Differentiability of Lieb functional in electronic density functional theory
NASA Astrophysics Data System (ADS)
Lammert, Paul E.
A solid understanding of the Lieb functional FL is important because of its centrality in the foundations of electronic density functional theory. A basic question is whether directional derivatives of FL at an ensemble-V-representable density are given by (minus) the potential. A widely accepted purported proof that FL is Gâteaux differentiable at EV-representable densities would say, ?yes.? But that proof is fallacious, as shown here. FL is not Gâteaux differentiable in the normal sense, nor is it continuous. By means of a constructive approach, however, we are able to show that the derivative of FL at an EV-representable density ?0 in the direction of ?1 is given by the potential if ?0 and ?1 are everywhere strictly greater than zero, and they and the ground state wave function have square integrable derivatives through second order.
Gutzwiller density functional theory for correlated electron systems
Ho, K. M.; Schmalian, J.; Wang, C. Z.
2008-02-04
We develop a density functional theory (DFT) and formalism for correlated electron systems by taking as reference an interacting electron system that has a ground state wave function which exactly obeys the Gutzwiller approximation for all one-particle operators. The solution of the many-electron problem is mapped onto the self-consistent solution of a set of single-particle Schroedinger equations, analogously to standard DFT-local density approximation calculations.
Optimal-transport formulation of electronic density-functional theory
NASA Astrophysics Data System (ADS)
Buttazzo, Giuseppe; De Pascale, Luigi; Gori-Giorgi, Paola
2012-06-01
The most challenging scenario for Kohn-Sham density-functional theory, that is, when the electrons move relatively slowly trying to avoid each other as much as possible because of their repulsion (strong-interaction limit), is reformulated here as an optimal transport (or mass transportation theory) problem, a well-established field of mathematics and economics. In practice, we show that to solve the problem of finding the minimum possible internal repulsion energy for N electrons in a given density ρ(r) is equivalent to find the optimal way of transporting N-1 times the density ρ into itself, with the cost function given by the Coulomb repulsion. We use this link to set the strong-interaction limit of density-functional theory on firm ground and to discuss the potential practical aspects of this reformulation.
The exact density functional for two electrons in one dimension
NASA Astrophysics Data System (ADS)
Cohen, Aron; Mori-Sanchez, Paula
The exact universal density functional F [ ρ ] is calculated for real space two-electron densities in one dimension ρ (x) with a soft-Coulomb interaction. It is calculated by the Levy constrained search F [ ρ ] =minΨ-->ρ < Ψ | \\Tcirc +\\Vcircee | Ψ > over wavefunctions of a two-dimensional Hilbert space Ψ (x1 ,x2) --> ρ (x1) and can be directly visualized. We do an approximate constrained search via density matrices and a direct approximation to natural orbitals. This allows us to make an accurate approximation to the exact functional that is calculated using a search over potentials. We investigate the exact functional and the performance of many approximations on some of the most challenging electronic structure in two-electron systems, from strongly-correlated electron transfer to the description of a localized-delocalized transition. The exact Kohn-Sham potential, vs (x) , and exact Kohn-Sham eigenvalues, ɛi, are calculated and this allows us to discuss the band-gap problem versus the perspective of the exact density functional F [ ρ ] for all numbers of electrons. We calculate the derivative discontinuity of the exact functional in an example of a Mott-Insulator, one-dimensional stretched H2.
Excitations and benchmark ensemble density functional theory for two electrons
Pribram-Jones, Aurora; Burke, Kieron; Yang, Zeng-hui; Ullrich, Carsten A.; Trail, John R.; Needs, Richard J.
2014-05-14
A new method for extracting ensemble Kohn-Sham potentials from accurate excited state densities is applied to a variety of two-electron systems, exploring the behavior of exact ensemble density functional theory. The issue of separating the Hartree energy and the choice of degenerate eigenstates is explored. A new approximation, spin eigenstate Hartree-exchange, is derived. Exact conditions that are proven include the signs of the correlation energy components and the asymptotic behavior of the potential for small weights of the excited states. Many energy components are given as a function of the weights for two electrons in a one-dimensional flat box, in a box with a large barrier to create charge transfer excitations, in a three-dimensional harmonic well (Hooke's atom), and for the He atom singlet-triplet ensemble, singlet-triplet-singlet ensemble, and triplet bi-ensemble.
Excess electrons in ice: a density functional theory study.
Bhattacharya, Somesh Kr; Inam, Fakharul; Scandolo, Sandro
2014-02-21
We present a density functional theory study of the localization of excess electrons in the bulk and on the surface of crystalline and amorphous water ice. We analyze the initial stages of electron solvation in crystalline and amorphous ice. In the case of crystalline ice we find that excess electrons favor surface states over bulk states, even when the latter are localized at defect sites. In contrast, in amorphous ice excess electrons find it equally favorable to localize in bulk and in surface states which we attribute to the preexisting precursor states in the disordered structure. In all cases excess electrons are found to occupy the vacuum regions of the molecular network. The electron localization in the bulk of amorphous ice is assisted by its distorted hydrogen bonding network as opposed to the crystalline phase. Although qualitative, our results provide a simple interpretation of the large differences observed in the dynamics and localization of excess electrons in crystalline and amorphous ice films on metals. PMID:24401958
Momentum distribution function of the electron gas at metallic densities
NASA Astrophysics Data System (ADS)
Takada, Yasutami; Yasuhara, H.
1991-10-01
The momentum distribution function n(k) of the electron gas is calculated in the effective-potential-expansion method at metallic densities. The recently established self-consistency relation between n(k) and the correlation energy [Y. Takada and T. Kita, J. Phys. Soc. Jpn. 60, 25 (1991)] is employed to check the accuracy of our results. This check shows that the effective-potential-expansion method provides probably the exact and at least more accurate results of n(k) than all the other methods that have given n(k) thus far.
Ionospheric mapping functions based on electron density fields
NASA Astrophysics Data System (ADS)
Zus, Florian; Deng, Zhiguo; Heise, Stefan; Wickert, Jens
2016-04-01
We developed an ionospheric Mapping Function (MF) for the Global Navigation Satellite System (GNSS) which is based on the electron density field of the International Reference Ionosphere (IRI). The station specific MF utilizes a look-up table which contains a set of ray-traced ionospheric delays. Hence, unlike the simple MFs that are currently in use, the developed MF depends on the time, location, elevation and azimuth angle. Ray-bending is taken into account, which implies that the MF depends on the carrier frequency as well. This frequency dependency of the MF can be readily used to examine higher-order ionospheric effects due to ray-bending. We compare the proposed MF with the so-called single layer model MF and find significant differences in particular around the equatorial anomaly. In so-far as the proposed MF is based on a realistic electron density field (IRI) our comparison shows the potential error of the single-layer model MF in practice. We conclude that the developed MF concept might be valuable in the GNSS Total Electron Content estimation. The frequency dependency of the MF can be used to mitigate higher-order ionospheric effects.
Electronic properties of graphene nanoribbons: A density functional investigation
Kumar, Sandeep Sharma, Hitesh
2015-05-15
Density functional theory calculations have been performed on graphene nano ribbons (GNRs) to investigate the electronic properties as a function of chirality, size and hydrogenation on the edges. The calculations were performed on GNRs with armchair and zigzag configurations with 28, 34, 36, 40, 50, 56, 62, 66 carbon atoms. The structural stability of AGNR and ZGNR increases with the size of nanoribbon where as hydrogenation of GNR tends to lowers their structural stability. All GNRs considered have shown semiconducting behavior with HOMO-LUMO gap decreasing with the increase in the GNR size. The hydrogenation of GNR decreases its HOMO-LUMO gap significantly. The results are in agreement with the available experimental and theoretical results.
Equation satisfied by the energy-density functional for electron-electron mutual Coulomb repulsion
Joubert, Daniel P.
2011-10-15
It is shown that the electron-electron mutual Coulomb repulsion energy-density functional V{sub ee}{sup {gamma}}[{rho}] satisfies the equationV{sub ee}{sup {gamma}}[{rho}{sub N}{sup 1}]-V{sub ee}{sup {gamma}}[{rho}{sub N-1}{sup {gamma}}]={integral}d{sup 3}r({delta}V{sub ee}{sup {gamma}}[{rho}{sub N}{sup 1}]/{delta}{rho}{sub N}{sup 1}(r))[{rho}{sub N}{sup 1}(r)-{rho}{sub N-1}{sup {gamma}}(r)], where {rho}{sub N}{sup 1}(r) and {rho}{sub N-1}{sup {gamma}}(r) are N-electron and (N-1)-electron densities determined from the same adiabatic scaled external potential of the N-electron system at coupling strength {gamma}.
NASA Astrophysics Data System (ADS)
Putaja, A.; Eich, F. G.; Baldsiefen, T.; Räsänen, E.
2016-03-01
Physically valid and numerically efficient approximations for the exchange and correlation energy are critical for reduced-density-matrix-functional theory to become a widely used method in electronic structure calculations. Here we examine the physical limits of power functionals of the form f (n ,n') =(nn')α for the scaling function in the exchange-correlation energy. To this end we obtain numerically the minimizing momentum distributions for the three- and two-dimensional homogeneous electron gas, respectively. In particular, we examine the limiting values for the power α to yield physically sound solutions that satisfy the Lieb-Oxford lower bound for the exchange-correlation energy and exclude pinned states with the condition n (k )<1 for all wave vectors k . The results refine the constraints previously obtained from trial momentum distributions. We also compute the values for α that yield the exact correlation energy and its kinetic part for both the three- and two-dimensional electron gas. In both systems, narrow regimes of validity and accuracy are found at α ≳0.6 and at rs≳10 for the density parameter, corresponding to relatively low densities.
NASA Astrophysics Data System (ADS)
Tachikawa, Hiroto; Iyama, Tetsuji; Kawabata, Hiroshi
2016-05-01
Electronic structures and formation mechanism of hydrogen functionalized carbon nanotube (CNT) have been investigated by means of density functional theory (DFT) method. The mechanism of hydrogen addition reaction to the CNT surface was also investigated. Pure and boron-nitrogen (BN) substituted CNT (denoted by CNT and BN-CNT, respectively) were examined as the carbon nanotubes. It was found that the additions of hydrogen atom to B (boron atom) and C (carbon atom) sites of BN-CNT proceed without activation barrier, whereas the hydrogenation of N (nitrogen atom) site needs the activation energy. The electronic states of hydrogen functionalized CNT and BN-CNT were discussed on the basis of theoretical results.
De Proft, F; Van Alsenoy, C; Peeters, A; Langenaeker, W; Geerlings, P
2002-09-01
In the Hirshfeld partitioning of the electron density, the molecular electron density is decomposed in atomic contributions, proportional to the weight of the isolated atom density in the promolecule density, constructed by superimposing the isolated atom electron densities placed on the positions the atoms have in the molecule. A maximal conservation of the information of the isolated atoms in the atoms-in-molecules is thereby secured. Atomic charges, atomic dipole moments, and Fukui functions resulting from the Hirshfeld partitioning of the electron density are computed for a large series of molecules. In a representative set of organic and hypervalent molecules, they are compared with other commonly used population analysis methods. The expected bond polarities are recovered, but the charges are much smaller compared to other methods. Condensed Fukui functions for a large number of molecules, undergoing an electrophilic or a nucleophilic attack, are computed and compared with the HOMO and LUMO densities, integrated over the Hirshfeld atoms in molecules. PMID:12116389
Electronic states of alkyl-radical-functionalized C20 fullerene using density functional theory
NASA Astrophysics Data System (ADS)
Abe, Shigeaki; Kawano, Shimpei; Toida, Yu; Nakamura, Mariko; Inoue, Satoshi; Sano, Hidehiko; Yoshida, Yasuhiro; Kawabata, Hiroshi; Tachikawa, Hiroto
2016-03-01
The structures and electronic states of alkyl-radical-functionalized C20 fullerenes (denoted by C20-R) have been investigated using density functional theory (DFT). The different alkyl radicals investigated were methyl, ethyl, propyl, and butyl radicals. The DFT calculation indicated that the alkyl radical binds to the carbon atom of C20 in the on-top site, thus forming a strong C-C single bond. The binding energies of the alkyl radicals to C20 were calculated to be 83.9-86.6 kcal/mol at the CAM-B3LYP/6-311G(d,p) level. The electronic states of the C20-R complex are discussed on the basis of the theoretical results.
Electronic states of aryl radical functionalized graphenes: Density functional theory study
NASA Astrophysics Data System (ADS)
Tachikawa, Hiroto; Kawabata, Hiroshi
2016-06-01
Functionalized graphenes are known as a high-performance molecular device. In the present study, the structures and electronic states of the aryl radical functionalized graphene have been investigated by the density functional theory (DFT) method to elucidate the effects of functionalization on the electronic states of graphene (GR). Also, the mechanism of aryl radical reaction with GR was investigated. The benzene, biphenyl, p-terphenyl, and p-quaterphenyl radicals [denoted by (Bz) n (n = 1–4), where n means numbers of benzene rings in aryl radical] were examined as aryl radicals. The DFT calculation of GR–(Bz) n (n = 1–4) showed that the aryl radical binds to the carbon atom of GR, and a C–C single bond was formed. The binding energies of aryl radicals to GR were calculated to be ca. 6.0 kcal mol‑1 at the CAM-B3LYP/6-311G(d,p) level. It was found that the activation barrier exists in the aryl radical addition: the barrier heights were calculated to be 10.0 kcal mol‑1. The electronic states of GR–(Bz) n were examined on the basis of theoretical results.
Many-electron expansion: A density functional hierarchy for strongly correlated systems
NASA Astrophysics Data System (ADS)
Zhu, Tianyu; de Silva, Piotr; van Aggelen, Helen; Van Voorhis, Troy
2016-05-01
Density functional theory (DFT) is the de facto method for the electronic structure of weakly correlated systems. But for strongly correlated materials, common density functional approximations break down. Here, we derive a many-electron expansion (MEE) in DFT that accounts for successive one-, two-, three-, ... particle interactions within the system. To compute the correction terms, the density is first decomposed into a sum of localized, nodeless one-electron densities (ρi). These one-electron densities are used to construct relevant two- (ρi+ρj ), three- (ρi+ρj+ρk ), ... electron densities. Numerically exact results for these few-particle densities can then be used to correct an approximate density functional via any of several many-body expansions. We show that the resulting hierarchy gives accurate results for several important model systems: the Hubbard and Peierls-Hubbard models in 1D and the pure Hubbard model in 2D. We further show that the method is numerically convergent for strongly correlated systems: applying successively higher order corrections leads to systematic improvement of the results. MEE thus provides a hierarchy of density functional approximations that applies to both weakly and strongly correlated systems.
Reimers, Jeffrey R; Solomon, Gemma C; Gagliardi, Alessio; Bilić, Ante; Hush, Noel S; Frauenheim, Thomas; Di Carlo, Aldo; Pecchia, Alessandro
2007-07-01
A review is presented of the nonequilibrium Green's function (NEGF) method "gDFTB" for evaluating elastic and inelastic conduction through single molecules employing the density functional tight-binding (DFTB) electronic structure method. This focuses on the possible advantages that DFTB implementations of NEGF have over conventional methods based on density functional theory, including not only the ability to treat large irregular metal-molecule junctions with high nonequilibrium thermal distributions but perhaps also the ability to treat dispersive forces, bond breakage, and open-shell systems and to avoid large band lineup errors. New results are presented indicating that DFTB provides a useful depiction of simple gold-thiol interactions. Symmetry is implemented in DFTB, and the advantages it brings in terms of large savings of computational resources with significant increase in numerical stability are described. The power of DFTB is then harnessed to allow the use of gDFTB as a real-time tool to discover the nature of the forces that control inelastic charge transport through molecules and the role of molecular symmetry in determining both elastic and inelastic transport. Future directions for the development of the method are discussed. PMID:17530826
Putz, Mihai V.
2009-01-01
The density matrix theory, the ancestor of density functional theory, provides the immediate framework for Path Integral (PI) development, allowing the canonical density be extended for the many-electronic systems through the density functional closure relationship. Yet, the use of path integral formalism for electronic density prescription presents several advantages: assures the inner quantum mechanical description of the system by parameterized paths; averages the quantum fluctuations; behaves as the propagator for time-space evolution of quantum information; resembles Schrödinger equation; allows quantum statistical description of the system through partition function computing. In this framework, four levels of path integral formalism were presented: the Feynman quantum mechanical, the semiclassical, the Feynman-Kleinert effective classical, and the Fokker-Planck non-equilibrium ones. In each case the density matrix or/and the canonical density were rigorously defined and presented. The practical specializations for quantum free and harmonic motions, for statistical high and low temperature limits, the smearing justification for the Bohr’s quantum stability postulate with the paradigmatic Hydrogen atomic excursion, along the quantum chemical calculation of semiclassical electronegativity and hardness, of chemical action and Mulliken electronegativity, as well as by the Markovian generalizations of Becke-Edgecombe electronic focalization functions – all advocate for the reliability of assuming PI formalism of quantum mechanics as a versatile one, suited for analytically and/or computationally modeling of a variety of fundamental physical and chemical reactivity concepts characterizing the (density driving) many-electronic systems. PMID:20087467
Sun, Jianwei; Perdew, John P; Yang, Zenghui; Peng, Haowei
2016-05-21
The uniform electron gas and the hydrogen atom play fundamental roles in condensed matter physics and quantum chemistry. The former has an infinite number of electrons uniformly distributed over the neutralizing positively charged background, and the latter only one electron bound to the proton. The uniform electron gas was used to derive the local spin density approximation to the exchange-correlation functional that undergirds the development of the Kohn-Sham density functional theory. We show here that the ground-state exchange-correlation energies of the hydrogen atom and many other 1- and 2-electron systems are modeled surprisingly well by a different local spin density approximation (LSDA0). LSDA0 is constructed to satisfy exact constraints but agrees surprisingly well with the exact results for a uniform two-electron density in a finite, curved three-dimensional space. We also apply LSDA0 to excited or noded 1-electron densities, where it works less well. Furthermore, we show that the localization of the exact exchange hole for a 1- or 2-electron ground state can be measured by the ratio of the exact exchange energy to its optimal lower bound. PMID:27208927
NASA Astrophysics Data System (ADS)
Sun, Jianwei; Perdew, John P.; Yang, Zenghui; Peng, Haowei
2016-05-01
The uniform electron gas and the hydrogen atom play fundamental roles in condensed matter physics and quantum chemistry. The former has an infinite number of electrons uniformly distributed over the neutralizing positively charged background, and the latter only one electron bound to the proton. The uniform electron gas was used to derive the local spin density approximation to the exchange-correlation functional that undergirds the development of the Kohn-Sham density functional theory. We show here that the ground-state exchange-correlation energies of the hydrogen atom and many other 1- and 2-electron systems are modeled surprisingly well by a different local spin density approximation (LSDA0). LSDA0 is constructed to satisfy exact constraints but agrees surprisingly well with the exact results for a uniform two-electron density in a finite, curved three-dimensional space. We also apply LSDA0 to excited or noded 1-electron densities, where it works less well. Furthermore, we show that the localization of the exact exchange hole for a 1- or 2-electron ground state can be measured by the ratio of the exact exchange energy to its optimal lower bound.
Guido, Ciro A. Cortona, Pietro; Adamo, Carlo; Institut Universitaire de France, 103 Bd Saint-Michel, F-75005 Paris
2014-03-14
We extend our previous definition of the metric Δr for electronic excitations in the framework of the time-dependent density functional theory [C. A. Guido, P. Cortona, B. Mennucci, and C. Adamo, J. Chem. Theory Comput. 9, 3118 (2013)], by including a measure of the difference of electronic position variances in passing from occupied to virtual orbitals. This new definition, called Γ, permits applications in those situations where the Δr-index is not helpful: transitions in centrosymmetric systems and Rydberg excitations. The Γ-metric is then extended by using the Natural Transition Orbitals, thus providing an intuitive picture of how locally the electron density changes during the electronic transitions. Furthermore, the Γ values give insight about the functional performances in reproducing different type of transitions, and allow one to define a “confidence radius” for GGA and hybrid functionals.
Nishimoto, Yoshio
2015-09-07
We develop a formalism for the calculation of excitation energies and excited state gradients for the self-consistent-charge density-functional tight-binding method with the third-order contributions of a Taylor series of the density functional theory energy with respect to the fluctuation of electron density (time-dependent density-functional tight-binding (TD-DFTB3)). The formulation of the excitation energy is based on the existing time-dependent density functional theory and the older TD-DFTB2 formulae. The analytical gradient is computed by solving Z-vector equations, and it requires one to calculate the third-order derivative of the total energy with respect to density matrix elements due to the inclusion of the third-order contributions. The comparison of adiabatic excitation energies for selected small and medium-size molecules using the TD-DFTB2 and TD-DFTB3 methods shows that the inclusion of the third-order contributions does not affect excitation energies significantly. A different set of parameters, which are optimized for DFTB3, slightly improves the prediction of adiabatic excitation energies statistically. The application of TD-DFTB for the prediction of absorption and fluorescence energies of cresyl violet demonstrates that TD-DFTB3 reproduced the experimental fluorescence energy quite well.
Nishimoto, Yoshio
2015-09-01
We develop a formalism for the calculation of excitation energies and excited state gradients for the self-consistent-charge density-functional tight-binding method with the third-order contributions of a Taylor series of the density functional theory energy with respect to the fluctuation of electron density (time-dependent density-functional tight-binding (TD-DFTB3)). The formulation of the excitation energy is based on the existing time-dependent density functional theory and the older TD-DFTB2 formulae. The analytical gradient is computed by solving Z-vector equations, and it requires one to calculate the third-order derivative of the total energy with respect to density matrix elements due to the inclusion of the third-order contributions. The comparison of adiabatic excitation energies for selected small and medium-size molecules using the TD-DFTB2 and TD-DFTB3 methods shows that the inclusion of the third-order contributions does not affect excitation energies significantly. A different set of parameters, which are optimized for DFTB3, slightly improves the prediction of adiabatic excitation energies statistically. The application of TD-DFTB for the prediction of absorption and fluorescence energies of cresyl violet demonstrates that TD-DFTB3 reproduced the experimental fluorescence energy quite well. PMID:26342360
Density functional theory for d- and f-electron materials and compounds
Mattson, Ann E.; Wills, John M.
2016-02-12
Here, the fundamental requirements for a computationally tractable Density Functional Theory-based method for relativistic f- and (nonrelativistic) d-electron materials and compounds are presented. The need for basing the Kohn–Sham equations on the Dirac equation is discussed. The full Dirac scheme needs exchange-correlation functionals in terms of four-currents, but ordinary functionals, using charge density and spin-magnetization, can be used in an approximate Dirac treatment. The construction of a functional that includes the additional confinement physics needed for these materials is illustrated using the subsystem-functional scheme. If future studies show that a full Dirac, four-current based, exchange-correlation functional is needed, the subsystemmore » functional scheme is one of the few schemes that can still be used for constructing functional approximations.« less
Long-range density-matrix-functional theory: Application to a modified homogeneous electron gas
Pernal, Katarzyna
2010-05-15
We propose a method that employs functionals of the one-electron reduced density matrix (density matrix) to capture long-range effects of electron correlation. The complementary short-range regime is treated with density functionals. In an effort to find approximations for the long-range density-matrix functional, a modified power functional is applied to the homogeneous electron gas with Coulomb interactions replaced by their corresponding long-range counterparts. For the power {beta}=1/2 and the range-separation parameter {omega}=1/r{sub s}, the functional reproduces the correlation and the kinetic correlation energies with a remarkable accuracy for intermediate and large values of r{sub s}. Analysis of the Euler equation corresponding to this functional reveals correct r{sub s} expansion of the correlation energy in the limit of large r{sub s}. The first expansion coefficient is in very good agreement with that obtained from the modified Wigner-Seitz model.
Lehtovaara, Lauri; Havu, Ville; Puska, Martti
2011-10-21
We present an all-electron method for time-dependent density functional theory which employs hierarchical nonuniform finite-element bases and the time-propagation approach. The method is capable of treating linear and nonlinear response of valence and core electrons to an external field. We also introduce (i) a preconditioner for the propagation equation, (ii) a stable way to implement absorbing boundary conditions, and (iii) a new kind of absorbing boundary condition inspired by perfectly matched layers. PMID:22029294
Density functional theory for low-energy electron-molecule scattering
NASA Astrophysics Data System (ADS)
Burke, Kieron; Wasserman, Adam
2004-09-01
Time-dependent density functional theory (TDDFT) is becoming popular as an approach to time-dependent electronic problems[1]. In the weak field regime, TDDFT predicts electronic transition frequencies and optical spectra of atoms, molecules, clusters, and solids, with an accuracy comparable to high-level wavefunction calculations at a fraction of the computational cost[2]. For large systems, TDDFT is the method of choice. Given the importance of correlation effects in low-energy electron-molecule scattering, extracting scattering amplitudes from TDDFT appears desirable. I will review this background, and outline how this can be done[3]. Detailed results will be shown by Wasserman in another talk. [1] Time-Dependent Density Functional Theory, M.A.L. Marques and E.K.U. Gross, Annu. Rev. Phys. Chem. 55, 427 (2004). [2] Time-dependent density functional theory in quantum chemistry, F. Furche and K. Burke, to appear in 1st vol. of Annu. Rev. of Computational Chemistry (2004) [3] Electron-molecule scattering from time-dependent density functional theory A. Wasserman, N.T. Maitra, and K. Burke, submitted (see http:dft.rutgers.edu/pubs/publist.html).
Employing homogeneity properties of density functionals to determine the total electronic energy
NASA Astrophysics Data System (ADS)
Morrison, Robert C.; Parr, Robert G.
1996-05-01
The exchange-correlation energy is calculated from expressions for (Exc+Tc)/2 and (Exc-Tc)/2. The expression for Exc+Tc is known exactly. The expression for Exc-Tc is not known exactly and we approximate Gxc-Tc, where Gxc=Exc+J/N, by assuming that it is a functional that is homogeneous of degree 1 in the electron density and that Tc is homogeneous of degree 0 in the electron density. The resulting formula for the total energy reduces the average of the errors in the Parr and Ghosh
NASA Astrophysics Data System (ADS)
Farzanehpour, Mehdi; Tokatly, Ilya; Nano-Bio Spectroscopy Group; ETSF Scientific Development Centre Team
2015-03-01
We present a rigorous formulation of the time-dependent density functional theory for interacting lattice electrons strongly coupled to cavity photons. We start with an example of one particle on a Hubbard dimer coupled to a single photonic mode, which is equivalent to the single mode spin-boson model or the quantum Rabi model. For this system we prove that the electron-photon wave function is a unique functional of the electronic density and the expectation value of the photonic coordinate, provided the initial state and the density satisfy a set of well defined conditions. Then we generalize the formalism to many interacting electrons on a lattice coupled to multiple photonic modes and prove the general mapping theorem. We also show that for a system evolving from the ground state of a lattice Hamiltonian any density with a continuous second time derivative is locally v-representable. Spanish Ministry of Economy and Competitiveness (Grant No. FIS2013-46159-C3-1-P), Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT578-13), COST Actions CM1204 (XLIC) and MP1306 (EUSpec).
Time-Dependent Electronic Populations in Fragment-Based Time-Dependent Density Functional Theory.
Mosquera, Martín A; Wasserman, Adam
2015-08-11
Conceiving a molecule as being composed of smaller molecular fragments, or subunits, is one of the pillars of the chemical and physical sciences and leads to productive methods in quantum chemistry. Using a fragmentation scheme, efficient algorithms can be proposed to address problems in the description of chemical bond formation and breaking. We present a formally exact time-dependent density functional theory for the electronic dynamics of molecular fragments with a variable number of electrons. This new formalism is an extension of previous work [Phys. Rev. Lett. 111, 023001 (2013)]. We also introduce a stable density-inversion method that is applicable to time-dependent and ground-state density functional theories and their extensions, including those discussed in this work. PMID:26574438
Sun, Shih-Jye; Lin, Ken-Huang; Li, Jia-Yun; Ju, Shin-Pon
2014-10-07
The simulated annealing basin-hopping method incorporating the penalty function was used to predict the lowest-energy structures for ultrathin tungsten nanowires and nanotubes of different sizes. These predicted structures indicate that tungsten one-dimensional structures at this small scale do not possess B.C.C. configuration as in bulk tungsten material. In order to analyze the relationship between multi-shell geometries and electronic transfer, the electronic and structural properties of tungsten wires and tubes including partial density of state and band structures which were determined and analyzed by quantum chemistry calculations. In addition, in order to understand the application feasibility of these nanowires and tubes on nano-devices such as field emitters or chemical catalysts, the electronic stability of these ultrathin tungsten nanowires was also investigated by density functional theory calculations.
Tait, E W; Ratcliff, L E; Payne, M C; Haynes, P D; Hine, N D M
2016-05-18
Experimental techniques for electron energy loss spectroscopy (EELS) combine high energy resolution with high spatial resolution. They are therefore powerful tools for investigating the local electronic structure of complex systems such as nanostructures, interfaces and even individual defects. Interpretation of experimental electron energy loss spectra is often challenging and can require theoretical modelling of candidate structures, which themselves may be large and complex, beyond the capabilities of traditional cubic-scaling density functional theory. In this work, we present functionality to compute electron energy loss spectra within the onetep linear-scaling density functional theory code. We first demonstrate that simulated spectra agree with those computed using conventional plane wave pseudopotential methods to a high degree of precision. The ability of onetep to tackle large problems is then exploited to investigate convergence of spectra with respect to supercell size. Finally, we apply the novel functionality to a study of the electron energy loss spectra of defects on the (1 0 1) surface of an anatase slab and determine concentrations of defects which might be experimentally detectable. PMID:27094207
NASA Astrophysics Data System (ADS)
Tait, E. W.; Ratcliff, L. E.; Payne, M. C.; Haynes, P. D.; Hine, N. D. M.
2016-05-01
Experimental techniques for electron energy loss spectroscopy (EELS) combine high energy resolution with high spatial resolution. They are therefore powerful tools for investigating the local electronic structure of complex systems such as nanostructures, interfaces and even individual defects. Interpretation of experimental electron energy loss spectra is often challenging and can require theoretical modelling of candidate structures, which themselves may be large and complex, beyond the capabilities of traditional cubic-scaling density functional theory. In this work, we present functionality to compute electron energy loss spectra within the onetep linear-scaling density functional theory code. We first demonstrate that simulated spectra agree with those computed using conventional plane wave pseudopotential methods to a high degree of precision. The ability of onetep to tackle large problems is then exploited to investigate convergence of spectra with respect to supercell size. Finally, we apply the novel functionality to a study of the electron energy loss spectra of defects on the (1 0 1) surface of an anatase slab and determine concentrations of defects which might be experimentally detectable.
Density functional calculation of the structural and electronic properties of germanium quantum dots
Anas, M. M.; Gopir, G.
2015-04-24
We apply first principles density functional computational methods to study the structures, densities of states (DOS), and higher occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) gaps of selected free-standing Ge semiconductor quantum dots up to 1.8nm. Our calculations are performed using numerical atomic orbital approach where linear combination of atomic orbital was applied. The surfaces of the quantum dots was passivized by hydrogen atoms. We find that surface passivation does affect the electronic properties associated with the changes of surface state, electron localization, and the energy gaps of germanium nanocrystals as well as the confinement of electrons inside the quantum dots (QDs). Our study shows that the energy gaps of germanium quantum dots decreases with the increasing dot diameter. The size-dependent variations of the computed HOMO-LUMO gaps in our quantum dots model were found to be consistent with the effects of quantum confinement reported in others theoretical and experimental calculation.
