for N2-N2, N2-H2, CH4-N2, (CH4)2, and CH4-Ar true bound dimers in Titan's atmosphere were calculated with the use of reliable ab initio PESs.
NASA Astrophysics Data System (ADS)
Zemen, J.; Mašek, J.; Kučera, J.; Mol, J. A.; Motloch, P.; Jungwirth, T.
2014-04-01
An empirical multiorbital (spd) tight binding (TB) model including magnetism and spin-orbit coupling is applied to calculations of magnetic anisotropy energy (MAE) in CoPt L10 structure. A realistic Slater-Koster parametrisation for single-element transition metals is adapted for the ordered binary alloy. Spin magnetic moment and density of states are calculated using a full-potential linearised augmented plane-wave (LAPW) ab initio method and our TB code with different variants of the interatomic parameters. Detailed mutual comparison of this data allows for determination of a subset of the compound TB parameters tuning of which improves the agreement of the TB and LAPW results. MAE calculated as a function of band filling using the refined parameters is in broad agreement with ab initio data for all valence states and in quantitative agreement with ab initio and experimental data for the natural band filling. Our work provides a practical basis for further studies of relativistic magnetotransport anisotropies by means of local Green's function formalism which is directly compatible with our TB approach.
NASA Astrophysics Data System (ADS)
Wang, Yimin; Bowman, Joel M.; Kamarchik, Eugene
2016-03-01
We report full-dimensional, ab initio-based potentials and dipole moment surfaces for NaCl, NaF, Na+H2O, F-H2O, and Cl-H2O. The NaCl and NaF potentials are diabatic ones that dissociate to ions. These are obtained using spline fits to CCSD(T)/aug-cc-pV5Z energies. In addition, non-linear least square fits using the Born-Mayer-Huggins potential are presented, providing accurate parameters based strictly on the current ab initio energies. The long-range behavior of the NaCl and NaF potentials is shown to go, as expected, accurately to the point-charge Coulomb interaction. The three ion-H2O potentials are permutationally invariant fits to roughly 20 000 coupled cluster CCSD(T) energies (awCVTZ basis for Na+ and aVTZ basis for Cl- and F-), over a large range of distances and H2O intramolecular configurations. These potentials are switched accurately in the long range to the analytical ion-dipole interactions, to improve computational efficiency. Dipole moment surfaces are fits to MP2 data; for the ion-ion cases, these are well described in the intermediate- and long-range by the simple point-charge expression. The performance of these new fits is examined by direct comparison to additional ab initio energies and dipole moments along various cuts. Equilibrium structures, harmonic frequencies, and electronic dissociation energies are also reported and compared to direct ab initio results. These indicate the high fidelity of the new PESs.
Wang, Yimin; Bowman, Joel M; Kamarchik, Eugene
2016-03-21
We report full-dimensional, ab initio-based potentials and dipole moment surfaces for NaCl, NaF, Na(+)H2O, F(-)H2O, and Cl(-)H2O. The NaCl and NaF potentials are diabatic ones that dissociate to ions. These are obtained using spline fits to CCSD(T)/aug-cc-pV5Z energies. In addition, non-linear least square fits using the Born-Mayer-Huggins potential are presented, providing accurate parameters based strictly on the current ab initio energies. The long-range behavior of the NaCl and NaF potentials is shown to go, as expected, accurately to the point-charge Coulomb interaction. The three ion-H2O potentials are permutationally invariant fits to roughly 20,000 coupled cluster CCSD(T) energies (awCVTZ basis for Na(+) and aVTZ basis for Cl(-) and F(-)), over a large range of distances and H2O intramolecular configurations. These potentials are switched accurately in the long range to the analytical ion-dipole interactions, to improve computational efficiency. Dipole moment surfaces are fits to MP2 data; for the ion-ion cases, these are well described in the intermediate- and long-range by the simple point-charge expression. The performance of these new fits is examined by direct comparison to additional ab initio energies and dipole moments along various cuts. Equilibrium structures, harmonic frequencies, and electronic dissociation energies are also reported and compared to direct ab initio results. These indicate the high fidelity of the new PESs. PMID:27004880
Xiong, L H; Yoo, H; Lou, H B; Wang, X D; Cao, Q P; Zhang, D X; Jiang, J Z; Xie, H L; Xiao, T Q; Jeon, S; Lee, G W
2015-01-28
X-ray diffraction and electrostatic levitation measurements, together with the ab initio molecular dynamics simulation of liquid Al(75)Cu(25) alloy have been performed from 800 to 1600 K. Experimental and ab initio molecular dynamics simulation results match well with each other. No abnormal changes were experimentally detected in the specific heat capacity over total hemispheric emissivity and density curves in the studied temperature range for a bulk liquid Al(75)Cu(25) alloy measured by the electrostatic levitation technique. The structure factors gained by the ab initio molecular dynamics simulation precisely coincide with the experimental data. The atomic structure analyzed by the Honeycutt-Andersen index and Voronoi tessellation methods shows that icosahedral-like atomic clusters prevail in the liquid Al(75)Cu(25) alloy and the atomic clusters evolve continuously. All results obtained here suggest that no liquid-liquid transition appears in the bulk liquid Al(75)Cu(25) alloy in the studied temperature range. PMID:25524926
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.