Density Functional Theory of Structural and Electronic Properties of III-N Semiconductors
Guerel, H. Hakan; Akinci, Oezden; Uenlue, Hilmi
2010-11-01
In this wok, we present the density functional theory (DFT) calculations of cubic III-N based semiconductors by using the full potential linear augmented plane-wave method plus local orbitals as implemented in the WIEN2k code. Our aim is to predict the pressure effect on structural and electronic properties of III-N binaries and ternaries. Results are given for structural properties (e.g., lattice constant, elastic constants, bulk modulus, and its pressure derivative) and electronic properties (e.g., band structure, density of states, band gaps and band widths) of GaAs, GaN, AlN, and InN binaries and GaAsN ternaries. The proposed model uses GGA exchange-correlation potential to determine band gaps of semiconductors at {Gamma}, L and X high symmetry points of Brillouin zone. The results are found in good agreement with available experimental data for structural and electronic properties of these semiconductors.
Density functional theory description of electronic properties of wurtzite zinc oxide
NASA Astrophysics Data System (ADS)
Franklin, L.; Ekuma, C. E.; Zhao, G. L.; Bagayoko, D.
2013-05-01
We report calculated, electronic properties of wurtzite zinc oxide (w-ZnO). We solved self-consistently the two inherently coupled equations of density functional theory (DFT), following the Bagayoko, Zhao, and Williams (BZW) method as enhanced by the work of Ekuma and Franklin (BZW-EF). We employed a local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO). Most of the calculated, electronic properties of w-ZnO are in excellent agreement with experiment, including our zero temperature band gap of 3.39 eV and the electron effective mass. The doubly self-consistent approach utilized in this work points to the ability of theory to predict accurately key properties of semiconductors and hence to inform and to guide the design and fabrication of semiconductor-based devices.
Angyan, Janos G.; Gerber, Iann C.; Savin, Andreas; Toulouse, Julien
2005-07-15
Long-range exchange and correlation effects, responsible for the failure of currently used approximate density functionals in describing van der Waals forces, are taken into account explicitly after a separation of the electron-electron interaction in the Hamiltonian into short- and long-range components. We propose a 'range-separated hybrid' functional based on a local density approximation for the short-range exchange-correlation energy, combined with a long-range exact exchange energy. Long-range correlation effects are added by a second-order perturbational treatment. The resulting scheme is general and is particularly well adapted to describe van der Waals complexes, such as rare gas dimers.
Density functional study on electronic properties of P-doped spinel silicon carbon nitride
NASA Astrophysics Data System (ADS)
Zhang, Yufen; Zhao, Xian; Cheng, Xiufeng; Mu, Yuguang
2008-08-01
We performed density functional calculations on the electronic properties of P-doped spinel silicon carbon nitride. When Si is replaced by C at the tetrahedral sites of P-doped c-Si 3N 4, the band gap can be adjusted, and an insulator-to-metal transition is predicted to occur at the C-to-Si ratio of 0.27. Finally, some possible examinations and potential applications for the large band-gap reduction are discussed.
NASA Astrophysics Data System (ADS)
Ruggenthaler, Michael; Flick, Johannes; Pellegrini, Camilla; Appel, Heiko; Tokatly, Ilya V.; Rubio, Angel
2014-07-01
In this work, we give a comprehensive derivation of an exact and numerically feasible method to perform ab initio calculations of quantum particles interacting with a quantized electromagnetic field. We present a hierarchy of density-functional-type theories that describe the interaction of charged particles with photons and introduce the appropriate Kohn-Sham schemes. We show how the evolution of a system described by quantum electrodynamics in Coulomb gauge is uniquely determined by its initial state and two reduced quantities. These two fundamental observables, the polarization of the Dirac field and the vector potential of the photon field, can be calculated by solving two coupled, nonlinear evolution equations without the need to explicitly determine the (numerically infeasible) many-body wave function of the coupled quantum system. To find reliable approximations to the implicit functionals, we present the appropriate Kohn-Sham construction. In the nonrelativistic limit, this density-functional-type theory of quantum electrodynamics reduces to the density-functional reformulation of the Pauli-Fierz Hamiltonian, which is based on the current density of the electrons and the vector potential of the photon field. By making further approximations, e.g., restricting the allowed modes of the photon field, we derive further density-functional-type theories of coupled matter-photon systems for the corresponding approximate Hamiltonians. In the limit of only two sites and one mode we deduce the appropriate effective theory for the two-site Hubbard model coupled to one photonic mode. This model system is used to illustrate the basic ideas of a density-functional reformulation in great detail and we present the exact Kohn-Sham potentials for our coupled matter-photon model system.
NASA Astrophysics Data System (ADS)
Gong, Sai; Liu, Bang-Gui
2012-05-01
TiO2 has been recently used to realize high-temperature ferromagnetic semiconductors. In fact, it has been widely used for a long time as white pigment and sunscreen because of its whiteness, high refractive index, and excellent optical properties. However, its electronic structures and the related properties have not been satisfactorily understood. Here, we use Tran and Blaha's modified Becke-Johnson (TB-mBJ) exchange potential (plus a local density approximation correlation potential) within the density functional theory to investigate electronic structures and optical properties of rutile and anatase TiO2. Our comparative calculations show that the energy gaps obtained from mBJ method agree better with the experimental results than that obtained from local density approximation (LDA) and generalized gradient approximation (GGA), in contrast with substantially overestimated values from many-body perturbation (GW) calculations. As for optical dielectric functions (both real and imaginary parts), refractive index, and extinction coefficients as functions of photon energy, our mBJ calculated results are in excellent agreement with the experimental curves. Our further analysis reveals that these excellent improvements are achieved because mBJ potential describes accurately the energy levels of Ti 3d states. These results should be helpful to understand the high temperature ferromagnetism in doped TiO2. This approach can be used as a standard to understand electronic structures and the related properties of such materials as TiO2.
Electron affinities for rare gases and some actinides from local-spin-density-functional theory
Guo, Y.; Wrinn, M.C.; Whitehead, M.A. )
1989-12-01
The negative ions of the rare gases (He, Ne, Ar, Kr, Xe, and Rn) and some actinides (Pu, Am, Bk, Cf, and Es) have been calculated self-consistently by the generalized exchange local-spin-density-functional theory with self-interaction correction and correlation. The electron affinities were obtained as the differences between the statistical total energies of the negative ions and neutral atoms; the electron affinities were positive around several millirydbergs. Consequently, the negative ions are predicted stable for the rare gases and actinides.
A hybrid density functional study on the electron and hole trap states in anatase titanium dioxide.
Yamamoto, Takenori; Ohno, Takahisa
2012-01-14
We present a theoretical study on electron and hole trap states in the bulk and (001) surface of anatase titanium dioxide using screened hybrid density functional calculations. In both the bulk and surface, calculations suggest that the neutral and ionized oxygen vacancies are possible electron traps. The doubly ionized oxygen vacancy is the most stable in the bulk, and is a candidate for a shallow donor in colorless anatase crystals. The hole trap states are localized at oxygen anions in both the bulk and surface. The self-trapped electron centered at a titanium cation cannot be produced in the bulk, but can be formed at the surface. The electron trap level at the surface oxygen vacancy is consistent with observations by photoelectron spectroscopy. The optical absorptions and luminescence in UV-irradiated anatase nanoparticles are found to come from the surface self-trapped hole and the surface oxygen vacancy. PMID:22127526
Corsini, Niccolò R. C. Greco, Andrea; Haynes, Peter D.; Hine, Nicholas D. M.; Molteni, Carla
2013-08-28
We present an implementation in a linear-scaling density-functional theory code of an electronic enthalpy method, which has been found to be natural and efficient for the ab initio calculation of finite systems under hydrostatic pressure. Based on a definition of the system volume as that enclosed within an electronic density isosurface [M. Cococcioni, F. Mauri, G. Ceder, and N. Marzari, Phys. Rev. Lett.94, 145501 (2005)], it supports both geometry optimizations and molecular dynamics simulations. We introduce an approach for calibrating the parameters defining the volume in the context of geometry optimizations and discuss their significance. Results in good agreement with simulations using explicit solvents are obtained, validating our approach. Size-dependent pressure-induced structural transformations and variations in the energy gap of hydrogenated silicon nanocrystals are investigated, including one comparable in size to recent experiments. A detailed analysis of the polyamorphic transformations reveals three types of amorphous structures and their persistence on depressurization is assessed.
NASA Astrophysics Data System (ADS)
Dimakis, Nicholas; Navarro, Nestor E.; Velazquez, Julian; Salgado, Andres
2015-04-01
Periodic density functional calculations on graphene monolayers with and without an iron adatom have been used to elucidate iron-graphene adsorption and its effects on graphene electronic and vibrational properties. Density-of-states calculations and charge density contour plots reveal charge transfer from the iron s orbitals to the d orbitals, in agreement with past reports. Adsorbed iron atoms covalently bind to the graphene substrate, verified by the strong hybridization of iron d-states with the graphene bands in the energy region just below the Fermi level. This adsorption is weak and compared to the well-analyzed CO adsorption on Pt: It is indicated by its small adsorption energy and the minimal change of the substrate geometry due to the presence of the iron adatoms. Graphene vibrational spectra are analyzed though a systematic variation of the graphene supercell size. The shifts of graphene most prominent infrared active vibrational modes due to iron adsorption are explored using normal mode eigenvectors.
Extracting the density profile of an electronic wave function in a quantum dot
NASA Astrophysics Data System (ADS)
Boyd, Erin E.; Westervelt, Robert M.
2011-11-01
We use a model of a one-dimensional nanowire quantum dot to demonstrate the feasibility of a scanning probe microscope (SPM) imaging technique that can extract both the energy of an electron state and the amplitude of its wave function using a single instrument. This imaging technique can probe electrons that are buried beneath the surface of a low-dimensional semiconductor structure and provide valuable information for the design of quantum devices. A conducting SPM tip, acting as a movable gate, measures the energy of an electron state using Coulomb blockade spectroscopy. When the tip is close to the nanowire dot, it dents the wave function Ψ(x) of the quantum state, changing the electron's energy by an amount proportional to |Ψ(x)|2. By recording the change in energy as the SPM tip is moved along the length of the dot, the density profile of the electronic wave function can be found along the length of the quantum dot.
NASA Astrophysics Data System (ADS)
Basurto, Luis; Zope, Rajendra R.; Baruah, Tunna
2016-05-01
We report an electronic structure study of a multichromophoric molecular complex containing two of each borondipyrromethane dye, Zn-tetraphenyl-porphyrin, bisphenyl anthracene and a fullerene. The snowflake shaped molecule behaves like an antenna capturing photon at different frequencies and transferring the photon energy to the porphyrin where electron transfer occurs from the porphyrin to the fullerene. The study is performed within density functional formalism using large polarized Guassian basis sets (12,478 basis functions in total). The energies of the HOMO and LUMO states in the complex, as adjudged by the ionization potential and the electron affinity values, show significant differences with respect to their values in participating subunits in isolation. These differences are also larger than the variations of the ionization potential and electron affinity values observed in non-bonded C60-ZnTPP complexes in co-facial arrangement or end-on orientations. An understanding of the origin of these differences is obtained by a systematic study of the effect of structural strain, the presence of ligands, the effect of orbital delocalization on the ionization energy and the electron affinity. Finally, a few lowest charge transfer energies involving electronic transitions from the porphyrin component to the fullerene subunit of the complex are predicted.
Applications of methods beyond density functional theory to the study of correlated electron systems
NASA Astrophysics Data System (ADS)
Sims, Hunter Robert
The difficulty in accurately treating systems in which electron-electron interactions are the dominant physics has plagued condensed matter physics for decades. Currently, there exist many different computational techniques designed to improve upon density functional theory to varying degrees of accuracy. To date, no unified, parameter-free method exists that is guaranteed to yield the correct answer for all materials. Consequently, proper treatment of such systems often requires a combination of several methods, allowing one to check them against one another when their regions of validity overlap and to expand one's reach when a single method cannot reliably describe all of the physics at work. In this dissertation, I present discussion and, when appropriate, brief derivations of several of the most prominent electronic structure methods currently in use---from the local density approximation through LDA+DMFT. I then present several investigations into the electronic and magnetic structure of materials of potential interest for information technology that also illustrate the current state of affairs in computational condensed matter physics. I explore the intersite exchange interactions in CrO2 within density functional theory (with and without Hubbard "+U" corrections) and evaluate these results through analytic and numerical means. I study the dependence of the mysterious magnetization of Fe16N2 on crystal and electronic structure and employ a wide range of techniques in an attempt to bring greater rigor and deeper understanding to the widely-varying reports on this material. In conjunction with others' careful experimental analysis, I provide a picture of the band structure of the magnetic insulator NiFe2O4 that reveals a novel hierarchy in its band gaps and suggests applications in spintronics and possibly other areas. Finally, I employ dynamical mean-field theory to study the behavior of impurity states in elemental semiconductors, using H impurities in Ge as
Density functional study on electronic properties of P-doped spinel silicon carbon nitride
Zhang Yufen; Zhao Xian Cheng Xiufeng; Mu Yuguang
2008-08-15
We performed density functional calculations on the electronic properties of P-doped spinel silicon carbon nitride. When Si is replaced by C at the tetrahedral sites of P-doped c-Si{sub 3}N{sub 4}, the band gap can be adjusted, and an insulator-to-metal transition is predicted to occur at the C-to-Si ratio of 0.27. Finally, some possible examinations and potential applications for the large band-gap reduction are discussed. - Graphical abstract: We performed density functional calculations to predict the insulator-to-metal transition by replacing Si by C at the tetrahedral sites of P-doped c-Si{sub 3}N{sub 4}.
NASA Astrophysics Data System (ADS)
Prasad, O.; Sinha, L.; Misra, N.; Narayan, V.; Kumar, N.; Kumar, A.
2010-09-01
The present work deals with the structural, electronic, and vibrational analysis of rivastigmine. Rivastigmine, an antidementia medicament, is credited with significant therapeutic effects on the cognitive, functional, and behavioural problems that are commonly associated with Alzheimer’s dementia. For rivastigmine, a number of minimum energy conformations are possible. The geometry of twelve possible conformers has been analyzed and the most stable conformer was further optimized at a higher basis set. The electronic properties and vibrational frequencies were then calculated using a density functional theory at the B3LYP level with the 6-311+G(d, p) basis set. The different molecular surfaces have also been drawn to understand the activity of the molecule. A narrower frontier orbital energy gap in rivastigmine makes it softer and more reactive than water and dimethylfuran. The calculated value of the dipole moment is 2.58 debye.
Li, Zun-Yun; Wang, Hai-Long; Lu, Tong-Tong; He, Tian-Jing; Liu, Fan-Chen; Chen, Dong-Ming
2007-08-01
The ground-state structure and electronic and vibrational spectra of octaethylporphyrin diacid (H4OEP2+) have been studied with the density functional theory. The geometrical parameters computed with B3LYP, PBE1PBE and mPW1PW91 functionals and 6-31G* basis sets are well consistent with the experimental values. Electronic absorption spectrum of H4OEP2+ has been studied with the time-dependent DFT method, and the calculated excitation energies and oscillator strengths are compared with the experimental results. The Raman and IR spectra of H4OEP2+ and the Raman spectrum of its N-deuterated analogue (D4OEP2+) were measured. The observed Raman and IR bands have been assigned based on the frequency calculations at the B3LYP/6-31G* level of theory. PMID:17142087
Hao, Feng Mattsson, Ann E.; Armiento, Rickard
2014-05-14
We have previously proposed that further improved functionals for density functional theory can be constructed based on the Armiento-Mattsson subsystem functional scheme if, in addition to the uniform electron gas and surface models used in the Armiento-Mattsson 2005 functional, a model for the strongly confined electron gas is also added. However, of central importance for this scheme is an index that identifies regions in space where the correction provided by the confined electron gas should be applied. The electron localization function (ELF) is a well-known indicator of strongly localized electrons. We use a model of a confined electron gas based on the harmonic oscillator to show that regions with high ELF directly coincide with regions where common exchange energy functionals have large errors. This suggests that the harmonic oscillator model together with an index based on the ELF provides the crucial ingredients for future improved semi-local functionals. For a practical illustration of how the proposed scheme is intended to work for a physical system we discuss monoclinic cupric oxide, CuO. A thorough discussion of this system leads us to promote the cell geometry of CuO as a useful benchmark for future semi-local functionals. Very high ELF values are found in a shell around the O ions, and take its maximum value along the Cu–O directions. An estimate of the exchange functional error from the effect of electron confinement in these regions suggests a magnitude and sign that could account for the error in cell geometry.
Actinide electronic structure based on the Dirac equation and density functional theory
NASA Astrophysics Data System (ADS)
Wills, John M.; Mattsson, Ann E.
2013-03-01
Density functional theory (DFT) provides a formally predictive basis for predicting the structural properties of actinides. Although available approximations to the exchange/correlation functional provide accurate predictions for many materials, they fail qualitatively and sometimes quantitatively when applied to actinides. Major contributors to this deficiency are an inadequate treatment of confinement physics and an incomplete treatment of relativity in the underlying equations. The development of a functional correctly incorporating confinement physics with a proper treatment of relativity would provide definitive, internally consistent predictions of actinide properties. To enable the development of such a functional and quantify the predictions of currently available functionals, we have developed an efficient first-principles electronic structure method based on the Dirac equation. Results are compared with current methods, and the implications for relativistic density functionals discussed. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
Dale, Stephen G.; Johnson, Erin R.
2015-11-14
Exploration of the solvated electron phenomena using density-functional theory (DFT) generally results in prediction of a localised electron within an induced solvent cavity. However, it is well known that DFT favours highly delocalised charges, rendering the localisation of a solvated electron unexpected. We explore the origins of this counterintuitive behaviour using a model Kevan-structure system. When a polarisable-continuum solvent model is included, it forces electron localisation by introducing a strong energetic bias that favours integer charges. This results in the formation of a large energetic barrier for charge-hopping and can cause the self-consistent field to become trapped in local minima thus converging to stable solutions that are higher in energy than the ground electronic state. Finally, since the bias towards integer charges is caused by the polarisable continuum, these findings will also apply to other classical polarisation corrections, as in combined quantum mechanics and molecular mechanics (QM/MM) methods. The implications for systems beyond the solvated electron, including cationic DNA bases, are discussed.
Two-electron Rabi oscillations in real-time time-dependent density-functional theory
Habenicht, Bradley F.; Tani, Noriyuki P.; Provorse, Makenzie R.; Isborn, Christine M.
2014-11-14
We investigate the Rabi oscillations of electrons excited by an applied electric field in several simple molecular systems using time-dependent configuration interaction (TDCI) and real-time time-dependent density-functional theory (RT-TDDFT) dynamics. While the TDCI simulations exhibit the expected single-electron Rabi oscillations at a single resonant electric field frequency, Rabi oscillations in the RT-TDDFT simulations are a two-electron process. The existence of two-electron Rabi oscillations is determined both by full population inversion between field-free molecular orbitals and the behavior of the instantaneous dipole moment during the simulations. Furthermore, the Rabi oscillations in RT-TDDFT are subject to an intensity threshold of the electric field, below which Rabi oscillations do not occur and above which the two-electron Rabi oscillations occur at a broad range of frequencies. It is also shown that at field intensities near the threshold intensity, the field frequency predicted to induce Rabi oscillations by linear response TDDFT only produces detuned Rabi oscillations. Instead, the field frequency that yields the full two-electron population inversion and Rabi oscillation behavior is shown to be the average of single-electron transition frequencies from the ground S{sub 0} state and the doubly-excited S{sub 2} state. The behavior of the two-electron Rabi oscillations is rationalized via two possible models. The first model is a multi-photon process that results from the electric field interacting with the three level system such that three level Rabi oscillations may occur. The second model suggests that the mean-field nature of RT-TDDFT induces paired electron propagation.
Two-electron Rabi oscillations in real-time time-dependent density-functional theory.
Habenicht, Bradley F; Tani, Noriyuki P; Provorse, Makenzie R; Isborn, Christine M
2014-11-14
We investigate the Rabi oscillations of electrons excited by an applied electric field in several simple molecular systems using time-dependent configuration interaction (TDCI) and real-time time-dependent density-functional theory (RT-TDDFT) dynamics. While the TDCI simulations exhibit the expected single-electron Rabi oscillations at a single resonant electric field frequency, Rabi oscillations in the RT-TDDFT simulations are a two-electron process. The existence of two-electron Rabi oscillations is determined both by full population inversion between field-free molecular orbitals and the behavior of the instantaneous dipole moment during the simulations. Furthermore, the Rabi oscillations in RT-TDDFT are subject to an intensity threshold of the electric field, below which Rabi oscillations do not occur and above which the two-electron Rabi oscillations occur at a broad range of frequencies. It is also shown that at field intensities near the threshold intensity, the field frequency predicted to induce Rabi oscillations by linear response TDDFT only produces detuned Rabi oscillations. Instead, the field frequency that yields the full two-electron population inversion and Rabi oscillation behavior is shown to be the average of single-electron transition frequencies from the ground S0 state and the doubly-excited S2 state. The behavior of the two-electron Rabi oscillations is rationalized via two possible models. The first model is a multi-photon process that results from the electric field interacting with the three level system such that three level Rabi oscillations may occur. The second model suggests that the mean-field nature of RT-TDDFT induces paired electron propagation. PMID:25399137
Hubert, Mickaël; Hedegård, Erik D; Jensen, Hans Jørgen Aa
2016-05-10
Computational methods that can accurately and effectively predict all types of electronic excitations for any molecular system are missing in the toolbox of the computational chemist. Although various Kohn-Sham density-functional methods (KS-DFT) fulfill this aim in some cases, they become inadequate when the molecule has near-degeneracies and/or low-lying double-excited states. To address these issues we have recently proposed multiconfiguration short-range density-functional theory-MC-srDFT-as a new tool in the toolbox. While initial applications for systems with multireference character and double excitations have been promising, it is nevertheless important that the accuracy of MC-srDFT is at least comparable to the best KS-DFT methods also for organic molecules that are typically of single-reference character. In this paper we therefore systematically investigate the performance of MC-srDFT for a selected benchmark set of electronic excitations of organic molecules, covering the most common types of organic chromophores. This investigation confirms the expectation that the MC-srDFT method is accurate for a broad range of excitations and comparable to accurate wave function methods such as CASPT2, NEVPT2, and the coupled cluster based CC2 and CC3. PMID:27058733
Double-hybrid density functional theory for excited electronic states of molecules
NASA Astrophysics Data System (ADS)
Grimme, Stefan; Neese, Frank
2007-10-01
Double-hybrid density functionals are based on a mixing of standard generalized gradient approximations (GGAs) for exchange and correlation with Hartree-Fock (HF) exchange and a perturbative second-order correlation part (PT2) that is obtained from the Kohn-Sham (GGA) orbitals and eigenvalues. This virtual orbital-dependent functional (dubbed B2PLYP) contains only two empirical parameters that describe the mixture of HF and GGA exchange (ax) and of the PT2 and GGA correlation (ac), respectively. Extensive testing has recently demonstrated the outstanding accuracy of this approach for various ground state problems in general chemistry applications. The method is extended here without any further empirical adjustments to electronically excited states in the framework of time-dependent density functional theory (TD-DFT) or the closely related Tamm-Dancoff approximation (TDA-DFT). In complete analogy to the ground state treatment, a scaled second-order perturbation correction to configuration interaction with singles (CIS(D)) wave functions developed some years ago by Head-Gordon et al. [Chem. Phys. Lett. 219, 21 (1994)] is computed on the basis of density functional data and added to the TD(A)-DFT/GGA excitation energy. The method is implemented by applying the resolution of the identity approximation and the efficiency of the code is discussed. Extensive tests for a wide variety of molecules and excited states (of singlet, triplet, and doublet multiplicities) including electronic spectra are presented. In general, rather accurate excitation energies (deviations from reference data typically <0.2eV) are obtained that are mostly better than those from standard functionals. Still, systematic errors are obtained for Rydberg (too low on average by about 0.3eV) and charge-transfer transitions but due to the relatively large ax parameter (0.53), B2PLYP outperforms most other functionals in this respect. Compared to conventional HF-based CIS(D), the method is more robust in
Khaira, Jobanpreet S.; Jain, Richa N.; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2015-06-24
The electronic structure of yttrium-doped Silicon Carbide Nanotubes has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom is bonded strongly on the surface of the nanotube with a binding energy of 2.37 eV and prefers to stay on the hollow site at a distance of around 2.25 Å from the tube. The semi-conducting nanotube with chirality (4, 4) becomes half mettalic with a magnetic moment of 1.0 µ{sub B} due to influence of Y atom on the surface. There is strong hybridization between d orbital of Y with p orbital of Si and C causing a charge transfer from d orbital of the Y atom to the tube. The Fermi level is shifted towards higher energy with finite Density of States for only upspin channel making the system half metallic and magnetic which may have application in spintronic devices.
Sjostrom, Travis; Daligault, Jérôme
2015-12-01
We validate the application of our recent orbital-free density functional theory (DFT) approach [Phys. Rev. Lett. 113, 155006 (2014);] for the calculation of ionic and electronic transport properties of dense plasmas. To this end, we calculate the self-diffusion coefficient, the viscosity coefficient, the electrical and thermal conductivities, and the reflectivity coefficient of hydrogen and aluminum plasmas. Very good agreement is found with orbital-based Kohn-Sham DFT calculations at lower temperatures. Because the computational costs of the method do not increase with temperature, we can produce results at much higher temperatures than is accessible by the Kohn-Sham method. Our results for warm dense aluminum at solid density are inconsistent with the recent experimental results reported by Sperling et al. [Phys. Rev. Lett. 115, 115001 (2015)]. PMID:26764850
Conformation and electronic structure of polyethylene: A density-functional approach
NASA Astrophysics Data System (ADS)
Miao, M. S.; van Camp, P. E.; van Doren, V. E.; Ladik, J. J.; Mintmire, J. W.
1996-10-01
Two different local-density approximations, the Gáspár-Kohn-Sham and the Perdew-Zunger approximations, of the density-functional method have been used to calculate structural and electronic properties of polyethylene systems with several different dihedral angles. For each system, the CC bond lengths and the CCC and HCH bond angles are optimized simultaneously. All the parameters appear to be strongly coupled with torsional freedom and vary with the change in dihedral angle in a pattern similar to that of the total energy. The total energy has an absolute minimum for the planar zigzag conformation but a distinct local minimum for the quasistable helical conformation. Another minimum occurs in the energy curve close to this gauche minimum. The calculated valence and conduction bands are discussed and compared with other theoretical calculations and experiment.
Ionic and electronic transport properties in dense plasmas by orbital-free density functional theory
NASA Astrophysics Data System (ADS)
Sjostrom, Travis; Daligault, Jérôme
2015-12-01
We validate the application of our recent orbital-free density functional theory (DFT) approach [Phys. Rev. Lett. 113, 155006 (2014), 10.1103/PhysRevLett.113.155006;] for the calculation of ionic and electronic transport properties of dense plasmas. To this end, we calculate the self-diffusion coefficient, the viscosity coefficient, the electrical and thermal conductivities, and the reflectivity coefficient of hydrogen and aluminum plasmas. Very good agreement is found with orbital-based Kohn-Sham DFT calculations at lower temperatures. Because the computational costs of the method do not increase with temperature, we can produce results at much higher temperatures than is accessible by the Kohn-Sham method. Our results for warm dense aluminum at solid density are inconsistent with the recent experimental results reported by Sperling et al. [Phys. Rev. Lett. 115, 115001 (2015), 10.1103/PhysRevLett.115.115001].
NASA Astrophysics Data System (ADS)
Khaira, Jobanpreet S.; Jain, Richa N.; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2015-06-01
The electronic structure of yttrium-doped Silicon Carbide Nanotubes has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom is bonded strongly on the surface of the nanotube with a binding energy of 2.37 eV and prefers to stay on the hollow site at a distance of around 2.25 Å from the tube. The semi-conducting nanotube with chirality (4, 4) becomes half mettalic with a magnetic moment of 1.0 µB due to influence of Y atom on the surface. There is strong hybridization between d orbital of Y with p orbital of Si and C causing a charge transfer from d orbital of the Y atom to the tube. The Fermi level is shifted towards higher energy with finite Density of States for only upspin channel making the system half metallic and magnetic which may have application in spintronic devices.
NASA Astrophysics Data System (ADS)
Kulwinder, Kaur; Ranjan, Kumar
2016-05-01
We study the effect of pressure on electronic and thermoelectric properties of Mg2Si using the density functional theory and Boltzmann transport equations. The variation of lattice constant, band gap, bulk modulus with pressure is also analyzed. Further, the thermoelectric properties (Seebeck coefficient, electrical conductivity, electronic thermal conductivity) have been studied as a function of temperature and pressure up to 1200 K. The results show that Mg2Si is an n-type semiconductor with a band gap of 0.21 eV. The negative value of the Seebeck coefficient at all pressures indicates that the conduction is due to electrons. With the increase in pressure, the Seebeck coefficient decreases and electrical conductivity increases. It is also seen that, there is practically no effect of pressure on the electronic contribution of thermal conductivity. The paper describes the calculation of the lattice thermal conductivity and figure of merit of Mg2Si at zero pressure. The maximum value of figure of merit is attained 1.83×10‑3 at 1000 K. The obtained results are in good agreement with the available experimental and theoretical results. Project supported by the Council of Scientific & Industrial Research (CSIR), India.
Ionic thermal effects on photo-electron emission within time-dependent density-functional theory
NASA Astrophysics Data System (ADS)
Gao, Cong-Zhang; Dinh, Phuong Mai; Reinhard, Paul-Gerhard; Suraud, Eric
2016-02-01
We study the impact of thermal fluctuations of cluster/molecule shape on photo-electron spectra (PES) and photo-electron angular distributions (PAD) using a detailed time-dependent simulation of the emission dynamics and thermal ionic motion. Basis of the description is time-dependent density-functional theory (TDDFT) coupled to molecular dynamics for ionic motion. Test cases are small Na clusters and the C3 molecule. For Na clusters, we find that PES signals are rather robust for one-photon processes while large smearing of the pattern are observed at lower frequencies in multi-photon processes. This effect can be related to the typical spectral response of the metal clusters. PAD are generally much more robust than PES. The C3 molecule produces a greater variety of thermal response. This happens because this molecule has eigenmodes with much different softness.
Electronic and magnetic properties of substituted BN sheets: A density functional theory study
Zhou, Yungang; Yang, Ping; Wang, Zhiguo; Zu, Xiaotao T.; Xiao, Hai Yan; Sun, Xin; Khaleel, Mohammad A.; Gao, Fei
2011-04-15
Using density functional calculations, we investigate the geometries, electronic structures and magnetic properties of hexagonal BN sheets with 3d transition metal (TM) and nonmetal atoms embedded in three types of vacancies: VB, VN, and VB+N. We show that some embedded configurations, except TM atoms in VN vacancy, are stable in BN sheet and yield interesting phenomena. For instance, the band gaps and magnetic moments of BN sheet can be tuned depending on the embedded dopant species and vacancy type. In particular, embedment such as Cr in VB+N, Co in VB, and Ni in VB leads to half-metallic BN sheets interesting for spin filter applications. From the investigation of Mn-chain (CMn) embedments, a regular 1D structure can be formed in BN sheet as an electron waveguide, a metal nanometer wire with a single atom thickness.
NASA Astrophysics Data System (ADS)
Xiao, Ling-Ping; Zeng, Zhi; Chen, Xiao-Jia
2016-06-01
The pressure effect on the geometrical and electronic structures of crystalline naphthalene is calculated up to 30 GPa by performing density functional calculations. The lattice parameters a, b, and c, decrease by 1.77 Å (-20.4%), 0.85 Å (-14.1%), and 0.91 Å (-8.2%), respectively, while the monoclinic angle β increases by 3.95° in this pressure region. At the highest pressure of 30 GPa the unit cell volume decreases by 62.7%. The detailed analysis of the molecular arrangement within crystal structure reveals that the molecular motion becomes more and more localized, and hints towards the evolution of intermolecular interaction with pressure. Moreover, the electronic structure of naphthalene under high pressure is also discussed. A pressure induced decrease of the band gap is observed.
NASA Astrophysics Data System (ADS)
Almeida, Nuno M. S.; McKinlay, Russell G.; Paterson, Martin J.
2015-01-01
The lowest excited electronic states of the permanganate ion MnO4- are calculated using a hierarchy of coupled cluster response approaches, as well as time-dependent density functional theory. It is shown that while full linear response coupled cluster with singles and doubles (or higher) performs well, that permanganate represents a stern test for approximate coupled cluster response models, and that problems can be traced to very large orbital relaxation effects. TD-DFT is reasonably robust although errors around 0.6 eV are still observed. In order to further investigate the strong correlations prevalent in the electronic ground state large-scale RASSCF calculations were also performed. Again very large orbital relaxation in the correlated wavefunction is observed. Although the system can qualitatively be described by a single configuration, multi-reference diagnostic values show that care must be taken in this and similar metal complexes.
Quantum chemistry without wave functions: two-electron reduced density matrices.
Mazziotti, David A
2006-03-01
For 50 years, progress toward the direct calculation of the ground-state two-electron reduced density matrix (2-RDM) was stymied from an inability to constrain the 2-RDM to represent an N-electron wave function. Recent advances in theory and optimization have realized the direct calculation of the 2-RDM. A variational 2-RDM procedure, using first-order semidefinite programming, has been shown to capture multireference correlation effects important at nonequilibrium geometries [Mazziotti, Phys. Rev. Lett. 2004, 93, No. 213001]. This method emerged from research on a nonvariational calculation of the 2-RDM by the contracted Schrödinger equation. Both approaches will be discussed and illustrated. PMID:16548509
Electron bubbles in liquid helium: Density functional calculations of infrared absorption spectra
Grau, Victor; Barranco, Manuel; Mayol, Ricardo; Pi, Marti
2006-02-01
Within density functional theory, we have calculated the energy of the transitions from the ground state to the first two excited states in the electron bubbles in liquid helium at pressures from zero to about the solidification pressure. For {sup 4}He at low temperatures, our results are in very good agreement with infrared absorption experiments. Above a temperature of {approx}2 K, we overestimate the energy of the 1s-1p transition. We attribute this to the break down of the Franck-Condon principle due to the presence of helium vapor inside the bubble. Our results indicate that the 1s-2p transition energies are sensitive not only to the size of the electron bubble, but also to its surface thickness. We also present results for the infrared transitions in the case of liquid {sup 3}He, for which we lack experimental data.
A revised electronic Hessian for approximate time-dependent density functional theory.
Ziegler, Tom; Seth, Michael; Krykunov, Mykhaylo; Autschbach, Jochen
2008-11-14
Time-dependent density functional theory (TD-DFT) at the generalized gradient level of approximation (GGA) has shown systematic errors in the calculated excitation energies. This is especially the case for energies representing electron transitions between two separated regions of space or between orbitals of different spatial extents. It will be shown that these limitations can be attributed to the electronic ground state Hessian G(GGA). Specifically, we shall demonstrate that the Hessian G(GGA) can be used to describe changes in energy due to small perturbations of the electron density (Deltarho), but it should not be applied to one-electron excitations involving the density rearrangement (Deltarho) of a full electron charge. This is in contrast to Hartree-Fock theory where G(HF) has a trust region that is accurate for both small perturbations and one-electron excitations. The large trust radius of G(HF) can be traced back to the complete cancellation of Coulomb and exchange terms in Hartree-Fock (HF) theory representing self-interaction (complete self-interaction cancellation, CSIC). On the other hand, it is shown that the small trust radius for G(GGA) can be attributed to the fact that CSIC is assumed for GGA in the derivation of G(GGA) although GGA (and many other approximate DFT schemes) exhibits incomplete self-interaction cancellation (ISIC). It is further shown that one can derive a new matrix G(R-DFT) with the same trust region as G(HF) by taking terms due to ISIC properly into account. Further, with TD-DFT based on G(R-DFT), energies for state-to-state transitions represented by a one-electron excitation (psi(i)-->psi(a)) are approximately calculated as DeltaE(ai). Here DeltaE(ai) is the energy difference between the ground state Kohn-Sham Slater determinant and the energy of a Kohn-Sham Slater determinant where psi(i) has been replaced by psi(a). We make use of the new Hessian in two numerical applications involving charge-transfer excitations. It is
Mezey, Paul G
2014-09-16
Conspectus Just as complete molecules have no boundaries and have "fuzzy" electron density clouds approaching zero density exponentially at large distances from the nearest nucleus, a physically justified choice for electron density fragments exhibits similar behavior. Whereas fuzzy electron densities, just as any fuzzy object, such as a thicker cloud on a foggy day, do not lend themselves to easy visualization, one may partially overcome this by using isocontours. Whereas a faithful representation of the complete fuzzy density would need infinitely many such isocontours, nevertheless, by choosing a selected few, one can still obtain a limited pictorial representation. Clearly, such images are of limited value, and one better relies on more complete mathematical representations, using, for example, density matrices of fuzzy fragment densities. A fuzzy density fragmentation can be obtained in an exactly additive way, using the output from any of the common quantum chemical computational techniques, such as Hartree-Fock, MP2, and various density functional approaches. Such "fuzzy" electron density fragments properly represented have proven to be useful in a rather wide range of applications, for example, (a) using them as additive building blocks leading to efficient linear scaling macromolecular quantum chemistry computational techniques, (b) the study of quantum chemical functional groups, (c) using approximate fuzzy fragment information as allowed by the holographic electron density theorem, (d) the study of correlations between local shape and activity, including through-bond and through-space components of interactions between parts of molecules and relations between local molecular shape and substituent effects, (e) using them as tools of density matrix extrapolation in conformational changes, (f) physically valid averaging and statistical distribution of several local electron densities of common stoichiometry, useful in electron density databank mining, for
A density functional theory investigation of the electronic structure and spin moments of magnetite
NASA Astrophysics Data System (ADS)
Noh, Junghyun; Osman, Osman I.; Aziz, Saadullah G.; Winget, Paul; Brédas, Jean-Luc
2014-08-01
We present the results of density functional theory (DFT) calculations on magnetite, Fe3O4, which has been recently considered as electrode in the emerging field of organic spintronics. Given the nature of the potential applications, we evaluated the magnetite room-temperature cubic Fd\\bar{3}m phase in terms of structural, electronic, and magnetic properties. We considered GGA (PBE), GGA + U (PBE + U), and range-separated hybrid (HSE06 and HSE(15%)) functionals. Calculations using HSE06 and HSE(15%) functionals underline the impact that inclusion of exact exchange has on the electronic structure. While the modulation of the band gap with exact exchange has been seen in numerous situations, the dramatic change in the valence band nature and states near the Fermi level has major implications for even a qualitative interpretation of the DFT results. We find that HSE06 leads to highly localized states below the Fermi level while HSE(15%) and PBE + U result in delocalized states around the Fermi level. The significant differences in local magnetic moments and atomic charges indicate that describing room-temperature bulk materials, surfaces and interfaces may require different functionals than their low-temperature counterparts.
Lowering the density of electronic defects on organic-functionalized Si(100) surfaces
NASA Astrophysics Data System (ADS)
Peng, Weina; DeBenedetti, William J. I.; Kim, Seonjae; Hines, Melissa A.; Chabal, Yves J.
2014-06-01
The electrical quality of functionalized, oxide-free silicon surfaces is critical for chemical sensing, photovoltaics, and molecular electronics applications. In contrast to Si/SiO2 interfaces, the density of interface states (Dit) cannot be reduced by high temperature annealing because organic layers decompose above 300 °C. While a reasonable Dit is achieved on functionalized atomically flat Si(111) surfaces, it has been challenging to develop successful chemical treatments for the technologically relevant Si(100) surfaces. We demonstrate here that recent advances in the chemical preparation of quasi-atomically-flat, H-terminated Si(100) surfaces lead to a marked suppression of electronic states of functionalized surfaces. Using a non-invasive conductance-voltage method to study functionalized Si(100) surfaces with varying roughness, a Dit as low as 2.5 × 1011 cm-2eV-1 is obtained for the quasi-atomically-flat surfaces, in contrast to >7 × 1011 cm-2eV-1 on atomically rough Si(100) surfaces. The interfacial quality of the organic/quasi-atomically-flat Si(100) interface is very close to that obtained on organic/atomically flat Si(111) surfaces, opening the door to applications previously thought to be restricted to Si(111).
NASA Astrophysics Data System (ADS)
Kweon, Kyoung E.; Hwang, Gyeong S.
2012-10-01
The structure and property prediction of metal oxides can significantly be improved by incorporating exact Hartree-Fock (HF) exchange into density functional theory (DFT), which is the so-called hybrid DFT. We explored the impact of HF exchange inclusion on the predicted structural, bonding, and electronic properties of bismuth vanadate (BiVO4), with particular attention to the difference between its monoclinic and tetragonal scheelite phases. The applied exchange-correlation (xc) functionals include the gradient corrected Perdew-Burke-Ernzerhof (PBE) and the PBE-HF hybrid functionals with HF exchange amounts of 10%, 25%, and 50%. We find that the PBE-HF25% yields a monoclinic structure in very close agreement with the experimentally determined structure, while the PBE-HF50% tends to overestimate the monoclinic distortion and the PBE/PBE-HF10% can hardly identify a distinct monoclinic configuration at ambient conditions. Electronic structure analysis reveals that the increasing monoclinic distortion with the amount of HF exchange is related to the enhancement of hybridization between Bi 6s-O 2p antibonding states and unoccupied Bi 6p states. The bonding mechanisms and band structures of the monoclinic and tetragonal phases of BiVO4 were also investigated, and we discuss how the predictions are sensitive to the xc functional choice.
Lowering the density of electronic defects on organic-functionalized Si(100) surfaces
Peng, Weina DeBenedetti, William J. I.; Kim, Seonjae; Chabal, Yves J.; Hines, Melissa A.
2014-06-16
The electrical quality of functionalized, oxide-free silicon surfaces is critical for chemical sensing, photovoltaics, and molecular electronics applications. In contrast to Si/SiO{sub 2} interfaces, the density of interface states (D{sub it}) cannot be reduced by high temperature annealing because organic layers decompose above 300 °C. While a reasonable D{sub it} is achieved on functionalized atomically flat Si(111) surfaces, it has been challenging to develop successful chemical treatments for the technologically relevant Si(100) surfaces. We demonstrate here that recent advances in the chemical preparation of quasi-atomically-flat, H-terminated Si(100) surfaces lead to a marked suppression of electronic states of functionalized surfaces. Using a non-invasive conductance-voltage method to study functionalized Si(100) surfaces with varying roughness, a D{sub it} as low as 2.5 × 10{sup 11} cm{sup −2}eV{sup −1} is obtained for the quasi-atomically-flat surfaces, in contrast to >7 × 10{sup 11} cm{sup −2}eV{sup −1} on atomically rough Si(100) surfaces. The interfacial quality of the organic/quasi-atomically-flat Si(100) interface is very close to that obtained on organic/atomically flat Si(111) surfaces, opening the door to applications previously thought to be restricted to Si(111)
Technology Transfer Automated Retrieval System (TEKTRAN)
Density functional theory/B3LYP has been employed to optimize the conformations of selected 4-arylflavan-3-ols and their phenolic methyl ether 3-O-acetates. The electronic circular dichroism spectra of the major conformers have been calculated using time-dependent density functional theory to valida...
Using time-dependent density functional theory in real time for calculating electronic transport
NASA Astrophysics Data System (ADS)
Schaffhauser, Philipp; Kümmel, Stephan
2016-01-01
We present a scheme for calculating electronic transport within the propagation approach to time-dependent density functional theory. Our scheme is based on solving the time-dependent Kohn-Sham equations on grids in real space and real time for a finite system. We use absorbing and antiabsorbing boundaries for simulating the coupling to a source and a drain. The boundaries are designed to minimize the effects of quantum-mechanical reflections and electrical polarization build-up, which are the major obstacles when calculating transport by applying an external bias to a finite system. We show that the scheme can readily be applied to real molecules by calculating the current through a conjugated molecule as a function of time. By comparing to literature results for the conjugated molecule and to analytic results for a one-dimensional model system we demonstrate the reliability of the concept.
Electronic transport properties of one dimensional lithium nanowire using density functional theory
Thakur, Anil; Kumar, Arun; Chandel, Surjeet; Ahluwalia, P. K.
2015-05-15
Single nanowire electrode devices are a unique platform for studying as energy storage devices. Lithium nanowire is of much importance in lithium ion batteries and therefore has received a great deal of attention in past few years. In this paper we investigated structural and electronic transport properties of Li nanowire using density functional theory (DFT) with SIESTA code. Electronic transport properties of Li nanowire are investigated theoretically. The calculations are performed in two steps: first an optimized geometry for Li nanowire is obtained using DFT calculations, and then the transport relations are obtained using NEGF approach. SIESTA and TranSIESTA simulation codes are used in the calculations correspondingly. The electrodes are chosen to be the same as the central region where transport is studied, eliminating current quantization effects due to contacts and focusing the electronic transport study to the intrinsic structure of the material. By varying chemical potential in the electrode regions, an I-V curve is traced which is in agreement with the predicted behavior. Agreement of bulk properties of Li with experimental values make the study of electronic and transport properties in lithium nanowires interesting because they are promising candidates as bridging pieces in nanoelectronics. Transmission coefficient and V-I characteristic of Li nano wire indicates that Li nanowire can be used as an electrode device.
Electronic transport properties of one dimensional lithium nanowire using density functional theory
NASA Astrophysics Data System (ADS)
Thakur, Anil; Kumar, Arun; Chandel, Surjeet; Ahluwalia, P. K.
2015-05-01
Single nanowire electrode devices are a unique platform for studying as energy storage devices. Lithium nanowire is of much importance in lithium ion batteries and therefore has received a great deal of attention in past few years. In this paper we investigated structural and electronic transport properties of Li nanowire using density functional theory (DFT) with SIESTA code. Electronic transport properties of Li nanowire are investigated theoretically. The calculations are performed in two steps: first an optimized geometry for Li nanowire is obtained using DFT calculations, and then the transport relations are obtained using NEGF approach. SIESTA and TranSIESTA simulation codes are used in the calculations correspondingly. The electrodes are chosen to be the same as the central region where transport is studied, eliminating current quantization effects due to contacts and focusing the electronic transport study to the intrinsic structure of the material. By varying chemical potential in the electrode regions, an I-V curve is traced which is in agreement with the predicted behavior. Agreement of bulk properties of Li with experimental values make the study of electronic and transport properties in lithium nanowires interesting because they are promising candidates as bridging pieces in nanoelectronics. Transmission coefficient and V-I characteristic of Li nano wire indicates that Li nanowire can be used as an electrode device.
NASA Astrophysics Data System (ADS)
Nawa, Kenji; Nakamura, Kohji; Akiyama, Toru; Ito, Tomonori; Weinert, Michael
2015-03-01
Interest in single organometallic molecule and that adsorbed on solid surfaces has rapidly increased because of possible novel applications. For molecules with transition metals (TMs), the d-electron configuration is an essential aspect of their electronic and magnetic properties, and correlation effects can not be excluded. Here, we investigate systematically the electron configuration and correlation effects for prototypical organometallic molecules of tridimensional metallocene (TMCp2) and planer phthalocyanine (TMPc). Calculations were carried out based on the constraint density functional theory (DFT) by using the full-potential linearized augmented plane wave method that incorporates an on-site Coulomb interaction correction + U . We find that these correlation effects play a key role in determining the ground state of the electron configuration for the organometallic molecules. The calculated ground states of TMCp2, where TM =Cr, Mn, Fe, Co, and Ni, obtained by constraint DFT with +U reproduce the experimentally determined structures of 3E2 g , 6A1 g , 1A1 g , 2E1 g , and 3A2 g , respectively. Results for the TMPc will be also presented.
NASA Astrophysics Data System (ADS)
Sun, Haitao; Tang, Ke; Li, Yanmin; Su, Chunfang; Zhou, Zhengyu; Wang, Zhizhong
The effect of hydrogen bond interactions on ionization potentials (IPs) and electron affinities (EAs) of thymine-formamide complexes (T-F) have been investigated employing the density functional theory B3LYP at 6-311++G(d, p) basis set level. All complexes experience a geometrical change on either electron detachment or attachment, and the change might be facilitated or hindered according to the strength of the hydrogen-bonding interaction involved. The strength of hydrogen bonds presents an opposite changing trend on the two processes. A more important role that H-bonding interaction plays in the process of electron attachment than in the process of electron detachment can be seen by a comparison of the IPs and EAs of complexes with that of isolated thymine. Futhermore, the EAs of isolated thymine are in good agreement with the experimental values (AEA is 0.79 eV, VEA is -0.29 eV [Wetmore et al., Chem Phys Lett 2000, 322, 129]). The calculated total NPA charge distributions reveal that nearly all the negative charges locate on thymine monomer in the anions and even in the cationic states, there are a few negative charges on thymine monomer. An analysis of dissociation energies predicts the processes T-F+→ T++ F and T-F- → T- + F to be the most energetically favorable for T-F+ and T-F-, respectively. Content:text/plain; charset="UTF-8"
Vassilev, Peter; Louwerse, Manuel J; Baerends, Evert Jan
2005-12-15
Ab initio density functional theory molecular dynamics simulations of the solvated states of the hydroxyl radical and hydroxide ion are performed using the Becke-Lee-Yang-Parr (BLYP) exchange-correlation functional (Becke, A. D. Phys. Rev. A 1988, 38, 3098. Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785). The structures of the solvation shells of the two species are examined. It is found that the OH radical forms a relatively well-defined solvation complex with four neighboring water molecules. Three of these molecules are hydrogen bonded to the OH, while the fourth is hemibonded via a three-electron two-centered bond between the oxygen atoms of the OH and water. The activity and the diffusion mechanism of the OH radical in water is discussed in comparison with the OH- ion. Although the results are partially influenced by the tendency of the BLYP density functional to overestimate hemibonded structure, the present simulations suggest that the widely accepted picture of rapid diffusion of OH radical in water through hydrogen exchange reaction may need to be reconsidered. PMID:16375337
NASA Astrophysics Data System (ADS)
Xiao, Bing; Sun, Jianwei; Ruzsinszky, Adrienn; Perdew, John P.
2014-08-01
We employ semilocal density functionals [local spin-density approximation (LSDA), Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA), and meta-GGAs)], LSDA plus Hubbard U (LSDA+U) theory, a nonlocal range-separated Heyd-Scuseria-Ernzerhof hybrid functional (HSE06), and the random-phase approximation (RPA) to assess their performances for the ground-state magnetism and electronic structure of a strongly correlated metal, rutile VO2. Using recent quantum Monte Carlo results as the benchmark, all tested semilocal and hybrid functionals as well as the RPA (with PBE inputs) predict the correct magnetic ground states for rutile VO2. The observed paramagnetism could arise from temperature-disordered local spin moments or from the thermal destruction of these moments. All semilocal functionals also give the correct ground-state metallicity for rutile VO2. However, in the ferromagnetic (FM) and antiferromagnetic (AFM) phases, LSDA+U and HSE06 incorrectly predict rutile VO2 to be a Mott-Hubbard insulator. For the computed electronic structures of FM and AFM phases, we find that the Tao-Perdew-Staroverov-Scuseria (TPSS) and revised TPSS (revTPSS) meta-GGAs give strong 2p-3d hybridizations, resulting in a depopulation of the 2p bands of O atoms, in comparison with other tested meta-GGAs. The regularized TPSS (regTPSS) and meta-GGAs made simple, i.e., MGGA_MS0 and MGGA_MS2, which are free of the spurious order-of-limits problem of TPSS and revTPSS, give electronic states close to those of the PBE GGA and LSDA. In comparison to experiment, semilocal functionals predict better equilibrium cell volumes for rutile VO2 in FM and AFM states than in the spin-unpolarized state. For meta-GGAs, a monotonic decrease of the exchange enhancement factor Fx(s,α) with α for small s, as in the MGGA_MS functionals, leads to large (probably too large) local magnetic moments in spin-polarized states.
NASA Astrophysics Data System (ADS)
Corsini, Niccolò R. C.; Greco, Andrea; Hine, Nicholas D. M.; Molteni, Carla; Haynes, Peter D.
2013-08-01
We present an implementation in a linear-scaling density-functional theory code of an electronic enthalpy method, which has been found to be natural and efficient for the ab initio calculation of finite systems under hydrostatic pressure. Based on a definition of the system volume as that enclosed within an electronic density isosurface [M. Cococcioni, F. Mauri, G. Ceder, and N. Marzari, Phys. Rev. Lett. 94, 145501 (2005)], 10.1103/PhysRevLett.94.145501, it supports both geometry optimizations and molecular dynamics simulations. We introduce an approach for calibrating the parameters defining the volume in the context of geometry optimizations and discuss their significance. Results in good agreement with simulations using explicit solvents are obtained, validating our approach. Size-dependent pressure-induced structural transformations and variations in the energy gap of hydrogenated silicon nanocrystals are investigated, including one comparable in size to recent experiments. A detailed analysis of the polyamorphic transformations reveals three types of amorphous structures and their persistence on depressurization is assessed.
Corsini, Niccolò R C; Greco, Andrea; Hine, Nicholas D M; Molteni, Carla; Haynes, Peter D
2013-08-28
We present an implementation in a linear-scaling density-functional theory code of an electronic enthalpy method, which has been found to be natural and efficient for the ab initio calculation of finite systems under hydrostatic pressure. Based on a definition of the system volume as that enclosed within an electronic density isosurface [M. Cococcioni, F. Mauri, G. Ceder, and N. Marzari, Phys. Rev. Lett. 94, 145501 (2005)], it supports both geometry optimizations and molecular dynamics simulations. We introduce an approach for calibrating the parameters defining the volume in the context of geometry optimizations and discuss their significance. Results in good agreement with simulations using explicit solvents are obtained, validating our approach. Size-dependent pressure-induced structural transformations and variations in the energy gap of hydrogenated silicon nanocrystals are investigated, including one comparable in size to recent experiments. A detailed analysis of the polyamorphic transformations reveals three types of amorphous structures and their persistence on depressurization is assessed. PMID:24006984
NASA Astrophysics Data System (ADS)
Ma, Liang-Cai; Zhang, Jian-Min; Xu, Ke-Wei
2013-02-01
By using first-principles calculations based on density-functional theory, we have systematically investigated the equilibrium structure and electronic properties of 22 free-standing copper (Cu) nanowires having different cross-sections with 1-14 Cu atoms per unit cell. The structural properties of the studied Cu nanowires were greatly different from those of face centered cubic bulk Cu. For each wire the equilibrium lattice constant was obtained. The binding energy increases with increasing atom number per unit cell in different structures. As for the polygonal structures of a fixed cross-section, the preferred structures should be the staggered ones which contain a linear chain along the wire axis that passes through the center of the polygons, where each chain atom is just located at a point equidistant from the planes of polygons. All the nanowires are metallic. The numbers of conduction channels were determined from the electronic band structures. In general, the number of conduction channels increases when the nanowire becomes thicker. The density of charge revealed delocalized metallic bonding for all studied Cu nanowires.
NASA Astrophysics Data System (ADS)
Terrier, Cyril; Vitorge, Pierre; Gaigeot, Marie-Pierre; Spezia, Riccardo; Vuilleumier, Rodolphe
2010-07-01
Structural and electronic properties of La3+ immersed in bulk water have been assessed by means of density functional theory (DFT)-based Car-Parrinello molecular dynamics (CPMD) simulations. Correct structural properties, i.e., La(III)-water distances and La(III) coordination number, can be obtained within the framework of Car-Parrinello simulations providing that both the La pseudopotential and conditions of the dynamics (fictitious mass and time step) are carefully set up. DFT-MD explicitly treats electronic densities and is shown here to provide a theoretical justification to the necessity of including polarization when studying highly charged cations such as lanthanoids(III) with classical MD. La3+ was found to strongly polarize the water molecules located in the first shell, giving rise to dipole moments about 0.5 D larger than those of bulk water molecules. Finally, analyzing Kohn-Sham orbitals, we found La3+ empty 4f orbitals extremely compact and to a great extent uncoupled from the water conduction band, while the 5d empty orbitals exhibit mixing with unoccupied states of water.
NASA Astrophysics Data System (ADS)
Gunasekaran, Sethu; Rajalakshmi, K.; Kumaresan, Subramanian
2013-08-01
The Fourier transform (FT-IR) spectrum of Levofloxacin was recorded in the region 4000-400 cm-1 and a complete vibrational assignment of fundamental vibrational modes of the molecule was carried out using density functional method. The observed fundamental modes have been compared with the harmonic vibrational frequencies computed using DFT (B3LYP) method by employing 6-31 G (d, p) basis sets. The most stable geometry of the molecule under investigation has been determined from the potential energy scan. The first-order hyperpolarizability (βo) and other related properties (μ, αo) of Levofloxacin are calculated using density functional theory (DFT) on a finite field approach. UV-vis spectrum of the molecule was recorded and the electronic properties, such as HOMO and LUMO energies were performed by DFT using 6-31 G (d, p) basis sets. Stability of the molecule arising from hyperconjugative interactions, charge delocalization have been analyzed using natural bond orbital analysis (NBO). The calculated HOMO and LUMO energies show that, the charge transfer occurs within the molecule. The other molecular properties like molecular electrostatic potential (MESP), Mulliken population analysis and thermodynamic properties of the title molecule have been calculated.
Gunasekaran, Sethu; Rajalakshmi, K; Kumaresan, Subramanian
2013-08-01
The Fourier transform (FT-IR) spectrum of Levofloxacin was recorded in the region 4000-400 cm(-1) and a complete vibrational assignment of fundamental vibrational modes of the molecule was carried out using density functional method. The observed fundamental modes have been compared with the harmonic vibrational frequencies computed using DFT (B3LYP) method by employing 6-31 G (d, p) basis sets. The most stable geometry of the molecule under investigation has been determined from the potential energy scan. The first-order hyperpolarizability (βo) and other related properties (μ, αo) of Levofloxacin are calculated using density functional theory (DFT) on a finite field approach. UV-vis spectrum of the molecule was recorded and the electronic properties, such as HOMO and LUMO energies were performed by DFT using 6-31 G (d, p) basis sets. Stability of the molecule arising from hyperconjugative interactions, charge delocalization have been analyzed using natural bond orbital analysis (NBO). The calculated HOMO and LUMO energies show that, the charge transfer occurs within the molecule. The other molecular properties like molecular electrostatic potential (MESP), Mulliken population analysis and thermodynamic properties of the title molecule have been calculated. PMID:23685802
Liu, Xia; Tan, Yingzi; Li, Xiuling; Wu, Xiaojun; Pei, Yong
2015-08-28
The electronic and magnetic properties of transition metal (TM = Sc, Ti, V, Cr and Mn) atom incorporated single and double one-dimensional (1D) styrene molecular wires confined on the hydrogen-terminated Si(100) surface are explored for the first time by means of spin-polarized density functional theory, denoted as Si-[TM(styrene)]. It is unveiled that TM atoms bind asymmetrically to the adjacent phenyl rings, which leads to novel electronic and magnetic properties in stark contrast to the well-studied gas phase TM-benzene molecular wires. Si-[Mn(styrene)]∞ and Si-[Cr(styrene)]∞ single molecular wires (SMWs) are a ferromagnetic semiconductor and half metal, respectively. Creation of H-atom defects on the silicon surface can introduce an impurity metallic band, which leads to novel half-metallic magnetism of a Si-[Mn(styrene)]∞ system. Moreover, double molecular wires (DMWs) containing two identical or hetero SMWs are theoretically designed. The [Mn(styrene)]∞-[Cr(styrene)]∞ DMW exhibits half-metallic magnetism where the spin-up and spin-down channels are contributed by two single molecular wires. Finally, we demonstrate that introducing a TM-defect may significantly affect the electronic structure and magnetic properties of molecular wires. These studies provide new insights into the structure and properties of surface supported 1-D sandwiched molecular wires and may inspire the future experimental synthesis of substrate confined organometallic sandwiched molecular wires. PMID:26219748
Electronic structure and defect properties of Tl6SeI4: Density functional calculations
NASA Astrophysics Data System (ADS)
Biswas, Koushik; Du, Mao-Hua; Singh, David J.
2012-10-01
We report density functional calculations of electronic structure, phase diagram, and dielectric, optical, and defect properties of Tl6SeI4. We discuss how electronic structure and defect properties affect resistivity and carrier mobility-lifetime (μτ) products in Tl6SeI4. We find large Born effective charges due to covalency involving Tl-6p states. High Born charges generally enhance the static dielectric constant. This provides a mechanism for effective screening of charged defects and impurities. We find that high resistivity can be obtained under near-stoichiometric growth conditions via Fermi level pinning near the middle of the band gap by shallow donors and acceptors, as opposed to deep traps that can give high resistivity, but at the expense of short carrier drift lengths. Defect calculations also reveal the presence of deep native donors that may cause electron trapping. The experimentally observed good μτ products may be explained by a combination of small effective masses and effective screening of charged defects. High resistivity and good μτ products make Tl6SeI4 a promising room-temperature radiation detector material. We also show the calculated defect diffusion barriers, which affect defect migration under external bias in a detector.
NASA Astrophysics Data System (ADS)
Ferri, Nicola; Distasio, Robert A., Jr.; Ambrosetti, Alberto; Car, Roberto; Scheffler, Matthias; Tkatchenko, Alexandre
2015-03-01
Ubiquitous long-range van der Waals (vdW) interactions play a fundamental role in the structure and stability of a wide range of systems. Within the DFT framework, the vdW energy represents a crucial, but tiny part of the total energy, hence its influence on the electronic density, n (r) , and electronic properties is typically assumed to be rather small. Here, we address this question via a fully self-consistent (SC) implementation of the interatomic Tkatchenko-Scheffler vdW functional and its extension to surfaces. Self-consistency leads to large changes in the binding energies and electrostatic moments of highly polarizable alkali metal dimers. For some metal surfaces, vdW interactions increase dipole moments and induce non-trivial charge rearrangements, leading to visible changes in the metal workfunctions. Similar behavior is observed for molecules adsorbed on metals. Our study reveals a non-trivial connection between electrostatics and long-range electron correlation effects.