NASA Astrophysics Data System (ADS)
Ming, Xing; Wang, Chun-Zhong; Fan, Hou-Gang; Hu, Fang; Wei, Ying-Jin; Huang, Zu-Fei; Meng, Xing; Chen, Gang
2008-10-01
Based on fully self-consistent first-principles density functional theory (DFT) calculations as well as classical spin analysis, we study the electronic structure and magnetic interactions of the spinel-related compound LiMnVO4. Four possible ordered spin states have been considered by spin-polarized generalized gradient approximation (GGA) calculations. The antiferromagnetic (AFM) configuration with both intra-chain and inter-chain AFM coupling interactions is energetically favorable among these magnetic ordered states. The calculated AFM solution agrees well with a series of experimental measurements. The intra-atomic exchange splitting of the Mn 3d spin-up and spin-down states results in the insulating behavior of LiMnVO4. The Heisenberg Hamiltonian is used to deduce the magnetic coupling parameters by adopting Noodleman's broken symmetry method. The intra-chain AFM interactions are much stronger than the inter-chain AFM interactions and thus LiMnVO4 can be described as a weakly coupled edge-sharing spin \\frac {5}{2} chain system. We propose that the presence of the side groups of edge-sharing LiO4 and VO4 tetrahedra are contributing to the intra-chain AFM interactions in the nearly 90° Mn-O-Mn bond configuration of the edge-sharing MnO4 chains.
Ab Initio Values of the Thermophysical Properties of Helium as Standards
Hurly, John J.; Moldover, Michael R.
2000-01-01
Recent quantum mechanical calculations of the interaction energy of pairs of helium atoms are accurate and some include reliable estimates of their uncertainty. We combined these ab initio results with earlier published results to obtain a helium-helium interatomic potential that includes relativistic retardation effects over all ranges of interaction. From this potential, we calculated the thermophysical properties of helium, i.e., the second virial coefficients, the dilute-gas viscosities, and the dilute-gas thermal conductivities of 3He, 4He, and their equimolar mixture from 1 K to 104 K. We also calculated the diffusion and thermal diffusion coefficients of mixtures of 3He and 4He. For the pure fluids, the uncertainties of the calculated values are dominated by the uncertainties of the potential; for the mixtures, the uncertainties of the transport properties also include contributions from approximations in the transport theory. In all cases, the uncertainties are smaller than the corresponding experimental uncertainties; therefore, we recommend the ab initio results be used as standards for calibrating instruments relying on these thermophysical properties. We present the calculated thermophysical properties in easy-to-use tabular form.
Novel high-pressure phase of ZrO{sub 2}: An ab initio prediction
Durandurdu, Murat
2015-10-15
The high-pressure behavior of the orthorhombic cotunnite type ZrO{sub 2} is explored using an ab initio constant pressure technique. For the first time, a novel hexagonal phase (Ni{sub 2}In type) within P6{sub 3}/mmc symmetry is predicted through the simulation. The Ni{sub 2}In type crystal is the densest high-pressure phase of ZrO{sub 2} proposed so far and has not been observed in other metal dioxides at high pressure before. The phase transformation is accompanied by a small volume drop and likely to occur around 380 GPa in experiment. - Graphical abstract: Post-cotunnite Ni{sub 2}In type hexagonal phase forms in zirconia at high pressure. - Highlights: • A post-cotunnite phase is predicted for ZrO{sub 2} through an ab initio simulation. • Cotunnite ZrO{sub 2} adopts the Ni{sub 2}In type structure at high pressure. • The Ni{sub 2}In type structure is the densest high-pressure phase of ZrO{sub 2} proposed so far. • The preferred mechanism in ZrO{sub 2} differs from the other metal dioxides.