NASA Astrophysics Data System (ADS)
Endo, Kazunaka
2016-02-01
In the Auger electron spectra (AES) simulations, we define theoretical modified kinetic energies of AES in the density functional theory (DFT) calculations. The modified kinetic energies correspond to two final-state holes at the ground state and at the transition-state in DFT calculations, respectively. This method is applied to simulate Auger electron spectra (AES) of 2nd periodic atom (Li, Be, B, C, N, O, F)-involving substances (LiF, beryllium, boron, graphite, GaN, SiO2, PTFE) by deMon DFT calculations using the model molecules of the unit cell. Experimental KVV (valence band electrons can fill K-shell core holes or be emitted during KVV-type transitions) AES of the (Li, O) atoms in the substances agree considerably well with simulation of AES obtained with the maximum kinetic energies of the atoms, while, for AES of LiF, and PTFE substance, the experimental F KVV AES is almost in accordance with the spectra from the transitionstate kinetic energy calculations.
Yuan, H. K.; Chen, H. Tian, C. L.; Kuang, A. L.; Wang, J. Z.
2014-04-21
Gadolinium-oxide clusters in various sizes and stoichiometries have been systematically studied by employing the density functional theory with the generalized gradient approximation. The clusters in bulk stoichiometry are relatively more stable and their binding energies increase with the increasing size. Stoichiometric (Gd{sub 2}O{sub 3}){sub n} clusters of n = 1–3 prefer cage-like structures, whereas the clusters of n = 4–30 prefer compact structures layered by wedge-like units and exhibit a rough feature toward the bulk-like arrangement with small disorders of atomic positions. The polyhedral-cages analogous to carbon-fullerenes are stable isomers yet not the minimum energy configurations. Their stabilities can be improved by embedding one oxygen atom or a suitable cage to form core-shell configurations. The mostly favored antiferromagnetic couplings between adjacent Gd atoms are nearly degenerated in energy with their ferromagnetic couplings, resulting in super-paramagnetic characters of gadolinium-oxide clusters. The Ruderman-Kittel-Kasuya-Yosida (RKKY)-type mechanism together with the superexchange-type mechanism plays cooperation role for the magnetic interactions in clusters. We present, as a function of n, calculated binding energies, ionization potential, electron affinity, and electronic dipole moment.
NASA Astrophysics Data System (ADS)
Yuan, H. K.; Chen, H.; Tian, C. L.; Kuang, A. L.; Wang, J. Z.
2014-04-01
Gadolinium-oxide clusters in various sizes and stoichiometries have been systematically studied by employing the density functional theory with the generalized gradient approximation. The clusters in bulk stoichiometry are relatively more stable and their binding energies increase with the increasing size. Stoichiometric (Gd2O3)n clusters of n = 1-3 prefer cage-like structures, whereas the clusters of n = 4-30 prefer compact structures layered by wedge-like units and exhibit a rough feature toward the bulk-like arrangement with small disorders of atomic positions. The polyhedral-cages analogous to carbon-fullerenes are stable isomers yet not the minimum energy configurations. Their stabilities can be improved by embedding one oxygen atom or a suitable cage to form core-shell configurations. The mostly favored antiferromagnetic couplings between adjacent Gd atoms are nearly degenerated in energy with their ferromagnetic couplings, resulting in super-paramagnetic characters of gadolinium-oxide clusters. The Ruderman-Kittel-Kasuya-Yosida (RKKY)-type mechanism together with the superexchange-type mechanism plays cooperation role for the magnetic interactions in clusters. We present, as a function of n, calculated binding energies, ionization potential, electron affinity, and electronic dipole moment.
NASA Astrophysics Data System (ADS)
Prabhaharan, M.; Prabakaran, A. R.; Srinivasan, S.; Gunasekaran, S.
2015-03-01
The present work has been carried out a combined experimental and theoretical study on molecular structure, vibrational spectra and NBO analysis of cyanuric acid. The FT-IR (100-4000 cm-1) and FT-Raman spectra (400-4000 cm-1) of cyanuric acid were recorded. In DFT methods, Becke's three parameter exchange-functional (B3) combined with gradient-corrected correlation functional of Lee, Yang and Parr (LYP) by implementing the split-valence polarized 6-31G(d,p) and 6-31++G(d,p) basis sets have been considered for the computation of the molecular structure optimization, vibrational frequencies, thermodynamic properties and energies of the optimized structures. The density functional theory (DFT) result complements the experimental findings. The electronic properties, such as HOMO-LUMO energies and molecular electrostatic potential (MESP) are also performed. Mulliken population analysis on atomic charges is also calculated. The first order hyperpolarizability (βtotal) of this molecular system and related properties (β, μ and Δα) are calculated using DFT/B3LYP/6-31G (d,p) and B3LYP/6-311++G(d,p) methods. The thermodynamic functions (heat capacity, entropy and enthalpy) from spectroscopic data by statistical methods were also obtained for the range of temperature 50-1000 K.
NASA Astrophysics Data System (ADS)
Romero, Nichols Anthony
Two decades after the discovery of C60, fullerenes continue to be intensely studied for their diverse properties, not the least of which is their unusually high superconducting transition temperature T c (up to 40 K in Cs3C60). Ab initio electronic structure methods are able to contribute to our understanding of these materials by providing electron densities, band structures, density-of-states, binding energies, and even the electron-phonon coupling. Because of the increasing computational feasibility of these large-scale calculations, these methods now play a prominent role in verifying and guiding experimental investigation of new materials. This dissertation presents results of a theoretical investigation of several scenarios where Tc enhancement exceeding those found in the alkali-doped fullerides could be exhibited: (1) field-effect doping of C60 layers, (2) C28-derived molecular solids and (3) covalently bonded C28 solids. The method employed is the Kohn-Sham formulation of density functional theory. Simpler tight-binding calculations are also used when appropriate. The study of field-effect doping was stimulated by the reports of large Tc enhancements by Schon et al., which were later retracted and found to be falsified. Even before the legitimacy of these reports came into question, we concluded that our calculations did not substantiate their claims. We present our electronic structure calculations and conclusions which are independent and potentially useful for future work on field-effect devices. The main part of this dissertation is a separate study on C28-based solids motivated by theoretical arguments suggesting that solids based on smaller fullerenes could exhibit a Tc enhancement. Among many molecular solids formed from closed-shell C28-derived molecules, we have found that solid C28H4 binds weakly and exhibit many of the salient features of solid C60, with an estimated Tc of 58 K. In this same spirit, we also study covalent solids based on
NASA Astrophysics Data System (ADS)
Di Valentin, M.; Salvadori, E.; Barone, V.; Carbonera, D.
2013-10-01
Advanced electron paramagnetic resonance (EPR) techniques, in combination with Density Functional theory (DFT), have been applied to the comparative study of carotenoid triplet states in two major photosynthetic antenna complexes, the Peridinin-chlorophyll a-protein of dinoflagellates and the light-harvesting complex II of higher plants. Carotenoid triplet states are populated by triplet-triplet energy transfer (TTET) from chlorophyll molecules to photoprotect the system from singlet oxygen formation under light-stress conditions. The TTET process is strongly dependent on the relative arrangement and on the electronic properties of the triplet states involved. The proposed spectroscopic approach exploits the concept of spin conservation during TTET, which leads to recognisable spin polarisation effects in the time-resolved and field-swept echo-detected EPR spectra. The electron spin polarisation produced at the carotenoid acceptor site depends on the initial polarisation of the chlorophyll donor and on the relative geometrical arrangement of the donor-acceptor zero-field splitting axes. We have demonstrated that a proper analysis of the spectra in the framework of spin angular momentum conservation allows to derive the pathways of TTET and to gain insight into the structural requirements of this mechanism for those antenna complexes, whose X-ray structure is available. We have further proved that this method, developed for natural antenna complexes of known X-ray structure, can be extended to systems lacking structural information in order to derive the relative arrangement of the partners in the energy transfer process. The structural requirements for efficient TTET, obtained from time-resolved and pulse EPR, have been complemented by a detailed description of the electronic structure of the carotenoid triplet state, provided by pulse Electron-Nuclear DOuble Resonance (ENDOR) experiments. Triplet-state hyperfine couplings of the α- and β-protons of the
Drew, Simon C; Young, Charles G; Hanson, Graeme R
2007-04-01
The electron paramagnetic resonance spin Hamiltonian parameters of mononuclear thiomolybdenyl complexes based upon the tris(pyrazolyl)borate ligand, together with their molybdenyl analogues, are calculated using density functional theory. The electronic g matrix and 95Mo hyperfine matrix are calculated as second-order response properties from the coupled-perturbed Kohn-Sham equations. The scalar relativistic zero-order regular approximation (ZORA) is used with an all-electron basis and an accurate mean-field spin-orbit operator which includes all one- and two-electron terms. The principal values and relative orientations of the g and A interaction matrices obtained from the experimental spectra in a previous EPR study are compared with those obtained from unrestricted Kohn-Sham calculations at the BP86 and B3LYP level, and the latter are found to be in good quantitative agreement. A quasi-restricted approach is used to analyze the influence of the various molecular orbitals on g and A. In all complexes the ground state magnetic orbital is dX2-Y2-based and the orientation of the A matrix is directly related to the orientation of this orbital. The largest single contribution to the orientation of the g matrix arises from the spin-orbit coupling of the dYZ-based lowest-unoccupied molecular orbital into the ground state. A number of smaller, cumulative charge-transfer contributions augment the d-d contributions. A comparison of the theoretical EPR parameters obtained using both crystallographic and gas-phase geometry-optimized structures of Tp*MoO(bdt) (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bdt = 1,2-benzenedithiolate) suggests a correspondence between the metal-dithiolate fold angle and the angle of noncoincidence between g and A. PMID:17305330
NASA Astrophysics Data System (ADS)
Kaewmaraya, T.; Ramzan, M.; Löfâs, H.; Ahuja, Rajeev
2013-01-01
In this article, we use hybrid density functional (HSE06) to study the crystal and electronic structures and optical properties of well known phase change memory material Ge2Sb2Te5. We calculate the structural parameters, band gaps, and dielectric functions of three stable structures of this material. We also analyze the electron charge distribution using the Bader's theory of charge analysis. We find that hybrid density functional slightly overestimates the value of "c" parameter. However, overall, our results calculated with the use of hybrid density functional (HSE06) are very close to available experimental values than calculated with the use of Perdew Burke-Ernzerhof functional. Specifically, the electronic band gap values of this material calculated with HSE06 are in good agreement with the available experimental data in the literature. Furthermore, we perform the charge analysis and find that naive ionic model fails to explain the charge distribution between the constituent atoms, showing the complex nature of this compound.
NASA Astrophysics Data System (ADS)
Liu, Xia; Tan, Yingzi; Li, Xiuling; Wu, Xiaojun; Pei, Yong
2015-08-01
The electronic and magnetic properties of transition metal (TM = Sc, Ti, V, Cr and Mn) atom incorporated single and double one-dimensional (1D) styrene molecular wires confined on the hydrogen-terminated Si(100) surface are explored for the first time by means of spin-polarized density functional theory, denoted as Si-[TM(styrene)]. It is unveiled that TM atoms bind asymmetrically to the adjacent phenyl rings, which leads to novel electronic and magnetic properties in stark contrast to the well-studied gas phase TM-benzene molecular wires. Si-[Mn(styrene)]∞ and Si-[Cr(styrene)]∞ single molecular wires (SMWs) are a ferromagnetic semiconductor and half metal, respectively. Creation of H-atom defects on the silicon surface can introduce an impurity metallic band, which leads to novel half-metallic magnetism of a Si-[Mn(styrene)]∞ system. Moreover, double molecular wires (DMWs) containing two identical or hetero SMWs are theoretically designed. The [Mn(styrene)]∞-[Cr(styrene)]∞ DMW exhibits half-metallic magnetism where the spin-up and spin-down channels are contributed by two single molecular wires. Finally, we demonstrate that introducing a TM-defect may significantly affect the electronic structure and magnetic properties of molecular wires. These studies provide new insights into the structure and properties of surface supported 1-D sandwiched molecular wires and may inspire the future experimental synthesis of substrate confined organometallic sandwiched molecular wires.The electronic and magnetic properties of transition metal (TM = Sc, Ti, V, Cr and Mn) atom incorporated single and double one-dimensional (1D) styrene molecular wires confined on the hydrogen-terminated Si(100) surface are explored for the first time by means of spin-polarized density functional theory, denoted as Si-[TM(styrene)]. It is unveiled that TM atoms bind asymmetrically to the adjacent phenyl rings, which leads to novel electronic and magnetic properties in stark contrast to
Uzunova, Ellie L; Mikosch, Hans
2012-03-29
The dimers of cobalt oxide (CoO)(2) with cyclic and open bent structure are studied with the B1LYP density functional; the ordering of states is validated by the CCSD(T) method. The D(2h)-symmetry rhombic dioxide Co(2)O(2) with antiferromagnetically ordered electrons on cobalt centers is the global minimum. The cyclic peroxide Co(2)(O(2)) with side-on-bonded dioxygen in (7)B(2) ground state is separated from the global minimum by an energy gap of 3.15 eV. The dioxide is highly reactive as indicated by the high value of proton affinity and chemical reactivity indices. The four-member ring structures are more stable than those with three-member ring or chain configuration. The thermodynamic stability toward dissociation to CoO increases upon carbonylation, whereas proton affinity and reactivity with release of molecular oxygen also increase. The global minimum of Co(2)O(2)(CO)(6) corresponds to a triplet state (3)A" with oxygen atoms shifted above the molecular plane of the rhombic dioxide Co(2)O(2). The SOMO-LUMO gap in the ground-state carbonylated dioxide is wider, compared to the same gap in the bare dicobalt dioxide. The peroxo-isomer Co(2)(O(2))(CO)(6) retains the planar Co(2)(O(2)) ring and is only stable in a high-spin state (7)A". The carbonylated clusters have increased reactivity in both redox and nucleophilic reactions, as a result of the increased electron density in the Co(2)O(2)-ring area. PMID:22397598
Domain boundaries in silicene: Density functional theory calculations on electronic properties
NASA Astrophysics Data System (ADS)
Xiao, Hong-Jun; Zhang, Li-Zhi; Du, Shi-Xuan; Gao, Hong-Jun
2015-08-01
By using density functional theory (DFT)-based first-principles calculations, the structural stability and electronic properties for two kinds of silicene domain boundaries, forming along armchair edge and zigzag edge, have been investigated. The results indicate that a linkage of tetragonal and octagonal rings (4|8) appears along the armchair edge, while a linkage of paired pentagonal and octagonal rings (5|5|8) appears along the zigzag edge. Different from graphene, the buckling properties of silicene lead to two mirror symmetrical edges of silicene line-defect. The formation energies indicate that the 5|5|8 domain boundary is more stable than the 4|8 domain boundary. Similar to graphene, the calculated electronic properties show that the 5|5|8 domain boundaries exhibit metallic properties and the 4|8 domain boundaries are half-metal. Both domain boundaries create the perfect one-dimensional (1D) metallic wires. Due to the metallic properties, these two kinds of nanowires can be used to build the silicene-based devices. Project supported by the National Natural Science Foundation of China (Grant Nos. 61390501 and 51325204), the National Basic Research Program of China (Grant Nos. 2011CB808401 and 2011CB921702), and the Tainjin Supercomputing Center, Chinese Academy of Sciences.
NASA Astrophysics Data System (ADS)
Andrzejak, Marcin; Sterzel, Mariusz; Pawlikowski, Marek T.
2005-07-01
The absorption spectra of the N-(2,5-di- tert-butylphenyl) phthalimide ( 1-), N-(2,5-di- tert-butylphenyl)-1,8-naphthalimide ( 2-) and N-(2,5-di- tert-butylphenyl)-perylene-3,4-dicarboximide ( 3-) anion radicals are studied in terms of time dependent density functional theory (TDDFT). For these anion radicals a large number electronic states (from 30 to 60) was found in the visible and near-IR regions (5000-45000 cm -1). In these regions the TD/B3LYP treatment at the 6-1+G* level is shown to reproduce satisfactorily the empirical absorption spectra of all three anion radicals studied. The most apparent discrepancies between purely electronic theory and the experiment could be found in the excitation region corresponding to D0→ D1 transitions in the 2- and 3- molecules. For these species we argue that the structures seen in the lowest energy part of the absorptions of the 2- and 3- species are very likely due to Franck-Condon (FC) activity of the totally symmetric vibrations not studied in this Letter.
NASA Astrophysics Data System (ADS)
Wang, Jun-Fei; Fu, Xiao-Nan; Zhang, Xiao-Dong; Wang, Jun-Tao; Li, Xiao-Dong; Jiang, Zhen-Yi
2016-08-01
The structural, elastic, electronic, and thermodynamic properties of thermoelectric material MgAgSb in γ,β,α phases are studied with first-principles calculations based on density functional theory. The optimized lattice constants accord well with the experimental data. According to the calculated total energy of the three phases, the phase transition order is determined from α to γ phase with cooling, which is in agreement with the experimental result. The physical properties such as elastic constants, bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and anisotropy factor are also discussed and analyzed, which indicates that the three structures are mechanically stable and each has a ductile feature. The Debye temperature is deduced from the elastic properties. The total density of states (TDOS) and partial density of states (PDOS) of the three phases are investigated. The TDOS results show that the γ phase is most stable with a pseudogap near the Fermi level, and the PDOS analysis indicates that the conduction band of the three phases is composed mostly of Mg-3s, Ag-4d, and Sb-5p. In addition, the changes of the free energy, entropy, specific heat, thermal expansion of γ-MgAgSb with temperature are obtained successfully. The obtained results above are important parameters for further experimental and theoretical tuning of doped MgAgSb as a thermoelectric material at high temperature. Project supported by the National Natural Science Foundation of China (Grant No. 11504088), the Fund from Henan University of Technology, China (Grant Nos. 2014YWQN08 and 2013JCYJ12), the Natural Science Fund from the Henan Provincial Education Department, China (Grant No. 16A140027), the Natural Science Foundation of Shaanxi Province of China (Grant Nos. 2013JQ1018 and 15JK1759), and the Science Foundation of Northwest University of China (Grant No. 14NW23).
Density-functional theory study of gramicidin A ion channel geometry and electronic properties
Todorović, Milica; Bowler, David R.; Gillan, Michael J.; Miyazaki, Tsuyoshi
2013-01-01
Understanding the mechanisms underlying ion channel function from the atomic-scale requires accurate ab initio modelling as well as careful experiments. Here, we present a density functional theory (DFT) study of the ion channel gramicidin A (gA), whose inner pore conducts only monovalent cations and whose conductance has been shown to depend on the side chains of the amino acids in the channel. We investigate the ground state geometry and electronic properties of the channel in vacuum, focusing on their dependence on the side chains of the amino acids. We find that the side chains affect the ground state geometry, while the electrostatic potential of the pore is independent of the side chains. This study is also in preparation for a full, linear scaling DFT study of gA in a lipid bilayer with surrounding water. We demonstrate that linear scaling DFT methods can accurately model the system with reasonable computational cost. Linear scaling DFT allows ab initio calculations with 10 000–100 000 atoms and beyond, and will be an important new tool for biomolecular simulations. PMID:24068174
Nagy, A; Amovilli, C
2008-03-21
In the ground state, the pair density n can be determined by solving a single auxiliary equation of a two-particle problem. Electron-electron cusp condition and asymptotic behavior for the Pauli potential of the effective potential of the two-particle equation are presented. PMID:18361562
Density-functional study of atomic and electronic structures of multivacancies in silicon carbide
NASA Astrophysics Data System (ADS)
Iwata, Jun-Ichi; Shinei, Chikara; Oshiyama, Atsushi
2016-03-01
We report the density-functional calculations that provide a firm theoretical framework to identify the multivacancies and unravel the underlying physics in the most stable silicon carbide polytype 4H-SiC. The calculations with the generalized gradient approximation (GGA) for the Si and C monovacancy, VSi and VC, have clarified the significantly lower formation energy of the C vacancy accompanied by the efficient pairing relaxation of the surrounding Si dangling bonds. Our GGA calculation also predicts a stable next-neighbor VCVC divacancy which is lower in the formation energy than the usual nearest-neighbor divacancy VCVSi discussed in the past. We also perform the calculations with the hybrid functional and confirm the stability of the next-neighbor VCVC divacancy. Our calculations indeed clarify that it is possible to detect the VCVC with its peculiar hyperfine coupling constants by the electron paramagnetic resonance (EPR) measurements. Based on the structural characteristics and the energetics for the monovacancy, we further propose an extended dangling-bond-counting (EDBC) model to choose the energetically favorable topological network of the vacant sites for the multivacancy. The GGA calculations combined with the EDBC model reveal that V3 and V6 are energetically favorable. The stable V3 is a nearest-neighbor complex of VC-VSi-VC , whereas the V6 is the high-symmetry V5 (the central VSi surrounded by four VC) plus a symmetry-breaking next-neighbor VC. We perform the GGA calculations for the electronic structure of such V3 and V6 and discuss the possibility of detecting these multivacancies. In particular, the EPR-detected ANN1 center is provisionally identified as the doubly positive trivacancy VC-VSi-VC .
Assessment of the ΔSCF density functional theory approach for electronic excitations in organic dyes
Kowalczyk, T.; Yost, S. R.; Van Voorhis, T.
2010-01-01
This paper assesses the accuracy of the ΔSCF method for computing low-lying HOMO→LUMO transitions in organic dye molecules. For a test set of vertical excitation energies of 16 chromophores, surprisingly similar accuracy is observed for time-dependent density functional theory and for ΔSCF density functional theory. In light of this performance, we reconsider the ad hoc ΔSCF prescription and demonstrate that it formally obtains the exact stationary density within the adiabatic approximation, partially justifying its use. The relative merits and future prospects of ΔSCF for simulating individual excited states are discussed.
Superheavy Element Chemistry by Relativistic Density Functional Theory Electronic Structure Modeling
NASA Astrophysics Data System (ADS)
Zaitsevskii, A. V.; Polyaev, A. V.; Demidov, Yu. A.; Mosyagin, N. S.; Lomachuk, Yu. V.; Titov, A. V.
2015-06-01
Two-component density functional theory in its non-collinear formulation combined with the accurate relativistic electronic structure model defined by shape-consistent small-core pseudopotentials (PP/RDFT) provides a robust basis of efficient computational schemes for predicting energetic and structural properties of complex polyatomic systems including superheavy elements (SHEs). Because of the exceptional role of thermochromatography in the experiments on the "chemical" identification of SHEs with atomic numbers Z ≥ 112, we focus on the description of the adsorption of single SHE atoms on the surfaces of solids through cluster modeling of adsorption complexes. In some cases our results differ significantly from those of previous theoretical studies. The results of systematic comparative studies on chemical bonding in simple molecules of binary compounds of SHEs and their nearest homologs with most common light elements, obtained at the PP/RDFT level and visualized through the "chemical graphs", provide the understanding of the general chemistry of SHEs which at present cannot be derived from the experimental data. These results are used to discuss the main trends in changing chemical properties of the elements in the given group of the periodic table and demonstrate the specificity of SHEs.
NASA Astrophysics Data System (ADS)
Turel, N.; Arikan, F.
2010-12-01
Ionospheric channel characterization is an important task for both HF and satellite communications. The inherent space-time variability of the ionosphere can be observed through total electron content (TEC) that can be obtained using GPS receivers. In this study, within-the-hour variability of the ionosphere over high-latitude, midlatitude, and equatorial regions is investigated by estimating a parametric model for the probability density function (PDF) of GPS-TEC. PDF is a useful tool in defining the statistical structure of communication channels. For this study, a half solar cycle data is collected for 18 GPS stations. Histograms of TEC, corresponding to experimental probability distributions, are used to estimate the parameters of five different PDFs. The best fitting distribution to the TEC data is obtained using the maximum likelihood ratio of the estimated parametric distributions. It is observed that all of the midlatitude stations and most of the high-latitude and equatorial stations are distributed as lognormal. A representative distribution can easily be obtained for stations that are located in midlatitude using solar zenith normalization. The stations located in very high latitudes or in equatorial regions cannot be described using only one PDF distribution. Due to significant seasonal variability, different distributions are required for summer and winter.
NASA Astrophysics Data System (ADS)
Chen, Hai-Hua; Bi, Yan; Cheng, Yan; Ji, Guangfu; Cai, Lingcang
2012-10-01
The elastic properties, electronic structure and thermodynamic behavior of the TaB have been investigated for the first time in this work. Using first-principles plane-wave ultrasoft-pseudopotential density functional theory (DFT), the ground state properties and equation of state of TaB have been obtained. The average zero-pressure bulk modulus of TaB is 302 GPa. By analyzing the elastically anisotropic behavior and the relative structure parameters of TaB, we found that the crystal cell along the b-axis was more compressible than along the a and c axes. The calculated ratio of bulk modulus and shear modulus (B/G) for TaB is 1.58, demonstrating that TaB is rather brittle. From the elastic stiffness constants, we found that TaB in the Cmcm phase is mechanically stable. The calculated hardness of TaB is 28.6 GPa which is close to the previous data. Moreover, using the Gibbs 2 model, the thermodynamic properties such as the thermal expansion and Debye temperature of TaB have been obtained firstly. At the ambient temperature, the Debye temperatures of TaB are 792 K and 845 K from GGA calculation and LDA calculation, respectively.
NASA Astrophysics Data System (ADS)
Abdulsattar, Mudar Ahmed
2016-04-01
Energetically stable Ge diamondoids are used as building blocks to investigate Ge nanocrystal properties using density functional theory (DFT). Energy gap, HOMO and LUMO of Ge diamondoids vary according to confinement theory with shape fluctuations. Ge diamondoid vibrational force constants reach 2.18 mDyne/Å which is between that of bulk silicon and tin. Ge-Ge vibrational frequencies and reduced masses reach 357.47 cm‑1 and 41.93 amu, respectively, which are higher than the values of bulk Ge. Size variation of UV-Vis shows that the maximum optical peak moves from 163 nm to nearly 290 nm as the size of Ge diamondoids and molecules increases. The higher peak value approaches that of experimental Ge quantum dots at 300 nm. NMR spectra of Ge diamondoids are analyzed as a function of diamondoid sizes. Our results show that all investigated spectroscopic tools are sensitive to molecular or nanocrystals size. NMR is particularly good size indicator. Natural bond orbital (NBO) population analysis shows present diamondoid bondings differ from ideal sp3 bonding. The bonding for Ge valence orbitals is in the range (4s1.244p2.60) ‑ (4s1.274p2.73) depending on distance between Ge atom and diamondoid surface. Highest Ge diamondoids vibrational longitudinal optical (LO) mode is blue shifted with respect to experimental bulk value which is the opposite case for C and Si. H surface atom effects on electronic and vibrational properties are discussed.
Gidopoulos, Nikitas I.; Gross, E. K. U.
2014-01-01
A novel treatment of non-adiabatic couplings is proposed. The derivation is based on a theorem by Hunter stating that the wave function of the complete system of electrons and nuclei can be written, without approximation, as a Born–Oppenheimer (BO)-type product of a nuclear wave function, X(R), and an electronic one, ΦR(r), which depends parametrically on the nuclear configuration R. From the variational principle, we deduce formally exact equations for ΦR(r) and X(R). The algebraic structure of the exact nuclear equation coincides with the corresponding one in the adiabatic approximation. The electronic equation, however, contains terms not appearing in the adiabatic case, which couple the electronic and the nuclear wave functions and account for the electron–nuclear correlation beyond the BO level. It is proposed that these terms can be incorporated using an optimized local effective potential. PMID:24516183
Visualization of electronic density
Grosso, Bastien; Cooper, Valentino R.; Pine, Polina; Hashibon, Adham; Yaish, Yuval; Adler, Joan
2015-04-22
An atom’s volume depends on its electronic density. Although this density can only be evaluated exactly for hydrogen-like atoms, there are many excellent numerical algorithms and packages to calculate it for other materials. 3D visualization of charge density is challenging, especially when several molecular/atomic levels are intertwined in space. We explore several approaches to 3D charge density visualization, including the extension of an anaglyphic stereo visualization application based on the AViz package to larger structures such as nanotubes. We will describe motivations and potential applications of these tools for answering interesting questions about nanotube properties.
Partition density functional theory
NASA Astrophysics Data System (ADS)
Nafziger, Jonathan
Partition density functional theory (PDFT) is a method for dividing a molecular electronic structure calculation into fragment calculations. The molecular density and energy corresponding to Kohn Sham density-functional theory (KS-DFT) may be exactly recovered from these fragments. Each fragment acts as an isolated system except for the influence of a global one-body 'partition' potential which deforms the fragment densities. In this work, the developments of PDFT are put into the context of other fragment-based density functional methods. We developed three numerical implementations of PDFT: One within the NWChem computational chemistry package using basis sets, and the other two developed from scratch using real-space grids. It is shown that all three of these programs can exactly reproduce a KS-DFT calculation via fragment calculations. The first of our in-house codes handles non-interacting electrons in arbitrary one-dimensional potentials with any number of fragments. This code is used to explore how the exact partition potential changes for different partitionings of the same system and also to study features which determine which systems yield non-integer PDFT occupations and which systems are locked into integer PDFT occupations. The second in-house code, CADMium, performs real-space calculations of diatomic molecules. Features of the exact partition potential are studied for a variety of cases and an analytical formula determining singularities in the partition potential is derived. We introduce an approximation for the non-additive kinetic energy and show how this quantity can be computed exactly. Finally a PDFT functional is developed to address the issues of static correlation and delocalization errors in approximations within DFT. The functional is applied to the dissociation of H2 + and H2.
Lykissa, Iliana; Li, Shu-Yi; Granqvist, Claes G.; Niklasson, Gunnar A.; Ramzan, Muhammad; Chakraborty, Sudip; Ahuja, Rajeev
2014-05-14
Thin films of V{sub 2}O{sub 5} were prepared by sputter deposition onto transparent and electrically conducting substrates and were found to be X-ray amorphous. Their electrochemical density of states was determined by chronopotentiometry and displayed a pronounced low-energy peak followed by an almost featureless contribution at higher energies. These results were compared with density functional theory calculations for amorphous V{sub 2}O{sub 5}. Significant similarities were found between measured data and computations; specifically, the experimental low-energy peak corresponds to a split-off part of the conduction band apparent in the computations. Furthermore, the calculations approximately reproduce the experimental band gap observed in optical measurements.
Structural and electronic properties of poly(vinyl alcohol) using density functional theory
Dabhi, Shweta Jha, Prafulla K.
2014-04-24
The first principles calculations have been carried out to investigate the structural, electronic band structure density of states along with the projected density of states for poly(vinyl alcohol). Our structural calculation suggests that the poly(vinyl alcohol) exhibits monoclinic structure. The calculated structural lattice parameters are in excellent agreement with available experimental values. The band structure calculations reveal that the direct and indirect band gaps are 5.55 eV and 5.363 eV respectively in accordance with experimental values.
Oshiyama, Atsushi Iwata, Jun-Ichi; Uchida, Kazuyuki; Matsushita, Yu-Ichiro
2015-03-21
We show that our real-space finite-difference scheme allows us to perform density-functional calculations for nanometer-scale targets containing more than 100 000 atoms. This real-space scheme is applied to twisted bilayer graphene, clarifying that Moiré pattern induced in the slightly twisted bilayer graphene drastically modifies the atomic and electronic structures.
Svane, A.; Trygg, J.; Johansson, B.; Eriksson, O. |
1997-09-01
Electronic-structure calculations of elemental praseodymium are presented. Several approximations are used to describe the Pr f electrons. It is found that the low-pressure, trivalent phase is well described using either the self-interaction corrected (SIC) local-spin-density (LSD) approximation or the generalized-gradient approximation (GGA) with spin and orbital polarization (OP). In the SIC-LSD approach the Pr f electrons are treated explicitly as localized with a localization energy given by the self-interaction of the f orbital. In the GGA+OP scheme the f-electron localization is described by the onset of spin and orbital polarization, the energetics of which is described by spin-moment formation energy and a term proportional to the total orbital moment, L{sub z}{sup 2}. The high-pressure phase is well described with the f electrons treated as band electrons, in either the LSD or the GGA approximations, of which the latter describes more accurately the experimental equation of state. The calculated pressure of the transition from localized to delocalized behavior is 280 kbar in the SIC-LSD approximation and 156 kbar in the GGA+OP approach, both comparing favorably with the experimentally observed transition pressure of 210 kbar. {copyright} {ital 1997} {ital The American Physical Society}
Visualization of electronic density
NASA Astrophysics Data System (ADS)
Grosso, Bastien; Cooper, Valentino R.; Pine, Polina; Hashibon, Adham; Yaish, Yuval; Adler, Joan
2015-10-01
The spatial volume occupied by an atom depends on its electronic density. Although this density can only be evaluated exactly for hydrogen-like atoms, there are many excellent algorithms and packages to calculate it numerically for other materials. Three-dimensional visualization of charge density is challenging, especially when several molecular/atomic levels are intertwined in space. In this paper, we explore several approaches to this, including the extension of an anaglyphic stereo visualization application based on the AViz package for hydrogen atoms and simple molecules to larger structures such as nanotubes. We will describe motivations and potential applications of these tools for answering interesting physical questions about nanotube properties.
Mentel, Ł M; van Meer, R; Gritsenko, O V; Baerends, E J
2014-06-01
For chemistry an accurate description of bond weakening and breaking is vital. The great advantage of density matrix functionals, as opposed to density functionals, is their ability to describe such processes since they naturally cover both nondynamical and dynamical correlation. This is obvious in the Löwdin-Shull functional, the exact natural orbital functional for two-electron systems. We present in this paper extensions of this functional for the breaking of a single electron pair bond in N-electron molecules, using LiH, BeH(+), and Li2 molecules as prototypes. Attention is given to the proper formulation of the functional in terms of not just J and K integrals but also the two-electron L integrals (K integrals with a different distribution of the complex conjugation of the orbitals), which is crucial for the calculation of response functions. Accurate energy curves are obtained with extended Löwdin-Shull functionals along the complete dissociation coordinate using full CI calculations as benchmark. PMID:24907988
NASA Astrophysics Data System (ADS)
Mentel, Ł. M.; van Meer, R.; Gritsenko, O. V.; Baerends, E. J.
2014-06-01
For chemistry an accurate description of bond weakening and breaking is vital. The great advantage of density matrix functionals, as opposed to density functionals, is their ability to describe such processes since they naturally cover both nondynamical and dynamical correlation. This is obvious in the Löwdin-Shull functional, the exact natural orbital functional for two-electron systems. We present in this paper extensions of this functional for the breaking of a single electron pair bond in N-electron molecules, using LiH, BeH+, and Li2 molecules as prototypes. Attention is given to the proper formulation of the functional in terms of not just J and K integrals but also the two-electron L integrals (K integrals with a different distribution of the complex conjugation of the orbitals), which is crucial for the calculation of response functions. Accurate energy curves are obtained with extended Löwdin-Shull functionals along the complete dissociation coordinate using full CI calculations as benchmark.
Mentel, Ł. M.; Meer, R. van; Gritsenko, O. V.; Baerends, E. J.
2014-06-07
For chemistry an accurate description of bond weakening and breaking is vital. The great advantage of density matrix functionals, as opposed to density functionals, is their ability to describe such processes since they naturally cover both nondynamical and dynamical correlation. This is obvious in the Löwdin-Shull functional, the exact natural orbital functional for two-electron systems. We present in this paper extensions of this functional for the breaking of a single electron pair bond in N-electron molecules, using LiH, BeH{sup +}, and Li{sub 2} molecules as prototypes. Attention is given to the proper formulation of the functional in terms of not just J and K integrals but also the two-electron L integrals (K integrals with a different distribution of the complex conjugation of the orbitals), which is crucial for the calculation of response functions. Accurate energy curves are obtained with extended Löwdin-Shull functionals along the complete dissociation coordinate using full CI calculations as benchmark.
NASA Astrophysics Data System (ADS)
van Meer, R.; Gritsenko, O. V.; Baerends, E. J.
2014-01-01
Time dependent density matrix functional theory in its adiabatic linear response formulation delivers exact excitation energies ωα and oscillator strengths fα for two-electron systems if extended to the so-called phase including natural orbital (PINO) theory. The Löwdin-Shull expression for the energy of two-electron systems in terms of the natural orbitals and their phases affords in this case an exact phase-including natural orbital functional (PILS), which is non-primitive (contains other than just J and K integrals). In this paper, the extension of the PILS functional to N-electron systems is investigated. With the example of an elementary primitive NO functional (BBC1) it is shown that current density matrix functional theory ground state functionals, which were designed to produce decent approximations to the total energy, fail to deliver a qualitatively correct structure of the (inverse) response function, due to essential deficiencies in the reconstruction of the two-body reduced density matrix (2RDM). We now deduce essential features of an N-electron functional from a wavefunction Ansatz: The extension of the two-electron Löwdin-Shull wavefunction to the N-electron case informs about the phase information. In this paper, applications of this extended Löwdin-Shull (ELS) functional are considered for the simplest case, ELS(1): one (dissociating) two-electron bond in the field of occupied (including core) orbitals. ELS(1) produces high quality ωα(R) curves along the bond dissociation coordinate R for the molecules LiH, Li2, and BH with the two outer valence electrons correlated. All of these results indicate that response properties are much more sensitive to deficiencies in the reconstruction of the 2RDM than the ground state energy, since derivatives of the functional with respect to both the NOs and the occupation numbers need to be accurate.
Meer, R. van; Gritsenko, O. V.; Baerends, E. J.
2014-01-14
Time dependent density matrix functional theory in its adiabatic linear response formulation delivers exact excitation energies ω{sub α} and oscillator strengths f{sub α} for two-electron systems if extended to the so-called phase including natural orbital (PINO) theory. The Löwdin-Shull expression for the energy of two-electron systems in terms of the natural orbitals and their phases affords in this case an exact phase-including natural orbital functional (PILS), which is non-primitive (contains other than just J and K integrals). In this paper, the extension of the PILS functional to N-electron systems is investigated. With the example of an elementary primitive NO functional (BBC1) it is shown that current density matrix functional theory ground state functionals, which were designed to produce decent approximations to the total energy, fail to deliver a qualitatively correct structure of the (inverse) response function, due to essential deficiencies in the reconstruction of the two-body reduced density matrix (2RDM). We now deduce essential features of an N-electron functional from a wavefunction Ansatz: The extension of the two-electron Löwdin-Shull wavefunction to the N-electron case informs about the phase information. In this paper, applications of this extended Löwdin-Shull (ELS) functional are considered for the simplest case, ELS(1): one (dissociating) two-electron bond in the field of occupied (including core) orbitals. ELS(1) produces high quality ω{sub α}(R) curves along the bond dissociation coordinate R for the molecules LiH, Li{sub 2}, and BH with the two outer valence electrons correlated. All of these results indicate that response properties are much more sensitive to deficiencies in the reconstruction of the 2RDM than the ground state energy, since derivatives of the functional with respect to both the NOs and the occupation numbers need to be accurate.
van Meer, R; Gritsenko, O V; Baerends, E J
2014-01-14
Time dependent density matrix functional theory in its adiabatic linear response formulation delivers exact excitation energies ωα and oscillator strengths fα for two-electron systems if extended to the so-called phase including natural orbital (PINO) theory. The Löwdin-Shull expression for the energy of two-electron systems in terms of the natural orbitals and their phases affords in this case an exact phase-including natural orbital functional (PILS), which is non-primitive (contains other than just J and K integrals). In this paper, the extension of the PILS functional to N-electron systems is investigated. With the example of an elementary primitive NO functional (BBC1) it is shown that current density matrix functional theory ground state functionals, which were designed to produce decent approximations to the total energy, fail to deliver a qualitatively correct structure of the (inverse) response function, due to essential deficiencies in the reconstruction of the two-body reduced density matrix (2RDM). We now deduce essential features of an N-electron functional from a wavefunction Ansatz: The extension of the two-electron Löwdin-Shull wavefunction to the N-electron case informs about the phase information. In this paper, applications of this extended Löwdin-Shull (ELS) functional are considered for the simplest case, ELS(1): one (dissociating) two-electron bond in the field of occupied (including core) orbitals. ELS(1) produces high quality ωα(R) curves along the bond dissociation coordinate R for the molecules LiH, Li2, and BH with the two outer valence electrons correlated. All of these results indicate that response properties are much more sensitive to deficiencies in the reconstruction of the 2RDM than the ground state energy, since derivatives of the functional with respect to both the NOs and the occupation numbers need to be accurate. PMID:24437859
NASA Astrophysics Data System (ADS)
Mata-Pinzon, Zaahel; Valladares, Ariel Alberto; Valladares, Alexander; Valladares, Renela Maria
2014-03-01
The properties of materials are strongly related to their atomic topology, especially when we compare properties related to the ordered and disordered phases. Using Density Functional Theory methods on 64-atom supercells we obtain the structure and calculate the electronic density of states (eDOS) as a function of the concentration of lead, thallium or antimony in an amorphous bismuth supercell. This is done to investigate how the eDOS affects the superconducting transition temperature (Tc), taking into account the measurements made by Shier and Ginsberg[2] on contaminated amorphous bismuth thin films. Supported by CONACYT and DGAPA (UNAM).
NASA Astrophysics Data System (ADS)
Kurashige, Yuki
2014-06-01
Recent advances in quantum chemical density matrix renormalisation group (DMRG) theory are presented. The DMRG, originally devised as an alternative to the exact diagonalisation in condensed matter physics, has become a powerful quantum chemical method for molecular systems that exhibit a multireference character, e.g., excited states, π-conjugated systems, transition metal complexes, and in particular for large systems by combining it with conventional multireference electron correlation methods. The capability of the current quantum chemical DMRG is demonstrated for an application involving the potential energy curve of the chromium dimer, which is one of the most demanding multireference systems and thus requires the best electronic structure treatment for non-dynamical and dynamical correlation as well as large basis sets.
Predicting a new photocatalyst and its electronic properties by density functional theory
NASA Astrophysics Data System (ADS)
Sarker, Pranab; Prasher, Dixit; Gaillard, Nicolas; Huda, Muhammad N.
2013-10-01
A new material CuBiW2O8 is reported here which is suitable for photocatalysts for solar-to-hydrogen generation by splitting water through photoelectrochemical approach. By density functional theory total energy calculations along with extensive mineral database search of relevant oxides, the crystal structures of CuBiW2O8 has been determined, which agrees well with the experimental result. We have analyzed the thermodynamical stability of this material. Its stability was found to be comparable to other well-known oxides, such as CuWO4. The band structure calculation reveals that it has a suitable band gap. In addition to this, density of states and optical absorption calculations show favorable features of a photocatalyst.
NASA Astrophysics Data System (ADS)
Ferri, Nicola; DiStasio, Robert A.; Ambrosetti, Alberto; Car, Roberto; Tkatchenko, Alexandre
2015-05-01
How strong is the effect of van der Waals (vdW) interactions on the electronic properties of molecules and extended systems? To answer this question, we derived a fully self-consistent implementation of the density-dependent interatomic vdW functional of Tkatchenko and Scheffler [Phys. Rev. Lett. 102, 073005 (2009)]. Not surprisingly, vdW self-consistency leads to tiny modifications of the structure, stability, and electronic properties of molecular dimers and crystals. However, unexpectedly large effects were found in the binding energies, distances, and electrostatic moments of highly polarizable alkali-metal dimers. Most importantly, vdW interactions induced complex and sizable electronic charge redistribution in the vicinity of metallic surfaces and at organic-metal interfaces. As a result, a substantial influence on the computed work functions was found, revealing a nontrivial connection between electrostatics and long-range electron correlation effects.
Andrés, Juan; Berski, Sławomir; Silvi, Bernard
2016-07-01
Probing the electron density transfers during a chemical reaction can provide important insights, making possible to understand and control chemical reactions. This aim has required extensions of the relationships between the traditional chemical concepts and the quantum mechanical ones. The present work examines the detailed chemical insights that have been generated through 100 years of work worldwide on G. N. Lewis's ground breaking paper on The Atom and the Molecule (Lewis, G. N. The Atom and the Molecule, J. Am. Chem. Soc. 1916, 38, 762-785), with a focus on how the determination of reaction mechanisms can be reached applying the bonding evolution theory (BET), emphasizing how curly arrows meet electron density transfers in chemical reaction mechanisms and how the Lewis structure can be recovered. BET that combines the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT) provides a powerful tool providing insight into molecular mechanisms of chemical rearrangements. In agreement with physical laws and quantum theoretical insights, BET can be considered as an appropriate tool to tackle chemical reactivity with a wide range of possible applications. Likewise, the present approach retrieves the classical curly arrows used to describe the rearrangements of chemical bonds for a given reaction mechanism, providing detailed physical grounds for this type of representation. The ideas underlying the valence-shell-electron pair-repulsion (VSEPR) model applied to non-equilibrium geometries provide simple chemical explanations of density transfers. For a given geometry around a central atom, the arrangement of the electronic domain may comply or not with the VSEPR rules according with the valence shell population of the considered atom. A deformation yields arrangements which are either VSEPR defective (at least a domain is missing to match the VSEPR arrangement corresponding to the geometry of the ligands), VSEPR compliant
Magnetic properties of f-electron systems in spin-polarized relativistic density functional theory
NASA Astrophysics Data System (ADS)
Yamagami, H.; Mavromaras, A.; Kübler, J.
1997-12-01
The magnetic ground state of the series of lanthanide and actinide trivalent ions is investigated by means of spin-polarized relativistic spin-density functional theory. In the local density functional approximation (LDA) an internal effective magnetic field due to exchange and correlation couples to the spin degrees of freedom. The resulting set of coupled Dirac equations yields ground-state multiplets that obey the well-known Hund's rules. This remarkable result comes about by the coupling of the j = l + 1/2 with the j = l - 1/2 states due to the exchange - correlation potential that is, as usual, the functional derivative of the exchange - correlation energy with respect to the spin magnetic moment. The effect of the coupling is shown to depend on the varying relative strengths of spin - orbit coupling and exchange splitting within the f series. Since in the f levels the internal exchange splitting dominates rather than the spin - orbit splitting, the energy level scheme is that of the Paschen - Back effect, and thus features of the Russell - Saunders coupling persist in spite of relativistic effects.
Itinerant Electron Magnets: Curie Temperature and Susceptibility in Density-Functional Theory
NASA Astrophysics Data System (ADS)
Mohn, Peter; Khmelevskyi, Sergei
Groundstate properties of solids are astonishingly well described by the local density functional approximation (LDA) [1]. This is also true of metallic magnets for which the situation was recently described by this author [2]. Excited-state properties of magnets (and other systems), however, are still a great challenge and it was believed until recently that the band-picture, based in the LDA, fails entirely in describing magnetism at elevated temperatures. We emphasize here that this is not so, attempting first to expose the reason why it was thought that the band-picture fails. Since historically the underlying physical picture was developed by Stoner and Wohlfarth, we begin with a discussion of their theory using, however, an approach that reveals the essential assumptions. This is Mermin's [3] finite-temperature density functional theory. Two points emerge: one is the essential noncollinearity of the magnetic moments at finite temperatures, the other is the form of the exchange-corr elation contribution to the thermodynamic potential. We know how to deal with noncollinear order in the LDA and we explain how we might use this knowledge to advance the issue. Exchange and correlation at finite temperatures are, however, at the present state not well understood. This statement not only applies to density functional theory but also to many-body treatments addressed at this workshop. We will show in particular that the theory of magnons in the band-LDA-picture at low temperatures is in good shape. At high temperatures we opt for a theory involving spin fluctuations and argue that, although broad features of the magnetic phase transition are described satisfactorily, many details await further improvements.
NASA Astrophysics Data System (ADS)
Skone, Jonathan; Govoni, Marco; Galli, Giulia
Dielectric-dependent hybrid [DDH] functionals have recently been shown to yield highly accurate energy gaps and dielectric constants for a wide variety of solids, at a computational cost considerably less than standard GW calculations. The fraction of exact exchange included in the definition of DDH functionals depends (self-consistently) on the dielectric constant of the material. In the present talk we introduce a range-separated (RS) version of DDH functionals where short and long-range components are matched using material dependent, non-empirical parameters. Comparing with state of the art GW calculations and experiment, we show that such RS hybrids yield accurate electronic properties of both molecules and solids, including energy gaps, photoelectron spectra and absolute ionization potentials. This work was supported by NSF-CCI Grant Number NSF-CHE-0802907 and DOE-BES.
NASA Astrophysics Data System (ADS)
Rathi, Somilkumar; Ray, Asok
2008-03-01
Ab initio calculations within the framework of hybrid density functional theory and finite cluster approximation have been performed for the electronic and geometric structures of three different types of armchair germanium carbide nanotubes from (3, 3) to (11, 11). Full geometry and spin optimizations with unrestricted symmetry have been performed. A detailed comparison of the structures and stabilities of the three types of nanotubes will be presented. The dependence of the electronic band gaps on the respective tube diameters, energy density of states, dipole moments as well as Mulliken charge distributions have been investigated. Radial buckling of tube along with bond length variations is also studied. All armchair GeC nanotubes investigated so far are semiconducting in nature. Applications in the field of nano-optoelectronic devices, molecular electronics and band gap engineering are envisioned for GeC nanotubes.
NASA Astrophysics Data System (ADS)
Nolan, Michael; Grigoleit, Sonja; Sayle, Dean C.; Parker, Stephen C.; Watson, Graeme W.
2005-02-01
We present periodic density functional theory (DFT) calculations of bulk ceria and its low index surfaces (1 1 1), (1 1 0) and (1 0 0). We find that the surface energies increase in the order (1 1 1) > (1 1 0) > (1 0 0), while the magnitude of the surface relaxations follows the inverse order. The electronic properties of the bulk and surfaces are analysed by means of the electronic density of states and the electron density. We demonstrate that the bonding in pure ceria is partially covalent and analysis of the resulting electronic states confirms the presence of localised Ce 4f states above the Fermi level. The surface atoms show only a small change in the charge distribution in comparison to the bulk and from the DOS the main differences are due to the changes in the oxygen 2p and cerium 5 d states. Investigation of the atomic and electronic structure of an oxygen vacancy on the (1 0 0) surface shows the problems DFT can have with the description of strongly localised systems, wrongly predicting electron delocalisation over all of the cerium atoms in the simulation cell. We demonstrate an improvement in the description of the strongly correlated cerium 4f states in partially reduced ceria by applying the DFT+U methodology, which leads to the appearance of a new gap state between the valence band and the empty Ce 4f band. Analysis of the partial charge density shows that these states are localised on the Ce III ions neighbouring the oxygen vacancy. In terms of classical defect chemistry, the vacancy is bound by two neighbouring Ce III ions, which have been reduced from Ce IV, i.e. VO··+2CeCe'. The remaining Ce ions are in the Ce IV oxidation state. The localisation of Ce 4f electrons modifies the predicted structure of the defective surface.
Ensling, David; Thissen, Andreas; Laubach, Stefan; Schmidt, Peter C.; Jaegermann, Wolfram
2010-11-15
The electronic properties of LiCoO{sub 2} have been studied by theoretical band-structure calculations (using density functional theory) and experimental methods (photoemission). Synchrotron-induced photoelectron spectroscopy, resonant photoemission spectroscopy (ResPES), and soft x-ray absorption (XAS) have been applied to investigate the electronic structure of both occupied and unoccupied states. High-quality PES spectra were obtained from stoichiometric and highly crystalline LiCoO{sub 2} thin films deposited ''in situ'' by rf magnetron sputtering. An experimental approach of separating oxygen- and cobalt-derived (final) states by ResPES in the valence-band region is presented. The procedure takes advantage of an antiresonant behavior of cobalt-derived states at the 3p-3d excitation threshold. Information about the unoccupied density of states has been obtained by O K XAS. The structure of the Co L absorption edge is compared to semiempirical charge-transfer multiplet calculations. The experimental results are furthermore compared with band-structure calculations considering three different exchange potentials [generalized gradient approximation (GGA), using a nonlocal Hubbard U (GGA+U) and using a hybrid functional (Becke, three-parameter, Lee-Yang-Parr [B3LYP])]. For these different approaches total density of states and partial valence-band density of states have been investigated. The best qualitative agreement with experimental results has been obtained by using a GGA+U functional with U=2.9 eV.
Effects of d-electrons in pseudopotential screened-exchange density functional calculations
NASA Astrophysics Data System (ADS)
Lee, Byounghak; Wang, Lin-Wang; Canning, Andrew
2008-06-01
We report a theoretical study on the role of shallow d states in the screened-exchange local density approximation (sX-LDA) band structure of binary semiconductor systems. We found that inaccurate pseudo-wave functions can lead to (1) an overestimation of the screened-exchange interaction between the localized d states and the delocalized higher energy s and p states, and (2) an underestimation of the screened-exchange interaction between the d states. The resulting sX-LDA band structures have substantially smaller band gaps compared with experiments. We correct the pseudo-wave functions of d states by including the semicore s and p states of the same shell in the valence states. The correction of pseudo-wave functions yields band gaps and d-state binding energies in good agreement with experiments and the full potential linearized augmented plane wave sX-LDA calculations. Compared with the quasiparticle GW method, our sX-LDA results shows not only similar quality on the band gaps but also much better d-state binding energies. Combined with its capability of ground-state structure calculation, the sX-LDA is expected to be a valuable theoretical tool for the II-VI and III-V (especially the III-N) bulk semiconductors and nanostructure studies.
Density Functionals of Chemical Bonding
Putz, Mihai V.
2008-01-01
The behavior of electrons in general many-electronic systems throughout the density functionals of energy is reviewed. The basic physico-chemical concepts of density functional theory are employed to highlight the energy role in chemical structure while its extended influence in electronic localization function helps in chemical bonding understanding. In this context the energy functionals accompanied by electronic localization functions may provide a comprehensive description of the global-local levels electronic structures in general and of chemical bonds in special. Becke-Edgecombe and author’s Markovian electronic localization functions are discussed at atomic, molecular and solid state levels. Then, the analytical survey of the main workable kinetic, exchange, and correlation density functionals within local and gradient density approximations is undertaken. The hierarchy of various energy functionals is formulated by employing both the parabolic and statistical correlation degree of them with the electronegativity and chemical hardness indices by means of quantitative structure-property relationship (QSPR) analysis for basic atomic and molecular systems. PMID:19325846
NASA Astrophysics Data System (ADS)
Çatıkkaş, Berna; Aktan, Ebru; Yalçın, Ergin
2016-08-01
This work deals with the optimized molecular structure, vibrational spectra, nonlinear optic (NLO) and frontier molecule orbital (FMO) properties of 1-Methyl-2-phenyl-3-(1,3,4-thiadiazol-2-yldiazenyl)-1H-indole (MPI) by quantum chemical calculations. The Fourier transform infrared (FT-MIR and FT-FIR) and Raman spectra of 1-Methyl-2-phenyl-3-(1,3,4-thiadiazol-2-yldiazenyl)-1H-indole (MPI) were recorded in the region (4000-400 cm-1 and 400-30 cm-1) and (3200-92 cm-1), respectively. The analysis and complete vibrational assignments of the fundamental modes of the MPI molecule were carried out by using the observed FT-IR and FT-Raman data and calculated Total Energy Distribution (TED) according to Scaled Quantum Mechanics procedure. The calculated geometrical parameters of the MPI molecule are in agreement with the obtained values from XRD studies. On the other hand, the difference between the scaled and observed wavenumber values of the most of the fundamentals are very small. 1H NMR and 13C NMR chemical shift values, and energy gap between LUMO-HOMO and molecular electrostatic potential (MEP) were investigated by using density functional theory (B3LYP) methods. UV/Visible spectra and λ maximum absorption values, the oscillator strengths in the chloroform, methanol and DMSO solvation in combination with different basis sets were calculated by using the time-dependent density functional theory (TD-DFT). Additionally, the predicted nonlinear optical (NLO) properties of the MPI are quite greater than that of urea at the B3LYP/6-31++G(d,p) level.
NASA Astrophysics Data System (ADS)
Hung, L.; Guedj, C.; Bernier, N.; Blaise, P.; Olevano, V.; Sottile, F.
2016-04-01
We present the valence electron energy-loss spectrum and the dielectric function of monoclinic hafnia (m -HfO2) obtained from time-dependent density-functional theory (TDDFT) predictions and compared to energy-filtered spectroscopic imaging measurements in a high-resolution transmission-electron microscope. Fermi's golden rule density-functional theory (DFT) calculations can capture the qualitative features of the energy-loss spectrum, but we find that TDDFT, which accounts for local-field effects, provides nearly quantitative agreement with experiment. Using the DFT density of states and TDDFT dielectric functions, we characterize the excitations that result in the m -HfO2 energy-loss spectrum. The sole plasmon occurs between 13 and 16 eV, although the peaks ˜28 and above 40 eV are also due to collective excitations. We furthermore elaborate on the first-principles techniques used, their accuracy, and remaining discrepancies among spectra. More specifically, we assess the influence of Hf semicore electrons (5 p and 4 f ) on the energy-loss spectrum, and find that the inclusion of transitions from the 4 f band damps the energy-loss intensity in the region above 13 eV. We study the impact of many-body effects in a DFT framework using the adiabatic local-density approximation (ALDA) exchange-correlation kernel, as well as from a many-body perspective using "scissors operators" matched to an ab initio G W calculation to account for self-energy corrections. These results demonstrate some cancellation of errors between self-energy and excitonic effects, even for excitations from the Hf 4 f shell. We also simulate the dispersion with increasing momentum transfer for plasmon and collective excitation peaks.
Borges, P. D. E-mail: lscolfaro@txstate.edu; Scolfaro, L. E-mail: lscolfaro@txstate.edu
2014-12-14
The thermoelectric properties of indium nitride in the most stable wurtzite phase (w-InN) as a function of electron and hole concentrations and temperature were studied by solving the semiclassical Boltzmann transport equations in conjunction with ab initio electronic structure calculations, within Density Functional Theory. Based on maximally localized Wannier function basis set and the ab initio band energies, results for the Seebeck coefficient are presented and compared with available experimental data for n-type as well as p-type systems. Also, theoretical results for electric conductivity and power factor are presented. Most cases showed good agreement between the calculated properties and experimental data for w-InN unintentionally and p-type doped with magnesium. Our predictions for temperature and concentration dependences of electrical conductivity and power factor revealed a promising use of InN for intermediate and high temperature thermoelectric applications. The rigid band approach and constant scattering time approximation were utilized in the calculations.
Johansson, Malin B; Baldissera, Gustavo; Valyukh, Iryna; Persson, Clas; Arwin, Hans; Niklasson, Gunnar A; Osterlund, Lars
2013-05-22
The optical and electronic properties of nanocrystalline WO3 thin films prepared by reactive dc magnetron sputtering at different total pressures (Ptot) were studied by optical spectroscopy and density functional theory (DFT) calculations. Monoclinic films prepared at low Ptot show absorption in the near infrared due to polarons, which is attributed to a strained film structure. Analysis of the optical data yields band-gap energies Eg ≈ 3.1 eV, which increase with increasing Ptot by 0.1 eV, and correlate with the structural modifications of the films. The electronic structures of triclinic δ-WO3, and monoclinic γ- and ε-WO3 were calculated using the Green function with screened Coulomb interaction (GW approach), and the local density approximation. The δ-WO3 and γ-WO3 phases are found to have very similar electronic properties, with weak dispersion of the valence and conduction bands, consistent with a direct band-gap. Analysis of the joint density of states shows that the optical absorption around the band edge is composed of contributions from forbidden transitions (>3 eV) and allowed transitions (>3.8 eV). The calculations show that Eg in ε-WO3 is higher than in the δ-WO3 and γ-WO3 phases, which provides an explanation for the Ptot dependence of the optical data. PMID:23614973
Maaloul, L.; Gangwar, R. K.; Morel, S.; Stafford, L.
2015-11-15
Langmuir probe and trace rare gases optical emission spectroscopy were used to analyze the spatial structure of the electron density and electron energy distribution function (EEDF) in a cylindrical Ar magnetron plasma reactor used for sputter-deposition of ZnO-based thin films. While a typical Bessel (zero order) diffusion profile was observed along the radial direction for the number density of charged particles at 21 cm from the ZnO target, a significant rise of these populations with respect to the Bessel function was seen in the center of the reactor at 4 cm from the magnetron surface. As for the EEDF, it was found to transform from a more or less Maxwellian far from the target to a two-temperature Maxwellian with a depletion of high-energy electrons where magnetic field confinement effects become important. No significant change in the behavior of the electron density and EEDF across a wide range of pressures (5–100 mTorr) and self-bias voltages (115–300 V) was observed during magnetron sputtering of Zn, ZnO, and In{sub 2}O{sub 3} targets. This indicates that sputtering of Zn, In, and O atoms do not play a very significant role on the electron particle balance and electron heating dynamics, at least over the range of experimental conditions investigated.
NASA Astrophysics Data System (ADS)
Polin, Daniel; Ziegler, Joshua; Malozovsky, Yuriy; Bagayoko, Diola
We present the findings of ab-initio calculations of electronic, transport, and structural properties of cubic sodium oxide (Na2O). These results were obtained using density functional theory (DFT), specifically a local density approximation (LDA) potential, and the linear combination of Gaussian orbitals (LCGO). Our implementation of LCGO followed the Bagayoko, Zhao, and Williams method as enhanced by the work of Ekuma and Franklin (BZW-EF). We describe the electronic band structure of Na2O with a direct band gap of 2.22 eV. Our results include predicted values for the electronic band structure and associated energy eigenvalues, the total and partial density of states (DOS and pDOS), the equilibrium lattice constant of Na2O, and the bulk modulus. We have also calculated the electron and holes effective masses in the Γ to L, Γ to X, and Γ to K directions. Acknowledgments: This work was funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy - National, Nuclear Security Administration (NNSA) (Award No. DE- NA0002630), LaSPACE, and LONI-SUBR.
Filatov, Michael; Huix-Rotllant, Miquel; Burghardt, Irene
2015-05-14
State-averaged (SA) variants of the spin-restricted ensemble-referenced Kohn-Sham (REKS) method, SA-REKS and state-interaction (SI)-SA-REKS, implement ensemble density functional theory for variationally obtaining excitation energies of molecular systems. In this work, the currently existing version of the SA-REKS method, which included only one excited state into the ensemble averaging, is extended by adding more excited states to the averaged energy functional. A general strategy for extension of the REKS-type methods to larger ensembles of ground and excited states is outlined and implemented in extended versions of the SA-REKS and SI-SA-REKS methods. The newly developed methods are tested in the calculation of several excited states of ground-state multi-reference systems, such as dissociating hydrogen molecule, and excited states of donor–acceptor molecular systems. For hydrogen molecule, the new method correctly reproduces the distance dependence of the lowest excited state energies and describes an avoided crossing between the doubly excited and singly excited states. For bithiophene–perylenediimide stacked complex, the SI-SA-REKS method correctly describes crossing between the locally excited state and the charge transfer excited state and yields vertical excitation energies in good agreement with the ab initio wavefunction methods.