NASA Astrophysics Data System (ADS)
Galler, Anna; Gunacker, Patrik; Tomczak, Jan; Thunström, Patrik; Held, Karsten
Recently, approaches such as the dynamical vertex approximation (D ΓA) or the dual-fermion method have been developed. These diagrammatic approaches are going beyond dynamical mean field theory (DMFT) by including nonlocal electronic correlations on all length scales as well as the local DMFT correlations. Here we present our efforts to extend the D ΓA methodology to ab-initio materials calculations (ab-initio D ΓA). Our approach is a unifying framework which includes both GW and DMFT-type of diagrams, but also important nonlocal correlations beyond, e.g. nonlocal spin fluctuations. In our multi-band implementation we are using a worm sampling technique within continuous-time quantum Monte Carlo in the hybridization expansion to obtain the DMFT vertex, from which we construct the reducible vertex function using the two particle-hole ladders. As a first application we show results for transition metal oxides. Support by the ERC project AbinitioDGA (306447) is acknowledged.
An ab Initio Benchmark and DFT Validation Study on Gold(I)-Catalyzed Hydroamination of Alkynes.
Ciancaleoni, Gianluca; Rampino, Sergio; Zuccaccia, Daniele; Tarantelli, Francesco; Belanzoni, Paola; Belpassi, Leonardo
2014-03-11
High level ab initio calculations have been carried out on an archetypal gold(I)-catalyzed reaction: hydroamination of ethyne. We studied up to 12 structures of possible gold(I)-coordinated species modeling different intermediates potentially present in a catalytic cycle for the addition of a protic nucleophile to an alkyne. The benchmark is used to evaluate the performances of some popular density functionals for describing geometries and relative energies of stationary points along the reaction profile. Most functionals (including hybrid or meta-hybrid) give accurate structures but large nonsystematic errors (4-12 kcal/mol) along the reaction energy profile. The double hybrid functional B2PLYP outperforms all considered functionals and compares very nicely with our reference ab initio benchmark energies. Moreover, we present an assessment of the accuracy of commonly used approaches to include relativistic effects, such as relativistic effective potentials and a scalar ZORA Hamiltonian, by a comparison with the results obtained using a relativistic all-electron four-component Dirac-Kohn-Sham method. The contribution of nonscalar relativistic effects in gold(I)-catalyzed reactions, as we investigated here, is expected to be on the order of 1 kcal/mol. PMID:26580180
Ab Initio Simulations of Hydrogen in Crystalline and Amorphous Metal Membranes
NASA Astrophysics Data System (ADS)
Huhn, William; Widom, Mike
2011-03-01
Solid metallic membranes are used to separate hydrogen from other gases for clean energy applications. In order to create cheaper, more effective membranes for hydrogen separation, it is desirable to model hydrogen transport through the membrane. Amorphous metal membranes in particular have potential for this type of application due to low expense and high theoretical hydrogen capacity. We computationally model hydrogen absorption and transport through materials in order to find materials that can be used to construct effective membranes for hydrogen capture. In this talk, we will obtain hydrogen binding sites and diffusion barriers in order to model the hydrogen diffusion through various nickel-based amorphous alloys and compare them to associated crystalline structures as well as elemental palladium, which is favored for this application despite its high expense. Ab initio methods (specifically the Vienna Ab Initio Simulation Package, VASP) are used to develop the hydrogen binding energy spectrum, from which thermodynamic models can be constructed. Kinetic Monte Carlo methods are used to model the hydrogen transport through the bulk, from which we can obtain the permeability.
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.