Yan, Xun-Wang; Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062; State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Science, Beijing 100190, China and School of Physics and Electrical Engineering, Anyang Normal University, Henan 455000 ; Huang, Zhongbing; Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062 ; Lin, Hai-Qing
2013-11-28
By the first principle calculations based on the van der Waals density functional theory, we study the crystal structures and electronic properties of La-doped phenanthrene. Two stable atomic geometries of La{sub 1}phenanthrene are obtained by relaxation of atomic positions from various initial structures. The structure-I is a metal with two energy bands crossing the Fermi level, while the structure-II displays a semiconducting state with an energy gap of 0.15 eV, which has an energy gain of 0.42 eV per unit cell compared to the structure-I. The most striking feature of La{sub 1}phenanthrene is that La 5d electrons make a significant contribution to the total density of state around the Fermi level, which is distinct from potassium doped phenanthrene and picene. Our findings provide an important foundation for the understanding of superconductivity in La-doped phenanthrene.
Ramesh Babu, K; Vaitheeswaran, G
2014-06-11
We report a systematic first principles density functional study on the electronic structure, elastic and optical properties of nitrogen based solid hydrogen storage materials LiNH2, NaNH2, KNH2, and RbNH2. The ground state structural properties are calculated by using standard density functional theory, and also dispersion corrected density functional theory. We find that van der Waals interactions are dominant in LiNH2 whereas they are relatively weak in other alkali metal amides. The calculated elastic constants show that all the compounds are mechanically stable and LiNH2 is found to be a stiffer material among the alkali metal amides. The melting temperatures are calculated and follow the order RbNH2 < KNH2 < NaNH2 < LiNH2. The electronic band structure is calculated by using the Tran–Blaha modified Becke–Johnson potential and found that all the compounds are insulators, with a considerable band gap. The [NH2]− derived states completely dominate in the entire valence band region while the metal atom states occupy the conduction band. The calculated band structure is used to analyze the different interband optical transitions occurring between valence and conduction bands. Our calculations show that these materials have considerable optical anisotropy. PMID:25932472
NASA Astrophysics Data System (ADS)
Gori-Giorgi, Paola; Perdew, John P.
2002-10-01
We construct analytic formulas that represent the coupling-constant-averaged pair distribution function gxc(rs,ζ,kFu) of a three-dimensional nonrelativistic ground-state electron gas constrained to a uniform density with density parameter rs=(9π/4)1/3/kF and relative spin polarization ζ over the whole range 0
The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory.
Cheng, Mu-Jeng; Nielsen, Robert J; Tahir-Kheli, Jamil; Goddard, William A
2011-05-28
We have studied the magnetic structure of the high symmetry vanadyl pyrophosphate ((VO)(2)P(2)O(7), VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J(OPO) = -156.8 K and J(O) = -68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J(layer) = 19.2 K and J(chain) = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J(OPO) originates from a super-exchange interaction through the bridging -O-P-O- unit. In contrast, J(O) results from a direct overlap of 3d(x(2)-y(2)) orbitals on two vanadium atoms in the same V(2)O(8) motif, making it very sensitive to structural fluctuations. Based on the variations in V-O bond length as a function of strain along a, we found that the V-O bonds of V-(OPO)(2)-V are covalent and rigid, whereas the bonds of V-(O)(2)-V are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J(O), changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J(O) to be a constant over the range of 2-300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O(2) can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride. PMID:21503330
NASA Astrophysics Data System (ADS)
Suhasini, M.; Sailatha, E.; Gunasekaran, S.; Ramkumaar, G. R.
2015-11-01
The Fourier transform infrared (FT-IR) and FT-Raman spectra of Lornoxicam were recorded in the region 4000-450 cm-1 and 4000-50 cm-1 respectively. Density functional theory (DFT) has been used to calculate the optimized geometrical parameters, atomic charges, and vibrational wavenumbers and intensity of the vibrational bands. The computed vibrational wave numbers were compared with the FT-IR and FT-Raman experimental data. The computational calculations at DFT/B3LYP level with 6-31G(d,p) and 6-31++G(d,p) basis sets. The complete vibrational assignments were performed on the basis of the potential energy distribution (PED) of the Vibrational modes calculated using Vibrational Energy Distribution Analysis (VEDA 4) program. The oscillator's strength calculated by TD-DFT and Lornoxicam is approach complement with the experimental findings. The NMR chemical shifts 13C and 1H were recorded and calculated using the gauge independent atomic orbital (GIAO) method. The Natural charges and intermolecular contacts have been interpreted using Natural Bond orbital (NBO) analysis and the HOMO-LUMO energy gap has been calculated. The thermodynamic properties like Entropy, Enthalpy, Specific heat capacity and zero vibrational energy have been calculated. Besides, molecular electrostatic potential (MEP) was investigated using theoretical calculations.
A van der Waals density functional built upon the electron-gas foundation
NASA Astrophysics Data System (ADS)
Hyldgaard, Per; Berland, Kristian; Schröder, Elsebeth
2015-03-01
The vdW-DF method is designed to be a systematic extension of the constraint-based generalized-gradient approximation (GGA) and can therefore serve as general purpose density functional [PRB 90, 075148 (2014)]. Yet the early versions can have issues both with bulk systems and with weak chemisorption. We present a recent nonempirical version, vdW-DF-cx [J. Chem. Phys. 140, 18A539 (2014), PRB 89, 035412 (2014)], that resolves these issues. The version is designed to have a consistent combination of exchange and correlation. We show that it performs well for inter-molecular binding and that it can even be better than PBE for describing cohesion and structure of molecules and solids. These results validate the robustness of the vdW-DF plasmon-pole model, which we show is closed linked to the exchange correlation hole of constraint-based GGA. The work was supported by the Swedish Research Council (VR), by the Chalmers Areas of Advance: Materials, and by the Swedish National Infrastructure for Computing.
A hybrid-exchange density functional study of the bonding and electronic structure in bulk CuFeS2.
Martínez-Casado, Ruth; Chen, Vincent H-Y; Mallia, Giuseppe; Harrison, Nicholas M
2016-05-14
The geometric, electronic, and magnetic properties of bulk chalcopyrite CuFeS2 have been investigated using hybrid-exchange density functional theory calculations. The results are compared with available theoretical and experimental data. The theoretical description of the bonding and electronic structure in CuFeS2 is analyzed in detail and compared to those computed for chalcocite (CuS2) and greigite (Fe3S4). In particular, the behavior of the 3d electrons of Fe(3+) is discussed in terms of the Hubbard-Anderson model in the strongly correlated regime and found to be similarly described in both materials by an on-site Coulomb repulsion (U) of ∼8.9 eV and a transfer integral (t) of ∼0.3 eV. PMID:27179495
A hybrid-exchange density functional study of the bonding and electronic structure in bulk CuFeS2
NASA Astrophysics Data System (ADS)
Martínez-Casado, Ruth; Chen, Vincent H.-Y.; Mallia, Giuseppe; Harrison, Nicholas M.
2016-05-01
The geometric, electronic, and magnetic properties of bulk chalcopyrite CuFeS2 have been investigated using hybrid-exchange density functional theory calculations. The results are compared with available theoretical and experimental data. The theoretical description of the bonding and electronic structure in CuFeS2 is analyzed in detail and compared to those computed for chalcocite (CuS2) and greigite (Fe3S4). In particular, the behavior of the 3d electrons of Fe3+ is discussed in terms of the Hubbard-Anderson model in the strongly correlated regime and found to be similarly described in both materials by an on-site Coulomb repulsion (U) of ˜8.9 eV and a transfer integral (t) of ˜0.3 eV.
NASA Astrophysics Data System (ADS)
Nawa, Kenji; Kitaoka, Yukie; Nakamura, Kohji; Imamura, Hiroshi; Akiyama, Toru; Ito, Tomonori; Weinert, M.
2016-07-01
The ground-state electronic configurations of the correlated organometallic metallocenes, M Cp2,M =V , Cr, Mn, Fe, Co, and Ni, are investigated using constraint density functional theory combined with nonempirical Ueff parameters determined from linear-response theory. The relative stability of the various d -orbital electronic configurations of these organometallic molecules is found to be sensitive to the amount of correlation. Using nonempirical values of Ueff, the calculated electronic configurations are in agreement with the experiments: 4A2 g ,3E2 g ,6A1 g ,1A1 g ,2E1 g , and 3A2 g for the VCp2,CrCp2,MnCp2,FeCp2,CoCp2 , and NiCp2, respectively.
NASA Astrophysics Data System (ADS)
Mo, Chongjie; Yang, Yu; Kang, Wei; Zhang, Ping
2016-04-01
The electronic structure and optical properties for the (U,Th)O2 compound are systematically studied by employing the Heyd-Scuseria-Ernzerh method (HSE) of screened hybrid density functional. The electronic band gap of (U,Th)O2 is predicted to be 3.06 eV, in the middle of the values of UO2 and ThO2. Based on wavefunction analysis, we conclude (U,Th)O2 to be a Mott insulator in its ground state. The frequency dependent dielectric functions and optical properties are then calculated and compared with those of ThO2 and UO2. At the visible light frequency range, the adsorption coefficients for ThO2, UO2 and (U,Th)O2 are totally different, which gives an accessible method to predict the proportion of U atoms in an arbitrary unknown (U,Th)O2 compounds from the adsorption spectrum of visible lights.
Interstellar Electron Density Spectra
NASA Astrophysics Data System (ADS)
Lambert, Hendrick Clark
This study concerns the investigation of the form of the wavenumber spectrum of the Galactic electron density fluctuations through an examination of the scattering of the radio pulses emitted by pulsars as they propagate through the diffuse ionized interstellar gas. A widely used model for the electron density spectrum is based on the simple power-law: Pne(q)∝ q-β, where β = 11/3 is usually assumed, corresponding to Kolmogorov's turbulence spectrum. The simple Kolmogorov model provides satisfactory agreement for observations along many lines of sight; however, major inconsistencies remain. The inconsistencies suggest that an increase in the ratio of the power between the high (10-8[ m]-1≤ q<=10-7[ m]-1) and low (10-13[ m]-1≤ q<=10-12[ m]-1) wavenumbers is needed. This enhancement in the ratio can in turn be achieved by either including an inner scale, corresponding to a dissipation scale for the turbulent cascade, in the Kolmogorov spectrum or by considering steeper spectra. Spectra with spectral exponents β > 4 have been in general rejected based on observations of pulsar refractive scintillations. The special case of β = 4 has been given little attention and is analyzed in detail. Physically, this 'β = 4' model corresponds to the random distribution, both in location and orientation, of discrete objects with relatively sharp boundaries across the line of sight. An outer scale is included in the model to account for the average size of such objects. We compare the predictions of the inner-scale and β = 4 models both with published observations and observations we made as part of this investigation. We conclude that the form of the wavenumber spectrum is dependent on the line of sight. We propose a composite spectrum featuring a uniform background turbulence in presence of randomly distributed discrete objects, as modeled by the β = model.
NASA Astrophysics Data System (ADS)
Grimme, Stefan
2013-06-01
Two approximations in the Tamm-Dancoff density functional theory approach (TDA-DFT) to electronically excited states are proposed which allow routine computations for electronic ultraviolet (UV)- or circular dichroism (CD) spectra of molecules with 500-1000 atoms. Speed-ups compared to conventional time-dependent DFT (TD-DFT) treatments of about two to three orders of magnitude in the excited state part at only minor loss of accuracy are obtained. The method termed sTDA ("s" for simplified) employs atom-centered Löwdin-monopole based two-electron repulsion integrals with the asymptotically correct 1/R behavior and perturbative single excitation configuration selection. It is formulated generally for any standard global hybrid density functional with given Fock-exchange mixing parameter ax. The method performs well for two standard benchmark sets of vertical singlet-singlet excitations for values of ax in the range 0.2-0.6. The mean absolute deviations from reference data are only 0.2-0.3 eV and similar to those from standard TD-DFT. In three cases (two dyes and one polypeptide), good mutual agreement between the electronic spectra (up to 10-11 eV excitation energy) from the sTDA method and those from TD(A)-DFT is obtained. The computed UV- and CD-spectra of a few typical systems (e.g., C60, two transition metal complexes, [7]helicene, polyalanine, a supramolecular aggregate with 483 atoms and about 7000 basis functions) compare well with corresponding experimental data. The method is proposed together with medium-sized double- or triple-zeta type atomic-orbital basis sets as a quantum chemical tool to investigate the spectra of huge molecular systems at a reliable DFT level.
Thakkar, Ajit J. Wu, Taozhe
2015-10-14
Static electronic dipole polarizabilities for 135 molecules are calculated using second-order Møller-Plesset perturbation theory and six density functionals recently recommended for polarizabilities. Comparison is made with the best gas-phase experimental data. The lowest mean absolute percent deviations from the best experimental values for all 135 molecules are 3.03% and 3.08% for the LC-τHCTH and M11 functionals, respectively. Excluding the eight extreme outliers for which the experimental values are almost certainly in error, the mean absolute percent deviation for the remaining 127 molecules drops to 2.42% and 2.48% for the LC-τHCTH and M11 functionals, respectively. Detailed comparison enables us to identify 32 molecules for which the discrepancy between the calculated and experimental values warrants further investigation.
Sato, Shunsuke A; Taniguchi, Yasutaka; Shinohara, Yasushi; Yabana, Kazuhiro
2015-12-14
We develop methods to calculate electron dynamics in crystalline solids in real-time time-dependent density functional theory employing exchange-correlation potentials which reproduce band gap energies of dielectrics; a meta-generalized gradient approximation was proposed by Tran and Blaha [Phys. Rev. Lett. 102, 226401 (2009)] (TBm-BJ) and a hybrid functional was proposed by Heyd, Scuseria, and Ernzerhof [J. Chem. Phys. 118, 8207 (2003)] (HSE). In time evolution calculations employing the TB-mBJ potential, we have found it necessary to adopt the predictor-corrector step for a stable time evolution. We have developed a method to evaluate electronic excitation energy without referring to the energy functional which is unknown for the TB-mBJ potential. For the HSE functional, we have developed a method for the operation of the Fock-like term in Fourier space to facilitate efficient use of massive parallel computers equipped with graphic processing units. We compare electronic excitations in silicon and germanium induced by femtosecond laser pulses using the TB-mBJ, HSE, and a simple local density approximation (LDA). At low laser intensities, electronic excitations are found to be sensitive to the band gap energy: they are close to each other using TB-mBJ and HSE and are much smaller in LDA. At high laser intensities close to the damage threshold, electronic excitation energies do not differ much among the three cases. PMID:26671367
NASA Astrophysics Data System (ADS)
Sato, Shunsuke A.; Taniguchi, Yasutaka; Shinohara, Yasushi; Yabana, Kazuhiro
2015-12-01
We develop methods to calculate electron dynamics in crystalline solids in real-time time-dependent density functional theory employing exchange-correlation potentials which reproduce band gap energies of dielectrics; a meta-generalized gradient approximation was proposed by Tran and Blaha [Phys. Rev. Lett. 102, 226401 (2009)] (TBm-BJ) and a hybrid functional was proposed by Heyd, Scuseria, and Ernzerhof [J. Chem. Phys. 118, 8207 (2003)] (HSE). In time evolution calculations employing the TB-mBJ potential, we have found it necessary to adopt the predictor-corrector step for a stable time evolution. We have developed a method to evaluate electronic excitation energy without referring to the energy functional which is unknown for the TB-mBJ potential. For the HSE functional, we have developed a method for the operation of the Fock-like term in Fourier space to facilitate efficient use of massive parallel computers equipped with graphic processing units. We compare electronic excitations in silicon and germanium induced by femtosecond laser pulses using the TB-mBJ, HSE, and a simple local density approximation (LDA). At low laser intensities, electronic excitations are found to be sensitive to the band gap energy: they are close to each other using TB-mBJ and HSE and are much smaller in LDA. At high laser intensities close to the damage threshold, electronic excitation energies do not differ much among the three cases.
Sato, Shunsuke A.; Taniguchi, Yasutaka; Shinohara, Yasushi; Yabana, Kazuhiro
2015-12-14
We develop methods to calculate electron dynamics in crystalline solids in real-time time-dependent density functional theory employing exchange-correlation potentials which reproduce band gap energies of dielectrics; a meta-generalized gradient approximation was proposed by Tran and Blaha [Phys. Rev. Lett. 102, 226401 (2009)] (TBm-BJ) and a hybrid functional was proposed by Heyd, Scuseria, and Ernzerhof [J. Chem. Phys. 118, 8207 (2003)] (HSE). In time evolution calculations employing the TB-mBJ potential, we have found it necessary to adopt the predictor-corrector step for a stable time evolution. We have developed a method to evaluate electronic excitation energy without referring to the energy functional which is unknown for the TB-mBJ potential. For the HSE functional, we have developed a method for the operation of the Fock-like term in Fourier space to facilitate efficient use of massive parallel computers equipped with graphic processing units. We compare electronic excitations in silicon and germanium induced by femtosecond laser pulses using the TB-mBJ, HSE, and a simple local density approximation (LDA). At low laser intensities, electronic excitations are found to be sensitive to the band gap energy: they are close to each other using TB-mBJ and HSE and are much smaller in LDA. At high laser intensities close to the damage threshold, electronic excitation energies do not differ much among the three cases.
Baruah, Tunna; Garnica, Amanda; Paggen, Marina; Basurto, Luis; Zope, Rajendra R
2016-04-14
We study the electronic structure of C60 fullerenes functionalized with a thiophene-diketo-pyrrolopyrrole-thiophene based chromophore using density functional theory combined with large polarized basis sets. As the attached chromophore has electron donor character, the functionalization of the fullerene leads to a donor-acceptor (DA) system. We examine in detail the effect of the linker and the addition site on the electronic structure of the functionalized fullerenes. We further study the electronic structure of these DA complexes with a focus on the charge transfer excitations. Finally, we examine the interface of the functionalized fullerenes with the widely used poly(3-hexylthiophene-2,5-diyl) (P3HT) donor. Our results show that all functionalized fullerenes with an exception of the C60-pyrrolidine [6,6], where the pyrrolidine is attached at a [6,6] site, have larger electron affinities relative to the pristine C60 fullerene. We also estimate the quasi-particle gap, lowest charge transfer excitation energy, and the exciton binding energies of the functionalized fullerene-P3MT model systems. Results show that the exciton binding energies in these model complexes are slightly smaller compared to a similarly prepared phenyl-C61-butyric acid methyl ester (PCBM)-P3MT complex. PMID:27083718
NASA Astrophysics Data System (ADS)
Baruah, Tunna; Garnica, Amanda; Paggen, Marina; Basurto, Luis; Zope, Rajendra R.
2016-04-01
We study the electronic structure of C60 fullerenes functionalized with a thiophene-diketo-pyrrolopyrrole-thiophene based chromophore using density functional theory combined with large polarized basis sets. As the attached chromophore has electron donor character, the functionalization of the fullerene leads to a donor-acceptor (DA) system. We examine in detail the effect of the linker and the addition site on the electronic structure of the functionalized fullerenes. We further study the electronic structure of these DA complexes with a focus on the charge transfer excitations. Finally, we examine the interface of the functionalized fullerenes with the widely used poly(3-hexylthiophene-2,5-diyl) (P3HT) donor. Our results show that all functionalized fullerenes with an exception of the C60-pyrrolidine [6,6], where the pyrrolidine is attached at a [6,6] site, have larger electron affinities relative to the pristine C60 fullerene. We also estimate the quasi-particle gap, lowest charge transfer excitation energy, and the exciton binding energies of the functionalized fullerene-P3MT model systems. Results show that the exciton binding energies in these model complexes are slightly smaller compared to a similarly prepared phenyl-C61-butyric acid methyl ester (PCBM)-P3MT complex.
Uzunova, Ellie L.; Mikosch, Hans
2014-01-14
The iron oxide dimers (FeO){sub 2} and their peroxide isomers are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. Among the bare clusters the planar four-member ring structures are more stable than the non-planar ones and the rhombic dioxide Fe{sub 2}O{sub 2} with antiferromagnetically ordered electrons on iron centers is the global minimum. Water adsorption on the bare diiron dioxide is exothermic, but dissociation does not occur. Carbonylation favors a non-planar Fe{sub 2}O{sub 2} ring for both the dioxides and the peroxides and high electron density at the Fe centers is induced, evidenced by the natural charge distribution, the high proton affinity, and the values of global electronegativity and hardness. The iron dioxide hexacarbonyl Fe{sub 2}O{sub 2}(CO){sub 6} is diamagnetic in the state of the global minimum. It is separated from the next low-lying triplet state by a small energy gap of 0.22 eV. Time-dependent density functional theory methods were applied to examine electron excitations from the ground state to the low-lying triplet states in the hexacarbonyls and their adsorption complexes with water. Singlet-to-triplet state excitations occur via ligand-to-metal charge transfer in the hexacarbonyls; in the adsorption complexes excitations from the oxygen lone pairs to the adsorption center also occur and they appear in the IR-visible region. The lowest energy singlet and triplet state reaction paths for water splitting were followed. On the singlet potential energy surface (PES), water splitting is spontaneous, while for the triplet PES an activation barrier of 14.1 kJ mol{sup −1} was determined.
NASA Astrophysics Data System (ADS)
Nawa, Kenji; Nakamura, Kohji; Akiyama, Toru; Ito, Tomonori; Weinert, Michael
Effective on-site Coulomb interactions (Ueff) and electron configurations in the localized d and f orbitals of metal complexes in transition-metal oxides and organometallic molecules, play a key role in the first-principles search for the true ground-state. However, wide ranges of values in the Ueff parameter of a material, even in the same ionic state, are often reported. Here, we revisit this issue from constraint density functional theory (DFT) by using the full-potential linearized augmented plane wave method. The Ueff parameters for prototypical transition-metal oxides, TMO (TM =Mn, Fe, Co, Ni), were calculated by the second derivative of the total energy functional with respect to the d occupation numbers inside the muffin-tin (MT) spheres as a function of the sphere radius. We find that the calculated Ueff values depend significantly on the MT radius, with a variation of more than 3 eV when the MT radius changes from 2.0 to 2.7 a.u., but importantly an identical valence band structure can be produced in all the cases, with an approximate scaling of Ueff. This indicates that a simple transferability of the Ueff value among different calculation methods is not allowed. We further extend the constraint DFT to treat various electron configurations of the localized d-orbitals in organometallic molecules, TMCp2 (TM =Cr, Mn, Fe, Co, Ni), and find that the calculated Ueff values can reproduce the experimentally determined ground-state electron configurations.
NASA Astrophysics Data System (ADS)
Marana, Naiara L.; Albuquerque, Anderson R.; La Porta, Felipe A.; Longo, Elson; Sambrano, Julio R.
2016-05-01
Periodic density functional theory calculations with the B3LYP hybrid functional and all-electron Gaussian basis set were performed to simulate the structural and electronic properties as well as the strain and formation energies of single-walled ZnO nanotubes (SWZnONTs) and Carbon nanotubes (SWCNTs) with different chiralities as functions of their diameters. For all SWZnONTs, the band gap, strain energy, and formation energy converge to ~4.5 eV, 0.0 eV/atom, and 0.40 eV/atom, respectively. This result suggests that the nanotubes are formed more easily from the surface than from the bulk. For SWCNTs, the strain energy is always positive, while the formation energy is negative for armchair and zigzag nanotubes, therefore suggesting that these types of nanotubes can be preferentially formed from the bulk. The electronic properties of SWCNTs depend on the chirality; all armchair nanotubes are metallic, while zigzag and chiral nanotubes can be metallic or semiconducting, depending on the n and m vectors.
NASA Astrophysics Data System (ADS)
Rivelino, Roberto; Malaspina, Thaciana; Fileti, Eudes E.
2009-01-01
We have investigated the stability, electronic properties, Rayleigh (elastic), and Raman (inelastic) depolarization ratios, infrared and Raman absorption vibrational spectra of fullerenols [C60(OH)n] with different degrees of hydroxylation by using all-electron density-functional-theory (DFT) methods. Stable arrangements of these molecules were found by means of full geometry optimizations using Becke’s three-parameter exchange functional with the Lee, Yang, and Parr correlation functional. This DFT level has been combined with the 6-31G(d,p) Gaussian-type basis set, as a compromise between accuracy and capability to treat highly hydroxylated fullerenes, e.g., C60(OH)36 . Thus, the molecular properties of fullerenols were systematically analyzed for structures with n=1 , 2, 3, 4, 8, 10, 16, 18, 24, 32, and 36. From the electronic structure analysis of these molecules, we have evidenced an important effect related to the weak chemical reactivity of a possible C60(OH)24 isomer. To investigate Raman scattering and the vibrational spectra of the different fullerenols, frequency calculations are carried out within the harmonic approximation. In this case a systematic study is only performed for n=1-4 , 8, 10, 16, 18, and 24. Our results give good agreements with the expected changes in the spectral absorptions due to the hydroxylation of fullerenes.
NASA Astrophysics Data System (ADS)
Kastlunger, Georg; Stadler, Robert
2015-03-01
To define the conductance of single-molecule junctions with a redox functionality in an electrochemical cell, two conceptually different electron transport mechanisms, namely, coherent tunneling and vibrationally induced hopping, compete with each other, where implicit parameters of the setup such as the length of the molecule and the applied gate voltage decide which mechanism is the dominant one. Although coherent tunneling is most efficiently described within Landauer theory and the common theoretical treatment of electron hopping is based on Marcus theory, both theories are adequate for the processes they describe without introducing accuracy-limiting approximations. For a direct comparison, however, it has to be ensured that the crucial quantities obtained from electronic structure calculations, i.e., the transmission function T (E ) in Landauer theory and the transfer integral V , the reorganization energy λ , and the driving force Δ G0 in Marcus theory, are derived from similar grounds, as pointed out by Nitzan and coworkers in a series of publications. In this paper our framework is a single-particle picture, for which we perform density functional theory calculations for the conductance corresponding to both transport mechanisms for junctions with the central molecule containing one, two, or three Ruthenium centers, from which we extrapolate our results in order to define the critical length of the transition point of the two regimes which we identify at 5.76nm for this type of molecular wire. We also discuss trends in the dependence on an electrochemically induced gate potential.
Density Functional Study of Structures and Electron Affinities of BrO4F/BrO4F−
Gong, Liangfa; Xiong, Jieming; Wu, Xinmin; Qi, Chuansong; Li, Wei; Guo, Wenli
2009-01-01
The structures, electron affinities and bond dissociation energies of BrO4F/BrO4F− species have been investigated with five density functional theory (DFT) methods with DZP++ basis sets. The planar F-Br…O2…O2 complexes possess 3A′ electronic state for neutral molecule and 4A′ state for the corresponding anion. Three types of the neutral-anion energy separations are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The EAad value predicted by B3LYP method is 4.52 eV. The bond dissociation energies De (BrO4F → BrO4-mF + Om) (m = 1–4) and De− (BrO4F− → BrO4-mF− + Om and BrO4F− → BrO4-mF + Om−) are predicted. The adiabatic electron affinities (EAad) were predicted to be 4.52 eV for F-Br…O2…O2 (3A′←4A′) (B3LYP method). PMID:19742128
NASA Astrophysics Data System (ADS)
Wang, Lu; Li, Qing-Fang; Yang, Cui-Hong; Wei, Yue-Ling; Zhu, Xing-Feng; Rao, Wei-Feng
2016-05-01
The geometric and electronic structures of the ethylene oxide (EO) molecule adsorbed on Si(100)-(2 × 1) surface were investigated by using the density-functional theory calculations. All possible adsorbed structures were considered and it was found that only four adsorption structures are stable. The calculations of the formation energy revealed the most stable conformation and demonstrated that the nature of Si-O bond significantly affects the stability of adsorption systems. The analysis of corresponding electronic structures showed that two adsorbed structures are still semiconductor compounds but the other two are not. In particular, the EO after adsorbing was found to be connected via a ring-opening reaction where the molecule forms a five-membered ring together with the surface of dimer silicon atoms, and the produced five-membered ring is almost perpendicular to the silicon surface.
Structural and electronic properties of Sb{sub n} (n=2-10) clusters using density-functional theory
Zhou Xiaolan; Zhao Jijun; Chen Xiaoshuang; Lu Wei
2005-11-15
Using a density-functional calculation within the generalized gradient approximation, we have investigated the structural and electronic properties of antimony clusters. The lowest-energy structures of neutral and anionic Sb{sub n} clusters up to n=10 were determined and compared with those of light-element group-V clusters (As{sub n},P{sub n}). The size dependence of binding energies, gaps between lowest-unoccupied and highest-occupied molecular orbitals, second-order difference of total energies, and adiabatic electron affinities (EAs) of Sb{sub n} clusters were discussed. Even-odd oscillation as well as magic-number behavior at Sb{sub 4} were observed. Qualitative agreement was found between theoretical and experimental EAs.
Li, Xifeng; Sevilla, Michael D; Sanche, Léon
2003-11-12
The discovery of DNA strand breaks induced by low energy secondary electrons sparks a necessity to elucidate the mechanism. Through theoretical studies based on a sugar-phosphate-sugar model that mimics a backbone section of the DNA strand, it is found that bond cleavages at 3' or 5'C-O sites after addition of an electron are possible with a ca. 10 kcal/mol activation barrier. Moreover, the potential energy surfaces show that dissociation at both sites is highly favorable thermodynamically. Although the phosphate group in DNA is not a favored site for electron attachment because of competitive electron transfer to the bases, any electrons which attach to phosphates on first encounter may induce strand breaks even when the electron energy is near zero eV. These findings have profound implication as low energy secondary electrons are abundantly generated in all types of ionization radiation. PMID:14599198
Prociuk, Alexander; Van Kuiken, Ben; Dunietz, Barry D
2006-11-28
Electronic transmission through a metal-molecule-metal system is calculated by employing a Green's function formalism in the scattering based scheme. Self-energy models representing the bulk and the potential bias are used to describe electron transport through the molecular system. Different self-energies can be defined by varying the partition between device and bulk regions of the metal-molecule-metal model system. In addition, the self-energies are calculated with different representations of the bulk through its Green's function. In this work, the dependence of the calculated transmission on varying the self-energy subspaces is benchmarked. The calculated transmission is monitored with respect to the different choices defining the self-energy model. In this report, we focus on one-dimensional model systems with electronic structures calculated at the density functional level of theory. PMID:17144733
Kuwahara, Riichi; Tadokoro, Yoichi; Ohno, Kaoru
2014-08-28
In this paper, we calculate kinetic and potential energy contributions to the electronic ground-state total energy of several isolated atoms (He, Be, Ne, Mg, Ar, and Ca) by using the local density approximation (LDA) in density functional theory, the Hartree-Fock approximation (HFA), and the self-consistent GW approximation (GWA). To this end, we have implemented self-consistent HFA and GWA routines in our all-electron mixed basis code, TOMBO. We confirm that virial theorem is fairly well satisfied in all of these approximations, although the resulting eigenvalue of the highest occupied molecular orbital level, i.e., the negative of the ionization potential, is in excellent agreement only in the case of the GWA. We find that the wave function of the lowest unoccupied molecular orbital level of noble gas atoms is a resonating virtual bound state, and that of the GWA spreads wider than that of the LDA and thinner than that of the HFA. PMID:25173006
Nordlund, Dennis; Odelius, Michael; Bluhm, Hendrik; Ogasawara, Hirohito; Pettersson, Lars G.M.; Nilsson, Anders
2008-04-29
We present valence photoelectron emission spectra of liquid water in comparison with gas-phase water, ice close to the melting point, low temperature amorphous and crystalline ice. All aggregation states have major electronic structure changes relative to the free molecule, with rehybridization and development of bonding and anti-bonding states accompanying the hydrogen bond formation. Sensitivity to the local structural order, most prominent in the shape and splitting of the occupied 3a{sub 1} orbital, is understood from the electronic structure averaging over various geometrical structures, and reflects the local nature of the orbital interaction.
Density Functional Theory Studies of the Electronic Structure of Solid State Actinide Oxides
Wen, Xiaodong; Martin, Richard L.; Henderson, Thomas M.; Scuseria, Gustavo E.
2013-02-13
The actinide oxides have been extensively studied in the context of the nuclear fuel cycle. They are also of fundamental interest as members of a class of strongly correlated materials, the Mott insulators. Their complex physical and chemical properties make them challenging systems to characterize, both experimentally and theoretically. Chiefly, this is because actinide oxides can exhibit both electronic localization and electronic delocalization and have partially occupied f orbitals, which can lead to multiple possibilities for ground states. Of particular concern for theoretical work is that the large number of competing states display strong correlations which are dffcult to capture with computationally tractable methods.
Espinosa-Torres, Néstor D; la Luz, David Hernández-de; Flores-Gracia, José Francisco J; Luna-López, José A; Martínez-Juárez, Javier; Vázquez-Valerdi, Diana E
2014-01-01
In systems in atomic scale and nanoscale such as clusters or agglomerates constituted by particles from a few to less than 100 atoms, quantum confinement effects are very important. Their optical and electronic properties are often dependent on the size of the systems and the way in which the atoms in these clusters are bonded. Generally, these nanostructures display optical and electronic properties significantly different to those found in corresponding bulk materials. Silicon agglomerates embedded in silicon rich oxide (SRO) films have optical properties, which have been reported to be directly dependent on silicon nanocrystal size. Furthermore, the room temperature photoluminescence (PL) of SRO has repeatedly generated a huge interest due to its possible applications in optoelectronic devices. However, a plausible emission mechanism has not been widely accepted in the scientific community. In this work, we present a short review about the experimental results on silicon nanoclusters in SRO considering different techniques of growth. We focus mainly on their size, Raman spectra, and photoluminescence spectra. With this as background, we employed the density functional theory with a functional B3LYP and a basis set 6-31G* to calculate the optical and electronic properties of clusters of silicon (constituted by 15 to 20 silicon atoms). With the theoretical calculation of the structural and optical properties of silicon clusters, it is possible to evaluate the contribution of silicon agglomerates in the luminescent emission mechanism, experimentally found in thin SRO films. PMID:25276105
2014-01-01
In systems in atomic scale and nanoscale such as clusters or agglomerates constituted by particles from a few to less than 100 atoms, quantum confinement effects are very important. Their optical and electronic properties are often dependent on the size of the systems and the way in which the atoms in these clusters are bonded. Generally, these nanostructures display optical and electronic properties significantly different to those found in corresponding bulk materials. Silicon agglomerates embedded in silicon rich oxide (SRO) films have optical properties, which have been reported to be directly dependent on silicon nanocrystal size. Furthermore, the room temperature photoluminescence (PL) of SRO has repeatedly generated a huge interest due to its possible applications in optoelectronic devices. However, a plausible emission mechanism has not been widely accepted in the scientific community. In this work, we present a short review about the experimental results on silicon nanoclusters in SRO considering different techniques of growth. We focus mainly on their size, Raman spectra, and photoluminescence spectra. With this as background, we employed the density functional theory with a functional B3LYP and a basis set 6-31G* to calculate the optical and electronic properties of clusters of silicon (constituted by 15 to 20 silicon atoms). With the theoretical calculation of the structural and optical properties of silicon clusters, it is possible to evaluate the contribution of silicon agglomerates in the luminescent emission mechanism, experimentally found in thin SRO films. PMID:25276105
NASA Astrophysics Data System (ADS)
Lacombe, Lionel; Dinh, P. Huong Mai; Reinhard, Paul-Gerhard; Suraud, Eric; Sanche, Leon
2015-08-01
We present an extension of standard time-dependent density functional theory (TDDFT) to include the evaluation of rare reaction channels, taking as an example of application the theoretical modelling of electron attachment to molecules. The latter process is of great importance in radiation-induced damage of biological tissue for which dissociative electron attachment plays a decisive role. As the attachment probability is very low, it cannot be extracted from the TDDFT propagation whose mean field provides an average over various reaction channels. To extract rare events, we augment TDDFT by a perturbative treatment to account for the occasional jumps, namely electron capture in our test case. We apply the modelling to electron attachment to H2O, H3O+, and (H2O)2. Dynamical calculations have been done at low energy (3-16 eV). We explore, in particular, how core-excited states of the targets show up as resonances in the attachment probability. Contribution to the Topical Issue "COST Action Nano-IBCT: Nano-scale Processes Behind Ion-Beam Cancer Therapy", edited by Andrey Solov'yov, Nigel Mason, Gustavo García, Eugene Surdutovich.
Morzan, Uriel N.; Ramírez, Francisco F.; Scherlis, Damián A. E-mail: mcgl@qb.ffyb.uba.ar; Lebrero, Mariano C. González E-mail: mcgl@qb.ffyb.uba.ar
2014-04-28
This article presents a time dependent density functional theory (TDDFT) implementation to propagate the Kohn-Sham equations in real time, including the effects of a molecular environment through a Quantum-Mechanics Molecular-Mechanics (QM-MM) hamiltonian. The code delivers an all-electron description employing Gaussian basis functions, and incorporates the Amber force-field in the QM-MM treatment. The most expensive parts of the computation, comprising the commutators between the hamiltonian and the density matrix—required to propagate the electron dynamics—, and the evaluation of the exchange-correlation energy, were migrated to the CUDA platform to run on graphics processing units, which remarkably accelerates the performance of the code. The method was validated by reproducing linear-response TDDFT results for the absorption spectra of several molecular species. Two different schemes were tested to propagate the quantum dynamics: (i) a leap-frog Verlet algorithm, and (ii) the Magnus expansion to first-order. These two approaches were confronted, to find that the Magnus scheme is more efficient by a factor of six in small molecules. Interestingly, the presence of iron was found to seriously limitate the length of the integration time step, due to the high frequencies associated with the core-electrons. This highlights the importance of pseudopotentials to alleviate the cost of the propagation of the inner states when heavy nuclei are present. Finally, the methodology was applied to investigate the shifts induced by the chemical environment on the most intense UV absorption bands of two model systems of general relevance: the formamide molecule in water solution, and the carboxy-heme group in Flavohemoglobin. In both cases, shifts of several nanometers are observed, consistently with the available experimental data.
NASA Astrophysics Data System (ADS)
Morzan, Uriel N.; Ramírez, Francisco F.; Oviedo, M. Belén; Sánchez, Cristián G.; Scherlis, Damián A.; Lebrero, Mariano C. González
2014-04-01
This article presents a time dependent density functional theory (TDDFT) implementation to propagate the Kohn-Sham equations in real time, including the effects of a molecular environment through a Quantum-Mechanics Molecular-Mechanics (QM-MM) hamiltonian. The code delivers an all-electron description employing Gaussian basis functions, and incorporates the Amber force-field in the QM-MM treatment. The most expensive parts of the computation, comprising the commutators between the hamiltonian and the density matrix—required to propagate the electron dynamics—, and the evaluation of the exchange-correlation energy, were migrated to the CUDA platform to run on graphics processing units, which remarkably accelerates the performance of the code. The method was validated by reproducing linear-response TDDFT results for the absorption spectra of several molecular species. Two different schemes were tested to propagate the quantum dynamics: (i) a leap-frog Verlet algorithm, and (ii) the Magnus expansion to first-order. These two approaches were confronted, to find that the Magnus scheme is more efficient by a factor of six in small molecules. Interestingly, the presence of iron was found to seriously limitate the length of the integration time step, due to the high frequencies associated with the core-electrons. This highlights the importance of pseudopotentials to alleviate the cost of the propagation of the inner states when heavy nuclei are present. Finally, the methodology was applied to investigate the shifts induced by the chemical environment on the most intense UV absorption bands of two model systems of general relevance: the formamide molecule in water solution, and the carboxy-heme group in Flavohemoglobin. In both cases, shifts of several nanometers are observed, consistently with the available experimental data.
Huang, Ruiqi; Wang, Qingxia; Cai, Xiaolin; Li, Chong; Jia, Yu; Wang, Fei; Wang, Sanjun
2015-07-15
Employing the first-principles combined with hybrid functional calculations, the electronic and magnetic properties of GaAs doped with a N{sub 2} molecule are investigated in this work. We find that in Ga{sub 32}As{sub 31}(N{sub 2}){sub As} the N-N split is able to saturate the dangling bond of Ga atom ,form sp{sup 3}-like hybridization, and simultaneously supply an extra localized electron, leading to a magnetic ground state with a magnetic moment of ∼1μ{sub B}. This magnetic ground state is different from previously nonmagnetic results predicted by PBE functional, which results from the self-interaction error inherent in semi-local density functional theory. Moreover, the band gap of magnetic ground state of Ga{sub 32}As{sub 31}(N{sub 2}){sub As} alloy decreases, which is relative to GaAs . Finally we discuss and explain why the magnetism is not discovered in previous experiments and theories.
Panek, Jaroslaw; Latajaka, Zdzislaw
2000-12-26
Results of DFT and MP4 calculations on AlNO2 and GaNO2 molecules are presented. One Cs and two C2v structures (two minima and one TS) are found and their energies and vibrational frequencies are reported and discussed. The minima are close in energy and lie ca. 70 kcal mol-1 below reactants (M+NO2). More insight is obtained via topological analysis of electron density and electron localization function (ELF). It is shown that the molecules are bound mainly via electrostatic interactions, and there is a significant charge transfer from metal atom to the NO2 moiety. Detailed analysis of the ELF shows that the loss of stability of gallium complexes with respect to aluminium structures is best explained by (antibonding) influence of gallium semi-cored electrons.
NASA Astrophysics Data System (ADS)
Fortunato, Leandro F.; Zubieta, Carolina E.; Fuente, Silvia A.; Belelli, Patricia G.; Ferullo, Ricardo M.
2016-11-01
We report a density functional theory (DFT) investigation on the interaction of tiny Aun (n = 1-5) clusters with the bare and hydroxylated (110) surfaces of goethite (α-FeOOH). Both adsorption and atom-by-atom nucleation processes were modeled. The adsorption is shown to be strong on the bare surface and takes place preferentially through the interaction of Au atoms with unsaturated surface oxygen anions, accompanied with an electronic charge transfer from the metal to the support. Au3, Au4 and Au5 planar structures resulted to be particularly stable due to polarization effects; indeed, Coulombic repulsion between basal Au atoms and surface oxygen anions promotes the displacement of the electronic density toward terminal Au atoms producing a Au+δ(basal)/Au-δ(terminal) polarization. On the hydroxylated surface, Au clusters adsorb more weakly with respect to the bare surface, mainly through monocoordinated surface hydroxyl groups and tricoordinated oxygen ions. Concerning the nucleation mechanism, while on the hydroxylated surface the nucleation energy is governed by the spin of the interacting systems, on the bare surface polarization effects seems to play a predominant role.
Effects of semicore d-electrons in screened-exchange density functional methods
NASA Astrophysics Data System (ADS)
Lee, Byounghak; Wang, Lin-Wang
2007-03-01
We report a theoretical study on the role of shallow d states in the screened-exchange local density approximation (sX-LDA) band structure of binary semiconductor systems. We found that the inaccurate pseudo-wavefunctions can lead to 1) an overestimation of the screened-exchange interaction between the localized d states and the delocalized higher energy s and p states and 2) an underestimation of the screened-exchange interaction between the d states. The resulting sX-LDA band structures have substantially smaller band gaps compared with experiments. We correct the pseudo-wavefunctions of d states by including the s and p states of the same shell in the valence states. The correction of pseudo-wavefunctions yields band gaps and the d state binding energy in good agreement with experiments. Compared with the quasi-particle GW method, our sX-LDA results shows not only similar quality band gaps but also much better d state binding energy. As an example, we present sX-LDA results of s-d coupling in zinc-blende semiconductors and compare them with LDA+U results. We also present an efficient method to correct the pseudo-wavefunction exchange-integral error by using projection of wavefunctions onto atomic orbitals.
Effects of d-electrons in pseudopotential screened-exchange density functional calculations
Lee, Byounghak; Canning, Andrew; Wang, Lin-Wang
2007-09-12
We report a theoretical study on the role of shallow d states in the screened-exchange local density approximation (sX-LDA) band structure of binary semiconductor systems.We found that inaccurate pseudo-wavefunctions can lead to 1) an overestimation of the screened-exchange interaction betweenthe localized d states and the delocalized higher energy s and p states and 2) an underestimation of the screened-exchange interaction between the d states. The resulting sX-LDA band structures have substantially smaller band gaps compared with experiments. We correct the pseudo-wavefunctions of d states by including the semicore s and p states of the same shell in the valence states. The correction of pseudo-wavefunctions yields band gaps and d state binding energies in good agreement with experiments and the full potential linearized augmented plane wave sX-LDA calculations. Compared with the quasi-particle GW method, our sX-LDA results shows not only similar quality on the band gaps but also much better d state binding energies. Combined with its capability of ground state structure calculation, the sX-LDA is expected to be a valuable theoretical tool for the II-VI and III-V (especially the III-N) bulk semiconductors and nanostructure studies.
Effects of d-electrons in pseudopotential screened-exchange density functional calculations
Wang, Lin-Wang; Lee, Byounghak; Canning, Andrew; Wang, Lin-Wang
2008-08-11
We report a theoretical study on the role of shallow d states in the screened-exchange local density approximation (sX-LDA) band structure of binary semiconductor systems. We found that the inaccurate pseudo-wavefunctions can lead to (1) an overestimation of the screened-exchange interaction between the localized d states and the delocalized higher energy s and p states and (2) an underestimation of the screened-exchange interaction between the d states. The resulting sX-LDA band structures have substantially smaller band gaps compared with experiments. We correct the pseudo-wavefunctions of d states by including the semicore s and p states of the same shell in the valence states. The correction of pseudo-wavefunctions yields band gaps and the d state binding energy with good agreements with experiments and the full potential linearized augmented planewave (FLAPW) calculations. Compared with the quasi-particle GW method, our sX-LDA results shows not only similar quality on the band gaps but also much better d state binding energy. Combined with its capability of ground state structure calculation, the sX-LDA is expected to be a valuable theoretical tool for the II-VI and III-V (especially the III-N) bulk semiconductors and nanostructure studies.
Ahangari, Morteza Ghorbanzadeh; Fereidoon, Abdolhosein; Ganji, Masoud Darvish
2013-08-01
We performed first principles calculations based on density functional theory (DFT) to investigate the effect of epoxy monomer content on the electronic and mechanical properties of single-walled carbon nanotubes (SWCNTs). Our calculation results reveal that interfacial interaction increases with increasing numbers of epoxy monomers on the surface of SWCNTs. Furthermore, density of states (DOS) results showed no orbital hybridization between the epoxy monomers and nanotubes. Mulliken charge analysis shows that the epoxy polymer carries a positive charge that is directly proportional to the number of monomers. The Young's modulus of the nanotubes was also studied as a function of monomer content. It was found that, with increasing number of monomers on the nanotubes, the Young's modulus first decreases and then approaches a constant value. The results of a SWCNT pullout simulation suggest that the interfacial shear stress of the epoxy/SWCNT complex is approximately 68 MPa. These results agreed well with experimental results, thus proving that the simulation methods used in this study are viable. PMID:23609226
Liu, Jie; Liang, WanZhen
2011-07-01
We present the analytical expression and computer implementation for the second-order energy derivatives of the electronic excited state with respect to the nuclear coordinates in the time-dependent density functional theory (TDDFT) with Gaussian atomic orbital basis sets. Here, the Tamm-Dancoff approximation to the full TDDFT is adopted, and therefore the formulation process of TDDFT excited-state Hessian is similar to that of configuration interaction singles (CIS) Hessian. However, due to the replacement of the Hartree-Fock exchange integrals in CIS with the exchange-correlation kernels in TDDFT, many quantitative changes in the derived equations are arisen. The replacement also causes additional technical difficulties associated with the calculation of a large number of multiple-order functional derivatives with respect to the density variables and the nuclear coordinates. Numerical tests on a set of test molecules are performed. The simulated excited-state vibrational frequencies by the analytical Hessian approach are compared with those computed by CIS and the finite-difference method. It is found that the analytical Hessian method is superior to the finite-difference method in terms of the computational accuracy and efficiency. The numerical differentiation can be difficult due to root flipping for excited states that are close in energy. TDDFT yields more exact excited-state vibrational frequencies than CIS, which usually overestimates the values. PMID:21744894
Proynov, Emil; Liu, Fenglai; Shao, Yihan; Kong, Jing
2012-01-01
In a recent letter [E. Proynov, Y. Shao, and J. Kong, Chem. Phys. Lett. 493, 381 (2010)10.1016/j.cplett.2010.05.029], Becke's B05 model of nondynamic electron correlation in density functional theory was implemented self-consistently with computational efficiency (the “SCF-RI-B05” scheme). Important modifications of the algorithm were done in order to make the self-consistency feasible. In the present work, we give a complete account of the SCF-RI-B05 algorithm, including all the formulae for the analytical representation of the B05 functional and for its self-consistent field (SCF) potential. The average performance of the SCF-RI-B05 method reported in the above letter was somewhat less accurate, compared to the original B05 implementation, mainly because the parameters of the original B05 model were optimized with post-local-spin-density calculations. In this work, we report improved atomization energies with SCF-RI-B05, based on a SCF re-optimization of its four linear parameters. The re-optimized SCF-RI-B05 scheme is validated also on reaction barriers, and on the subtle energetics of NO dimer, an exemplary system of strong nondynamic correlation. It yields both the binding energy and the singlet-triplet splitting of the NO dimer correctly, and close to the benchmarks reported in the literature. PMID:22280739
All-electron Kohn–Sham density functional theory on hierarchic finite element spaces
Schauer, Volker; Linder, Christian
2013-10-01
In this work, a real space formulation of the Kohn–Sham equations is developed, making use of the hierarchy of finite element spaces from different polynomial order. The focus is laid on all-electron calculations, having the highest requirement onto the basis set, which must be able to represent the orthogonal eigenfunctions as well as the electrostatic potential. A careful numerical analysis is performed, which points out the numerical intricacies originating from the singularity of the nuclei and the necessity for approximations in the numerical setting, with the ambition to enable solutions within a predefined accuracy. In this context the influence of counter-charges in the Poisson equation, the requirement of a finite domain size, numerical quadratures and the mesh refinement are examined as well as the representation of the electrostatic potential in a high order finite element space. The performance and accuracy of the method is demonstrated in computations on noble gases. In addition the finite element basis proves its flexibility in the calculation of the bond-length as well as the dipole moment of the carbon monoxide molecule.
Yang, Zhi; Xiong, Shi-Jie
2008-09-28
The geometries stability, electronic properties, and magnetism of Y(n)O clusters up to n=14 are systematically studied with density functional theory. In the lowest-energy structures of Y(n)O clusters, the equilibrium site of the oxygen atom gradually moves from an outer site of the cluster, via a surface site, and finally, to an interior site as the number of the Y atoms increases from 2 to 14. Starting from n=12, the O atom falls into the center of the cluster with the Y atoms forming the outer frame. The results show that clusters with n=2, 4, 8, and 12 are more stable than their respective neighbors, and that the total magnetic moments of Y(n)O clusters are all quite small except Y(12)O cluster. The lowest-energy structure of Y(12)O cluster is a perfect icosahedron with a large magnetic moment 6mu(B). In addition, we find that the total magnetic moments are quenched for n=2, 6, and 8 due to the closed-shell electronic configuration. The calculated ionization potentials and electron affinities are in good agreement with the experimental results, which imply that the present theoretical treatments are satisfactory. PMID:19045024
NASA Astrophysics Data System (ADS)
Zhong, X.; Rungger, I.; Zapol, P.; Heinonen, O.
2015-03-01
Understanding electronic properties of substoichiometric phases of titanium oxide such as Magnéli phase T i4O7 is crucial in designing and modeling resistive switching devices. Here we present our study on Magnéli phase T i4O7 together with rutile Ti O2 and T i2O3 using density functional theory methods with atomic-orbital-based self-interaction correction (ASIC). We predict a new antiferromagnetic (AF) ground state in the low temperature (LT) phase, and we explain energy difference with a competing AF state using a Heisenberg model. The predicted energy ordering of these states in the LT phase is calculated to be robust in a wide range of modeled isotropic strain. We have also investigated the dependence of the electronic structures of the Ti-O phases on stoichiometry. The splitting of titanium t2 g orbitals is enhanced with increasing oxygen deficiency as Ti-O is reduced. The electronic properties of all these phases can be reasonably well described by applying ASIC with a "standard" value for transition metal oxides of the empirical parameter α of 0.5 representing the magnitude of the applied self-interaction correction.
Regeta, Khrystyna; Bannwarth, Christoph; Grimme, Stefan; Allan, Michael
2015-06-28
The technique of low energy (0-30 eV) electron impact spectroscopy, originally developed for gas phase molecules, is applied to room temperature ionic liquids (IL). Electron energy loss (EEL) spectra recorded near threshold, by collecting 0-2 eV electrons, are largely continuous, assigned to excitation of a quasi-continuum of high overtones and combination vibrations of low-frequency modes. EEL spectra recorded by collecting 10 eV electrons show predominantly discrete vibrational and electronic bands. The vibrational energy-loss spectra correspond well to IR spectra except for a broadening (∼0.04 eV) caused by the liquid surroundings, and enhanced overtone activity indicating a contribution from resonant excitation mechanism. The spectra of four representative ILs were recorded in the energy range of electronic excitations and compared to density functional theory multireference configuration interaction (DFT/MRCI) calculations, with good agreement. The spectra up to about 8 eV are dominated by π-π* transitions of the aromatic cations. The lowest bands were identified as triplet states. The spectral region 2-8 eV was empty in the case of a cation without π orbitals. The EEL spectrum of a saturated solution of methylene green in an IL band showed the methylene green EEL band at 2 eV, indicating that ILs may be used as a host to study nonvolatile compounds by this technique in the future. PMID:26018044
NASA Astrophysics Data System (ADS)
Huang, Y.-T.; Diehl, R. D.; Pulkkinen, A.; Pussi, K.
2015-09-01
Alkali metal adsorption systems provide important models for chemisorption. Low-energy electron diffraction experiments and density functional theory calculations were carried out for the adsorption of potassium on Pb(1 0 0). The stable structure for all submonolayer coverages was found to be the commensurate c(2 × 2) structure, with potassium atoms located in substitutional sites in the top substrate layer. This structure is temperature activated and occurs for adsorption or annealing of the film above 200 K. This finding is consistent with an earlier theory that proposed that for substrates with low energies of vacancy formation, substitutional structures can be the most stable. The structural and vibrational parameters deduced from the experiment are in agreement with the calculated values, and these values fit well into and add to the database of alkali metal adsorption properties.
NASA Astrophysics Data System (ADS)
Zhang, Xiao; Guo, Ling; Guo, Jian; Ren, Ning-Ning
2014-03-01
The binding energy, dissociation energy, ionization potentials, electron affinities, gap and stability of small Al n Pt ( n = 1-15) clusters, in comparison with pure aluminum clusters have been systematically investigated by means of density functional calculations at the B3LYP level. The growth patten for Al n Pt clusters is that the Pt atom substituted the surface atom of the Al n + 1 clusters for n < 13. Starting from n = 13, the Pt atom completely falls into the center of the Al-frame. The Pt atom substituted the center atom of the Al n + 1 clusters to form the Pt-encapsulated Aln geometries for n > 13. We also find that the impurity Pt atom causes local structural distortion due to different atomic radii and different bonding characteristics. The clusters with total atom numbers of 2, 7, and 11 exhibit high stability.
Huang, Y-T; Diehl, R D; Pulkkinen, A; Pussi, K
2015-09-01
Alkali metal adsorption systems provide important models for chemisorption. Low-energy electron diffraction experiments and density functional theory calculations were carried out for the adsorption of potassium on Pb(1 0 0). The stable structure for all submonolayer coverages was found to be the commensurate c(2 × 2) structure, with potassium atoms located in substitutional sites in the top substrate layer. This structure is temperature activated and occurs for adsorption or annealing of the film above 200 K. This finding is consistent with an earlier theory that proposed that for substrates with low energies of vacancy formation, substitutional structures can be the most stable. The structural and vibrational parameters deduced from the experiment are in agreement with the calculated values, and these values fit well into and add to the database of alkali metal adsorption properties. PMID:26202693
Density-functional study of the electronic and optical properties of the spinel compound CuIr2S4
NASA Astrophysics Data System (ADS)
Sarkar, Soumyajit; de Raychaudhury, Molly; Saha-Dasgupta, T.
2009-03-01
Using first-principles density-functional calculations we have computed the electronic and optical properties of spinel compound CuIr2S4 , which undergoes a structural phase transition, accompanied by a metal-insulator transition at a temperature of about 230 K. The nature of this transition has been discussed in the literature in terms of both the correlated singlet formation picture as well as the orbitally driven Peierls transition picture. Our first-principles calculations find little role of correlation. Our calculated reflectivity and conductivity data for both the high-temperature and low-temperature phases are found to be in good agreement with measured data by Wang [Phys. Rev. B 69, 153104 (2004)].
NASA Astrophysics Data System (ADS)
Hahn, Torsten; Rückerl, Florian; Liebing, Simon; Pederson, Mark
We present our experimental and theoretical results on novel Picene/F4TCNQ and Manganese-Phthalocyanine/F4TCNQ donor / acceptor systems. We apply the recently developed Fermi-orbital based approach for self-interaction corrected density functional theory (FO-SIC DFT) to these materials and compare the results to standard DFT calculations and to experimental data obtained by photoemission spectroscopy. We focus our analysis on the description of the magnitude of the ground state charge transfer and on the details of the formed hybrid orbitals. Further, we show that for weakly bound donor / acceptor systems the FO-SIC approach delivers a more realistic description of the electronic structure compared to standard DFT calculations Support by DFG FOR1154 is greatly acknowledged.
NASA Astrophysics Data System (ADS)
Zheng, Y.; Neville, J. J.; Brion, C. E.; Wang, Y.; Davidson, E. R.
1994-11-01
The binding energy spectra and momentum distributions of all valence orbitals of acetone have been studied by electron momentum spectroscopy (EMS) and SCF, MRSD-CI, and density functional theory (DFT) calculations. The experiment was performed using a multichannel EMS spectrometer at a total energy of 1200 eV. Binding energy spectra measured in the energy range of 6-60 eV are compared with the results of OVGF and 2ph-TDA many-body Green's function calculations. In the inner valence region strong splitting of the 5a 1 and 4a 1 orbitals due to final state electron correlation is observed. The distribution of energies and pole strengths predicted by the Green's function calculations deviates considerably from the measured ionization energies and strengths in the innervalence region. The measured momentum distributions are compared with calculations at the level of the target Hartree-Fock approximation (THFA) using the SCF method and the target Kohn-Sham approximation (TKSA) using DFT and the local-density approximation. Basis sets used for the SCF calculations ranged from the simplest (STO-3G) to large (204-GTO) and for the DFT calculations very large atomic natural orbital (ANO) basis sets were used. The effects of electron correlation and relaxation are also investigated in MRSD-CI calculations of the full ion-neutral overlap amplitude using large and saturated basis sets. In general, the THFA model with an intermediate basis set and very diffuse functions (6-311 + +G ∗∗) and with a near Hartree-Fock limit SCF wavefunction (204-GTO), and the TKSA-DFT model with an ANO basis set all provide reasonable predictions of momentum distributions for most orbitals. However, none of these calculations gives a completely satisfactory description of the momentum distribution of the HOMO (5b 2) orbital.
The mapping of electronic energy distributions using experimental electron density.
Tsirelson, Vladimir G
2002-08-01
It is demonstrated that the approximate kinetic energy density calculated using the second-order gradient expansion with parameters of the multipole model fitted to experimental structure factors reproduces the main features of this quantity in a molecular or crystal position space. The use of the local virial theorem provides an appropriate derivation of approximate potential energy density and electronic energy density from the experimental (model) electron density and its derivatives. Consideration of these functions is not restricted by the critical points in the electron density and provides a comprehensive characterization of bonding in molecules and crystals. PMID:12149553
Bykov, Dmytro; Neese, Frank
2015-10-01
In this Forum Article, an extensive discussion of the mechanism of six-electron, seven-proton nitrite reduction by the cytochrome c nitrite reductase enzyme is presented. On the basis of previous studies, the entire mechanism is summarized and a unified picture of the most plausible sequence of elementary steps is presented. According to this scheme, the mechanism can be divided into five functional stages. The first phase of the reaction consists of substrate binding and N-O bond cleavage. Here His277 plays a crucial role as a proton donor. In this step, the N-O bond is cleaved heterolytically through double protonation of the substrate. The second phase of the mechanism consists of two proton-coupled electron-transfer events, leading to an HNO intermediate. The third phase involves the formation of hydroxylamine, where Arg114 provides the necessary proton for the reaction. The second N-O bond is cleaved in the fourth phase of the mechanism, again triggered by proton transfer from His277. The Tyr218 side chain governs the fifth and last phase of the mechanism. It consists of radical transfer and ammonia formation. Thus, this mechanism implies that all conserved active-site side chains work in a concerted way in order to achieve this complex chemical transformation from nitrite to ammonia. The Forum Article also provides a detailed discussion of the density functional theory based cluster model approach to bioinorganic reactivity. A variety of questions are considered: the resting state of enzyme and substrate binding modes, interaction with the metal site and with active-site side chains, electron- and proton-transfer events, substrate dissociation, etc. PMID:26237518
Gulans, Andris; Kontur, Stefan; Meisenbichler, Christian; Nabok, Dmitrii; Pavone, Pasquale; Rigamonti, Santiago; Sagmeister, Stephan; Werner, Ute; Draxl, Claudia
2014-09-10
Linearized augmented planewave methods are known as the most precise numerical schemes for solving the Kohn-Sham equations of density-functional theory (DFT). In this review, we describe how this method is realized in the all-electron full-potential computer package, exciting. We emphasize the variety of different related basis sets, subsumed as (linearized) augmented planewave plus local orbital methods, discussing their pros and cons and we show that extremely high accuracy (microhartrees) can be achieved if the basis is chosen carefully. As the name of the code suggests, exciting is not restricted to ground-state calculations, but has a major focus on excited-state properties. It includes time-dependent DFT in the linear-response regime with various static and dynamical exchange-correlation kernels. These are preferably used to compute optical and electron-loss spectra for metals, molecules and semiconductors with weak electron-hole interactions. exciting makes use of many-body perturbation theory for charged and neutral excitations. To obtain the quasi-particle band structure, the GW approach is implemented in the single-shot approximation, known as G(0)W(0). Optical absorption spectra for valence and core excitations are handled by the solution of the Bethe-Salpeter equation, which allows for the description of strongly bound excitons. Besides these aspects concerning methodology, we demonstrate the broad range of possible applications by prototypical examples, comprising elastic properties, phonons, thermal-expansion coefficients, dielectric tensors and loss functions, magneto-optical Kerr effect, core-level spectra and more. PMID:25135665
π vs σ-Radical States of One-Electron Oxidized DNA/RNA Bases: A Density Functional Theory Study
Kumar, Anil; Sevilla, Michael D.