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 adiabatic and diabatic energies and dipole moments of the KH molecule
NASA Astrophysics Data System (ADS)
Khelifi, Neji; Oujia, Brahim; Gadea, Florent Xavier
2002-02-01
An ab initio adiabatic and diabatic study of the KH molecule is performed for all states below the ionic limit [i.e., K (4s, 4p, 5s, 3d, 5p, 4d, 6s, and 4f)+H(1s)] in 1Σ+ and 3Σ+ symmetries. Adiabatic results are also reported for 1Π, 3Π, 1Δ, and 3Δ symmetries. The ab initio calculations rely on pseudopotential, operatorial core valence correlation, and full valence CI approaches, combined to an efficient diabatization procedure. For the low-lying states, our vibrational level spacings and spectroscopic constants are in very good agreement with the available experimental data. Diabatic potentials and dipoles moments are analyzed, revealing the strong imprint of the ionic state in the 1Σ+ adiabatic states while improving the results. The undulations of the diabatic curves and of the triplet-singlet diabatic energy difference which we found positive, as in Hund's rule, are related to the Rydberg functions. As for LiH, the vibrational spacing of the A state is bracketed by our results with and without the improvement taking into account the diabatic representation. Experimental suggestions are also given.
Ab initio calculations of the optical properties of crystalline and liquid InSb
NASA Astrophysics Data System (ADS)
Sano, Haruyuki; Mizutani, Goro
2015-11-01
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 simulations of nonstoichiometric CdxTe1-x liquids
NASA Astrophysics Data System (ADS)
Ko, Eunjung; Alemany, M. M. G.; Derby, Jeffrey J.; Chelikowsky, James R.
2005-08-01
We present ab initio molecular-dynamics simulations for CdxTe1-x liquids where the composition is nonstoichiometric. The simulations are performed following Born-Oppenheimer molecular dynamics. The required forces are obtained from a solution of the Kohn-Sham equation using ab initio pseudopotentials. We consider stoichiometries of the form: CdxTe1-x, where x =0.2, 0.4, 0.6, and 0.8. For each composition of the melt, we consider a range of temperatures near the experimentally determined liquid temperatures. We examine the microstructural properties of the melt, the viscosity, and self-diffusion properties of the liquid as a function of the stoichiometry and temperature. We also perform an analysis of the distribution of the electronic density of states in these liquids. We find that structural changes in the local order, experimentally predicted to occur when the concentration of Cd is increased, are closely related to changes in the electronic properties of the melt.
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).
Accelerating Ab Initio Path Integral Simulations via Imaginary Multiple-Timestepping.
Cheng, Xiaolu; Herr, Jonathan D; Steele, Ryan P
2016-04-12
This work investigates the use of multiple-timestep schemes in imaginary time for computationally efficient ab initio equilibrium path integral simulations of quantum molecular motion. In the simplest formulation, only every n(th) path integral replica is computed at the target level of electronic structure theory, whereas the remaining low-level replicas still account for nuclear motion quantum effects with a more computationally economical theory. Motivated by recent developments for multiple-timestep techniques in real-time classical molecular dynamics, both 1-electron (atomic-orbital basis set) and 2-electron (electron correlation) truncations are shown to be effective. Structural distributions and thermodynamic averages are tested for representative analytic potentials and ab initio molecular examples. Target quantum chemistry methods include density functional theory and second-order Møller-Plesset perturbation theory, although any level of theory is formally amenable to this framework. For a standard two-level splitting, computational speedups of 1.6-4.0x are observed when using a 4-fold reduction in time slices; an 8-fold reduction is feasible in some cases. Multitiered options further reduce computational requirements and suggest that quantum mechanical motion could potentially be obtained at a cost not significantly different from the cost of classical simulations. PMID:26966920
Operator evolution for ab initio electric dipole transitions of 4He
Schuster, Micah D.; Quaglioni, Sofia; Johnson, Calvin W.; Jurgenson, Eric D.; Navartil, Petr
2015-07-24
A goal of nuclear theory is to make quantitative predictions of low-energy nuclear observables starting from accurate microscopic internucleon forces. A major element of such an effort is applying unitary transformations to soften the nuclear Hamiltonian and hence accelerate the convergence of ab initio calculations as a function of the model space size. The consistent simultaneous transformation of external operators, however, has been overlooked in applications of the theory, particularly for nonscalar transitions. We study the evolution of the electric dipole operator in the framework of the similarity renormalization group method and apply the renormalized matrix elements to the calculationmore » of the 4He total photoabsorption cross section and electric dipole polarizability. All observables are calculated within the ab initio no-core shell model. Furthermore, we find that, although seemingly small, the effects of evolved operators on the photoabsorption cross section are comparable in magnitude to the correction produced by including the chiral three-nucleon force and cannot be neglected.« less