2013-01-01
As a result of their inherent planarity, DNA base radicals generated by one electron oxidation/reduction or bond cleavage form π- or σ-radicals. While most DNA base systems form π-radicals there are a number of nucleobase analogs such as one-electron oxidized 6-azauraci1, 6-azacytosine, and 2-thiothymine or one-electron reduced 5-bromouracil that form more reactive σ-radicals. Elucidating the availability of these states within DNA, base radical electronic structure is important to the understanding of the reactivity of DNA base radicals in different environments. In this work, we address this question by the calculation of the relative energies of π- and σ-radical states in DNA/RNA bases and their analogs. We used density functional theory B3LYP/6-31++G** method to optimize the geometries of π- and σ-radicals in Cs symmetry (i.e., planar) in the gas phase and in solution using the polarized continuum model (PCM). The calculations predict that σ- and π-radical states in one electron oxidized bases of thymine, T(N3-H)•, and uracil, U(N3-H)• are very close in energy, i.e., the π-radical is only ca. 4 kcal/mol more stable than the σ-radical. For the one electron oxidized radicals of cytosine, C•+, C(N4-H)•, adenine, A•+, A(N6-H)•, and guanine, G•+, G(N2-H)•, G(N1-H)• the π-radicals are ca. 16 to 41 kcal/mol more stable than their corresponding σ-radicals. Inclusion of solvent (PCM) is found to stabilize the π- over σ-radical of each of the systems. U(N3-H)• with three discrete water molecules in the gas phase, is found to form a three-electron σ bond between N3 atom of uracil and O atom of a water molecule but on inclusion of full solvation and discrete hydration the π-radical remains most stable.. PMID:24000793
NASA Astrophysics Data System (ADS)
Govindasamy, P.; Gunasekaran, S.; Ramkumaar, G. R.
2014-09-01
The Fourier transform infrared (FT-IR) and FT-Raman spectra of N-(4-hydroxy phenyl) acetamide (N4HPA) of painkiller agent were recorded in the region 4000-450 cm-1 and 4000-50 cm-1 respectively. Density functional theory (DFT) has been used to calculate the optimized geometrical parameter, atomic charges, and vibrational wavenumbers and intensity of the vibrational bands. The computed vibrational wave numbers were compared with the FT-IR and FT-Raman experimental data. The computational calculations at DFT/B3LYP level with 6-31G(d,p), 6-31++G(d,p), 6-311G(d,p) and 6-311++G(d,p) basis sets. The complete vibrational assignments were performed on the basis of the potential energy distribution (PED) of the vibrational modes calculated using Vibrational energy distribution analysis (VEDA 4) program. The oscillator’s strength calculated by TD-DFT and N4HPA is approach complement with the experimental findings. The NMR chemical shifts 13C and 1H were recorded and calculated using the gauge independent atomic orbital (GIAO) method. The molecular electrostatic potential (MESP) and electron density surfaces of the molecule were constructed. The Natural charges and intermolecular contacts have been interpreted using Natural Bond orbital (NBO) analysis the HOMO-LUMO energy gap has been calculated. The thermodynamic properties like entropy, heat capacity and zero vibrational energy have been calculated.
NASA Astrophysics Data System (ADS)
Chae, Kisung; Kim, Hanchul
2013-02-01
We perform spin-polarized density functional theory calculations for a Zn vacancy on the ZnO(10bar 10) surface. Two stable configurations of the surface Zn vacancy are found, and the activation energy barrier is estimated to be ˜0.01 eV. The lower energy configuration has a newly formed surface Zn-O bond to restore the bulk-like structure on the surface. Due to the newly formed bond, the vacancy state in the band gap is characterized by a complicated hybridization of neighboring surface and subsurface atoms and by a more extended electron density. Despite such a hybridization, the surface Zn vacancy is found to have a robust magnetic moment of 1 μ B , implying that surface Zn vacancies may be responsible for the ferromagnetism observed in ZnO thin films and nanoparticles. Simulated scanning tunneling microscope images show that the two structures of the surface Zn vacancy can be distinguished in the filled-state images.
NASA Astrophysics Data System (ADS)
Goings, Joshua J.; Li, Xiaosong
2016-06-01
One of the challenges of interpreting electronic circular dichroism (ECD) band spectra is that different states may have different rotatory strength signs, determined by their absolute configuration. If the states are closely spaced and opposite in sign, observed transitions may be washed out by nearby states, unlike absorption spectra where transitions are always positive additive. To accurately compute ECD bands, it is necessary to compute a large number of excited states, which may be prohibitively costly if one uses the linear-response time-dependent density functional theory (TDDFT) framework. Here we implement a real-time, atomic-orbital based TDDFT method for computing the entire ECD spectrum simultaneously. The method is advantageous for large systems with a high density of states. In contrast to previous implementations based on real-space grids, the method is variational, independent of nuclear orientation, and does not rely on pseudopotential approximations, making it suitable for computation of chiroptical properties well into the X-ray regime.
NASA Astrophysics Data System (ADS)
Rathi, Somilkumar J.; Ray, Asok K.
2008-08-01
Ab initio calculations within the framework of hybrid density functional theory and the finite cluster approximation have been performed for the electronic and geometric structures of three different types of armchair germanium carbide nanotube, from (3, 3) to (11, 11). Full geometry and spin optimizations with unrestricted symmetry have been performed. Physically pertinent quantities of interest such as the cohesive energies, band gaps, radial buckling, density of states, dipole moments, and Mulliken charge distributions have been investigated in detail for all nanotubes. For type I nanotubes, the largest cohesive energy obtained is 4.092 eV/atom, whereas for type II and type III nanotubes, the values are 3.987 eV/atom and 3.968 eV/atom, respectively. For optimized type I nanotubes, Ge atoms moved toward the tube axis and C atoms moved in the opposite direction after relaxation, opposite to the trends observed in types II and III. The band gaps for type I nanotubes are larger than the bulk 3C-GeC gap, varying between 2.666 and 3.016 eV, while type II and type III nanotubes have significantly lower band gaps, with all nanotubes being semiconducting in nature. Mulliken charge analysis indicates primarily ionic behavior for type I GeC nanotubes and a mixed ionic with covalent behavior for the other two types. None of the tubes appear to be magnetic. Applications in the field of nano-optoelectronic devices, molecular electronics, and band gap engineering are envisioned for GeC nanotubes.
Rathi, Somilkumar J; Ray, Asok K
2008-08-20
Ab initio calculations within the framework of hybrid density functional theory and the finite cluster approximation have been performed for the electronic and geometric structures of three different types of armchair germanium carbide nanotube, from (3, 3) to (11, 11). Full geometry and spin optimizations with unrestricted symmetry have been performed. Physically pertinent quantities of interest such as the cohesive energies, band gaps, radial buckling, density of states, dipole moments, and Mulliken charge distributions have been investigated in detail for all nanotubes. For type I nanotubes, the largest cohesive energy obtained is 4.092 eV/atom, whereas for type II and type III nanotubes, the values are 3.987 eV/atom and 3.968 eV/atom, respectively. For optimized type I nanotubes, Ge atoms moved toward the tube axis and C atoms moved in the opposite direction after relaxation, opposite to the trends observed in types II and III. The band gaps for type I nanotubes are larger than the bulk 3C-GeC gap, varying between 2.666 and 3.016 eV, while type II and type III nanotubes have significantly lower band gaps, with all nanotubes being semiconducting in nature. Mulliken charge analysis indicates primarily ionic behavior for type I GeC nanotubes and a mixed ionic with covalent behavior for the other two types. None of the tubes appear to be magnetic. Applications in the field of nano-optoelectronic devices, molecular electronics, and band gap engineering are envisioned for GeC nanotubes. PMID:21730632
Density functional study of structural and electronic properties of Al{sub n}@C{sub 60}
Dhiman, Shobhna; Kumar, Ranjan; Dharamvir, Keya
2014-04-24
Fullerene derivatives have been shown to make contributions in many types of applications. Ab initio investigation of structural and electronic properties of aluminum doped endohedral fullerene has been performed using numerical atomic orbital density functional theory. We have obtained ground state structures for Al{sub n}@C{sub 60} (n=1–10). Which shows that C{sub 60} molecule can accommodate maximum of nine aluminum atoms, for n > 9 the cage eventually break. Encapsulated large number of aluminum atoms leads to deformation of cage with diameter varies from 7.16Å to 7.95Å. Binding energy/Al atom is found to increase till n = 4 and after that it decreases with the number of Al atoms with a sudden increase for n=10 due to breakage of C{sub 60} cage and electronic affinity first increases till n=4 then it decreases up to n=9 with a sharp increase for n=10. Ionization potential also first increases and then decreases. Homo-Lumo gap decreases till n=3 with a sharp increase for n=4, after that it shows an oscillatory nature. The results obtained are consistent with available theoretical and experimental results. The ab-initio calculations were performed using SIESTA code with generalized gradient approximation (GGA)
Walsh, Aron; Wei, S.-H.; Yan Yanfa; Al-Jassim, M. M.; Turner, John A.; Woodhouse, Michael; Parkinson, B. A.
2007-10-15
A systematic study of nine binary and ternary spinel oxides formed from Co, Al, and Fe is presented by means of density functional theory. Analysis of the structural, magnetic, and electronic properties through the series of materials is carried out. Preference for the octahedral spinel sites are found in the order Fe
A finite-temperature density functional study of electron self-trapping in 3He and 4He.
Jin, Dafei; Guo, Wei
2012-06-28
We introduce a compact finite-temperature density functional model to study electron self-trapping in both liquid and vapor (3)He and (4)He. This model can quantitatively reproduce the most essential thermodynamic properties of (3)He and (4)He along their liquid-vapor coexistence lines. The structures and energetics of self-trapped electron bubbles on the 1S ground state and 1P excited state are particularly investigated. Our results show that 1S and 1P bubbles exist in liquid at any temperature, whereas 1S bubbles exist in vapor only above 1.6 K in (3)He and above 2.8 K in (4)He, 1P bubbles exist in vapor only above 2.5 K in (3)He and 4.0 K in (4)He. An initially spherical 1P bubble is unstable against deformation towards a peanut shape. In liquid, a peanut-shaped 1P bubble is held from fission by surface tension until reaching the liquid-vapor critical point, whereas in vapor it always splits into two smaller bubbles. The existence of 1P bubbles in finite-temperature liquid helium and their fission instability in helium vapor reveal interesting physics in this system. PMID:22755590
NASA Astrophysics Data System (ADS)
Sun, Xu; Gu, You-song; Wang, Xue-qiang; Zhang, Yue
2012-06-01
The electronic properties and stability of Li-doped ZnO with various defects have been studied by calculating the electronic structures and defect formation energies via first-principles calculations using hybrid Hartree-Fock and density functional methods. The results from formation energy calculations show that Li pair complexes have the lowest formation energy in most circumstances and they consume most of the Li content in Li doped ZnO, which make the p-type conductance hard to obtain. The formation of Li pair complexes is the main obstacle to realize p-type conductance in Li doped ZnO. However, the formation energy of LiZn decreases as environment changes from Zn-rich to O-rich and becomes more stable than that of Li-pair complexes at highly O-rich environment. Therefore, p-type conductance can be obtained by Li doped ZnO grown or post annealed in oxygen rich atmosphere.
Gedanken densities and exact constraints in density functional theory
Perdew, John P.; Ruzsinszky, Adrienn; Sun, Jianwei; Burke, Kieron
2014-05-14
Approximations to the exact density functional for the exchange-correlation energy of a many-electron ground state can be constructed by satisfying constraints that are universal, i.e., valid for all electron densities. Gedanken densities are designed for the purpose of this construction, but need not be realistic. The uniform electron gas is an old gedanken density. Here, we propose a spherical two-electron gedanken density in which the dimensionless density gradient can be an arbitrary positive constant wherever the density is non-zero. The Lieb-Oxford lower bound on the exchange energy can be satisfied within a generalized gradient approximation (GGA) by bounding its enhancement factor or simplest GGA exchange-energy density. This enhancement-factor bound is well known to be sufficient, but our gedanken density shows that it is also necessary. The conventional exact exchange-energy density satisfies no such local bound, but energy densities are not unique, and the simplest GGA exchange-energy density is not an approximation to it. We further derive a strongly and optimally tightened bound on the exchange enhancement factor of a two-electron density, which is satisfied by the local density approximation but is violated by all published GGA's or meta-GGA’s. Finally, some consequences of the non-uniform density-scaling behavior for the asymptotics of the exchange enhancement factor of a GGA or meta-GGA are given.
Eljarrat, Alberto; Sastre, Xavier; Peiró, Francesca; Estradé, Sónia
2016-06-01
In the present work, the dielectric response of III-nitride semiconductors is studied using density functional theory (DFT) band structure calculations. The aim of this study is to improve our understanding of the features in the low-loss electron energy-loss spectra of ternary alloys, but the results are also relevant to optical and UV spectroscopy results. In addition, the dependence of the most remarkable features with composition is tested, i.e. applying Vegard's law to band gap and plasmon energy. For this purpose, three wurtzite ternary alloys, from the combination of binaries AlN, GaN, and InN, were simulated through a wide compositional range (i.e., Al x Ga1-x N, In x Al1-x N, and In x Ga1-x N, with x=[0,1]). For this DFT calculations, the standard tools found in Wien2k software were used. In order to improve the band structure description of these semiconductor compounds, the modified Becke-Johnson exchange-correlation potential was also used. Results from these calculations are presented, including band structure, density of states, and complex dielectric function for the whole compositional range. Larger, closer to experimental values, band gap energies are predicted using the novel potential, when compared with standard generalized gradient approximation. Moreover, a detailed analysis of the collective excitation features in the dielectric response reveals their compositional dependence, which sometimes departs from a linear behavior (bowing). Finally, an advantageous method for measuring the plasmon energy dependence from these calculations is explained. PMID:26868876
Pinjari, Rahul V; Gejji, Shridhar P
2009-02-19
Inverted cucurbit[n]uril (i(x)CB[n], x = 1, 2; n = 6-8), the enantiomers of cucurbit[n]uril (CB[n]) comprising one or more inverted glycouril units, show distinct selectivity in recognition toward the guest by the virtue of shape and dimensions of its cavity. The iCB[n] (x = 1 and n = 6, 7) are isolated as intermediates during the synthesis of CB[n]. In this work, density functional theory using the hybrid B3LYP functional has been employed to derive the electronic structure and the NMR chemical shifts in the i(x)CB[n] hosts. The present calculations have shown that the inversion of the glycouril unit of CB[6] and CB[7] engenders a destabilization by 4.2 and 5.7 kJ mol(-1), respectively, and, as opposed to this, the iCB[8] is favored by 18.6 kJ mol(-1) over the corresponding CB[8] host. Likewise, i2CB[7] possessing two inverted glycourils are highly destabilized over CB[7]. A large separation of the inverted glycouril units reduces the repulsion between methine protons inside the cavity, rendering the 1,4-i2CB[n] (n = 7 or 8) to be of lowest energy. Stabilization energies from the self-consistent reaction field (SCRF) theory are calculated with water, ethanol, and tetrahydrofuran (THF) as solvents. Unlike in gas phase and other solvents, the stabilization hierarchy iCB[6] < iCB[7] < iCB[8] has been predicted in THF. Molecular electrostatic potential (MESP) was used to gauge the cavity shape of these hosts. Consequently the iCB[6] reveals a half-sprocket-like cavity; an additional tooth for each glycouril in the succeeding iCB[n] homologue was noticed. In the case of the 1,5-i2CB[8] enantiomer, the cavity turns out to be rectangular. The deeper MESP minima near the ureido oxygens suggest strong electrostatic interactions with the guest at the iCB[6] portals. The electron-rich region within the cavity explains the large affinity of CB[n] toward the electron deficient guests. The electronic distribution and shape and size of the cavity thus derived provide insights
NASA Astrophysics Data System (ADS)
Suhai, Sándor
1995-06-01
Structural and energetic aspects of the Peierls-type lattice dimerization were investigated in infinite, one-dimensional, periodic trans-polyacetylene (t-PA) using many-body perturbation theory (MBPT) and density-functional theory (DFT). Cohesive properties and dimerization parameters were obtained first for the classical Coulomb potential in the Hartree approximation and then by gradually turning on exchange and correlation potentials. Besides the nonlocal Hartree-Fock exchange, several other exchange functionals were used incorporating gradient corrections as well. For MBPT, electron correlation was included up to the fourth order of the Mo/ller-Plesset scheme and the behavior of lattice sums for different PT terms was analyzed in detail. The electrostatic part of the infinite lattice sums was computed by the multipole expansion technique. In solving the polymer Kohn-Sham equations, the performance of several different correlation potentials was studied again including different gradient corrections. Atomic basis sets of systematically increasing size, in the range of double-zeta to triple-zeta (TZ) up to TZ (3df,3p2d), were used in all calculations to construct the symmetry-adapted (Bloch-type) polymer wave functions, to fully optimize the structures, and to extrapolate different physical quantities to the limit of a hypothetical infinite basis set. Comparison of the different DFT results with MBPT and with experiments demonstrated the importance of gradient terms both for exchange and correlation. On the other hand, the best DFT functional, using a medium-size atomic basis set, excellently reproduced the cohesive and dimerization energies obtained for infinite t-PA at the MP4/TZ(3d2f,3p2d) level and provided dimerization parameters close to experiment. The experimentally observed lattice spacing of 2.46+/-0.01 Å will be correctly predicted both at the MBPT and DFT levels with 2.48 and 2.44 Å, respectively.
Ching, W. Y.; Aryal, Sitram; Rulis, Paul; Schnick, Wolfgang
2011-04-15
Using density-functional-theory-based ab initio methods, the electronic structure and physical properties of the newly synthesized nitride BeP{sub 2}N{sub 4} with a phenakite-type structure and the predicted high-pressure spinel phase of BeP{sub 2}N{sub 4} are studied in detail. It is shown that both polymorphs are wide band-gap semiconductors with relatively small electron effective masses at the conduction-band minima. The spinel-type phase is more covalently bonded due to the increased number of P-N bonds for P at the octahedral sites. Calculations of mechanical properties indicate that the spinel-type polymorph is a promising superhard material with notably large bulk, shear, and Young's moduli. Also calculated are the Be K, P K, P L{sub 3}, and N K edges of the electron energy-loss near-edge structure for both phases. They show marked differences because of the different local environments of the atoms in the two crystalline polymorphs. These differences will be very useful for the experimental identification of the products of high-pressure syntheses targeting the predicted spinel-type phase of BeP{sub 2}N{sub 4}.
NASA Astrophysics Data System (ADS)
Lee, Ming-Hsien; Liu, Po-Liang; Hong, Yung-An; Chou, Yen-Ting; Hong, Jia-Yang; Siao, Yu-Jin
2013-02-01
We conduct first-principles total-energy density functional calculations to study the band structures in Ge1-xSnx infrared semiconductor alloys. The norm-conserving optimized pseudopotentials of Ge and Sn have been constructed for electronic structure calculations. The composition-bandgap relationships in Ge1-xSnx lattices are evaluated by a detailed comparison of structural models and their electronic band structures. The critical Sn composition related to the transition from indirect- to direct-gap in Ge1-xSnx alloys is estimated to be as low as x ˜ 0.016 determined from the parametric fit. Our results show that the crossover Sn concentration occurs at a lower critical Sn concentration than the values predicted from the absorption measurements. However, early results indicate that the reliability of the critical Sn concentration from such measurements is hard to establish, since the indirect gap absorption is much weaker than the direct gap absorption. We find that the direct band gap decreases exponentially with the Sn composition over the range 0
Severino, Joyce Ferreira; Goodman, Bernard A; Kay, Christopher W M; Stolze, Klaus; Tunega, Daniel; Reichenauer, Thomas G; Pirker, Katharina F
2009-04-15
Electron paramagnetic resonance spectroscopy and density functional theory calculations have been used to investigate the redox properties of the green tea polyphenols (GTPs) (-)-epigallocatechin gallate (EGCG), (-)-epigallocatechin (EGC), and (-)-epicatechin gallate (ECG). Aqueous extracts of green tea and these individual phenols were autoxidized at alkaline pH and oxidized by superoxide anion (O(2)(-)) radicals in dimethyl sulfoxide. Several new aspects of the free radical chemistry of GTPs were revealed. EGCG can be oxidized on both the B and the D ring. The B ring was the main oxidation site during autoxidation, but the D ring was the preferred site for O(2)(-) oxidation. Oxidation of the D ring was followed by structural degradation, leading to generation of a radical identical to that of oxidized gallic acid. Alkaline autoxidation of green tea extracts produced four radicals that were related to products of the oxidation of EGCG, EGC, ECG, and gallic acid, whereas the spectra from O(2)(-) oxidation could be explained solely by radicals generated from EGCG. Assignments of hyperfine coupling constants were made by DFT calculations, allowing the identities of the radicals observed to be confirmed. PMID:19439236
Akiyama, Toru; Nakamura, Kohji; Ito, Tomonori; Freeman, Arthur J.
2014-03-31
The electronic band structures and excited states of III-V semiconductors such as GaP, AlP, AlAs, and AlSb for various polytypes are determined employing the screened-exchange density functional calculations implemented in the full-potential linearized augmented plane-wave methods. We demonstrate that GaP and AlSb in the wurtzite (WZ) structure have direct gap while III-V semiconductors in the zinc blende, 4H, and 6H structures considered in this study exhibit an indirect gap. Furthermore, we find that inclusion of Al atoms less than 17% and 83% in the hexagonal Al{sub x}Ga{sub 1−x}P and Al{sub x}Ga{sub 1−x}As alloys, respectively, leads to a direct transition with a gap energy of ∼2.3 eV. The feasibility of III-V semiconductors with a direct gap in WZ structure offers a possible crystal structure engineering to tune the optical properties of semiconductor materials.
Park, Ji-Sang; Kang, Joongoo; Yang, Ji-Hui; Metzger, Wyatt; Wei, Su-Huai
2015-01-15
Using first-principles density functional calculations, we investigate the relative stability and electronic structure of the grain boundaries (GBs) in zinc-blende CdTe. Among the low-Σ-value symmetric tilt Σ3 (111), Σ3 (112), Σ5 (120), and Σ5 (130) GBs, we show that the Σ3 (111)GB is always the most stable due to the absence of dangling bonds and wrong bonds. The Σ5 (120) GBs, however, are shown to be more stable than the Σ3 (112) GBs, even though the former has a higher Σ value, and the latter is often used as a model system to study GB effects in zinc-blende semiconductors. Furthermore,more » we find that although containing wrong bonds, the Σ5 (120) GBs are electrically benign due to the short wrong bond lengths, and thus are not as harmful as the Σ3 (112) GBs also having wrong bonds but with longer bond lengths.« less
NASA Astrophysics Data System (ADS)
Akiyama, Toru; Nakamura, Kohji; Ito, Tomonori; Freeman, Arthur J.
2014-03-01
The electronic band structures and excited states of III-V semiconductors such as GaP, AlP, AlAs, and AlSb for various polytypes are determined employing the screened-exchange density functional calculations implemented in the full-potential linearized augmented plane-wave methods. We demonstrate that GaP and AlSb in the wurtzite (WZ) structure have direct gap while III-V semiconductors in the zinc blende, 4H, and 6H structures considered in this study exhibit an indirect gap. Furthermore, we find that inclusion of Al atoms less than 17% and 83% in the hexagonal AlxGa1-xP and AlxGa1-xAs alloys, respectively, leads to a direct transition with a gap energy of ˜2.3 eV. The feasibility of III-V semiconductors with a direct gap in WZ structure offers a possible crystal structure engineering to tune the optical properties of semiconductor materials.
NASA Astrophysics Data System (ADS)
Nagae, Yuki; Kurosawa, Masashi; Shibayama, Shigehisa; Araidai, Masaaki; Sakashita, Mitsuo; Nakatsuka, Osamu; Shiraishi, Kenji; Zaima, Shigeaki
2016-08-01
We have carried out density functional theory (DFT) calculation for Si1‑ x Sn x alloy and investigated the effect of the displacement of Si and Sn atoms with strain relaxation on the lattice constant and E– k dispersion. We calculated the formation probabilities for all atomic configurations of Si1‑ x Sn x according to the Boltzmann distribution. The average lattice constant and E– k dispersion were weighted by the formation probability of each configuration of Si1‑ x Sn x . We estimated the displacement of Si and Sn atoms from the initial tetrahedral site in the Si1‑ x Sn x unit cell considering structural relaxation under hydrostatic pressure, and we found that the breaking of the degenerated electronic levels of the valence band edge could be caused by the breaking of the tetrahedral symmetry. We also calculated the E– k dispersion of the Si1‑ x Sn x alloy by the DFT+U method and found that a Sn content above 50% would be required for the indirect–direct transition.
Qian, Zekan; Li, Rui; Hou, Shimin; Xue, Zengquan; Sanvito, Stefano
2007-11-21
An efficient self-consistent approach combining the nonequilibrium Green's function formalism with density functional theory is developed to calculate electron transport properties of molecular devices with quasi-one-dimensional (1D) electrodes. Two problems associated with the low dimensionality of the 1D electrodes, i.e., the nonequilibrium state and the uncertain boundary conditions for the electrostatic potential, are circumvented by introducing the reflectionless boundary conditions at the electrode-contact interfaces and the zero electric field boundary conditions at the electrode-molecule interfaces. Three prototypical systems, respectively, an ideal ballistic conductor, a high resistance tunnel junction, and a molecular device, are investigated to illustrate the accuracy and efficiency of our approach. PMID:18035901
NASA Astrophysics Data System (ADS)
Wopperer, P.; Dinh, P. M.; Reinhard, P.-G.; Suraud, E.
2015-02-01
There are various ways to analyze the dynamical response of clusters and molecules to electromagnetic perturbations. Particularly rich information can be obtained from measuring the properties of electrons emitted in the course of the excitation dynamics. Such an analysis of electron signals covers observables such as total ionization, Photo-Electron Spectra (PES), Photoelectron Angular Distributions (PAD), and ideally combined PES/PAD. It has a long history in molecular physics and was increasingly used in cluster physics as well. Recent progress in the design of new light sources (high intensity, high frequency, ultra short pulses) opens new possibilities for measurements and thus has renewed the interest on these observables, especially for the analysis of various dynamical scenarios, well beyond a simple access to electronic density of states. This, in turn, has motivated many theoretical investigations of the dynamics of electronic emission for molecules and clusters up to such a complex and interesting system as C60. A theoretical tool of choice is here Time-Dependent Density Functional Theory (TDDFT) propagated in real time and on a spatial grid, and augmented by a Self-Interaction Correction (SIC). This provides a pertinent, robust, and efficient description of electronic emission including the detailed pattern of PES and PAD. A direct comparison between experiments and well founded elaborate microscopic theories is thus readily possible, at variance with more demanding observables such as for example fragmentation or dissociation cross sections. The purpose of this paper is to describe the theoretical tools developed on the basis of real-time and real-space TDDFT and to address in a realistic manner the analysis of electronic emission following irradiation of clusters and molecules by various laser pulses. After a general introduction, we shall present in a second part the available experimental results motivating such studies, starting from the simplest
NASA Astrophysics Data System (ADS)
Badaeva, Ekaterina; Feng, Yong; Gamelin, Daniel R.; Li, Xiaosong
2008-05-01
The electronic structures of pure and Co2+-doped ZnO quantum dots (QDs) with sizes up to 300 atoms were investigated using three different density functional theory approximations: local spin density approximation (LSDA), gradient-corrected Perdew-Burke-Ernzerhof (PBE) and the hybrid PBE1 functionals with LANL2DZ pseudo-potential and associated basis set. Qualitative agreement among the three methods is found for the pure ZnO nanostructures, but only the hybrid functional reproduces the correct bandgap energies quantitatively. For Co2+-doped ZnO QDs, both LSDA and PBE incorrectly model interactions between Co2+ d levels and the valence band of ZnO, which will strongly impair predictions of dopant-carrier magnetic exchange interactions based on such computational results. Experimental observations are reproduced well in calculations at the hybrid PBE1 level of theory, making this the method of choice for exploring the magnetism of transition metal ions in ZnO QDs computationally. The qualitative features of the Co2+ 3d levels do not change appreciably with changes in cluster size over the range examined, leading to size-dependent dopant-band edge energy differences. The results presented here thus provide an experimentally calibrated framework for future ab initio descriptions of dopant-carrier and dopant-dopant magnetic exchange interactions in diluted magnetic semiconductors (DMS) nanocrystals.
Wavelet analysis of electron-density maps.
Main, P; Wilson, J
2000-05-01
The wavelet transform is a powerful technique in signal processing and image analysis and it is shown here that wavelet analysis of low-resolution electron-density maps has the potential to increase their resolution. Like Fourier analysis, wavelet analysis expresses the image (electron density) in terms of a set of orthogonal functions. In the case of the Fourier transform, these functions are sines and cosines and each one contributes to the whole of the image. In contrast, the wavelet functions (simply called wavelets) can be quite localized and may only contribute to a small part of the image. This gives control over the amount of detail added to the map as the resolution increases. The mathematical details are outlined and an algorithm which achieves a resolution increase from 10 to 7 A using a knowledge of the wavelet-coefficient histograms, electron-density histogram and the observed structure amplitudes is described. These histograms are calculated from the electron density of known structures, but it seems likely that the histograms can be predicted, just as electron-density histograms are at high resolution. The results show that the wavelet coefficients contain the information necessary to increase the resolution of electron-density maps. PMID:10771431
Berland, Kristian; Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ; Arter, Calvin A.; Thonhauser, T.; Cooper, Valentino R.; Lee, Kyuho; Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720 ; Lundqvist, Bengt I.; Schröder, Elsebeth; Hyldgaard, Per
2014-05-14
The theoretical description of sparse matter attracts much interest, in particular for those ground-state properties that can be described by density functional theory. One proposed approach, the van der Waals density functional (vdW-DF) method, rests on strong physical foundations and offers simple yet accurate and robust functionals. A very recent functional within this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89, 035412 (2014)] stands out in its attempt to use an exchange energy derived from the same plasmon-based theory from which the nonlocal correlation energy was derived. Encouraged by its good performance for solids, layered materials, and aromatic molecules, we apply it to several systems that are characterized by competing interactions. These include the ferroelectric response in PbTiO{sub 3}, the adsorption of small molecules within metal-organic frameworks, the graphite/diamond phase transition, and the adsorption of an aromatic-molecule on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well suited to tackle these challenging systems. In addition to being a competitive density functional for sparse matter, the vdW-DF-cx construction presents a more robust general-purpose functional that could be applied to a range of materials problems with a variety of competing interactions.
Berland, Kristian; Arter, Calvin A; Cooper, Valentino R; Lee, Dr. Kyuho; Lundqvist, Prof. Bengt I.; Schroder, Prof. Elsebeth; Thonhauser, Prof. Timo; Hyldgaard, Per
2014-01-01
The theoretical description of sparse matter attracts much interest, in particular for those groundstate properties that can be described by density functional theory (DFT). One proposed approach, the van der Waals density functional (vdW-DF) method, rests on strong physical foundations and offers simple yet accurate and robust functionals. A very recent functional within this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B, in print] stands out in its attempt to use an exchange energy derived from the same plasmon-based theory from which the nonlocal correlation energy was derived. Encouraged by its good performance for solids, layered materials, and aromatic molecules, we apply it to several systems that are characterized by competing interactions. These include the ferroelectric response in PbTiO3, the adsorption of small molecules within metal-organic frameworks (MOFs), the graphite/diamond phase transition, and the adsorption of an aromaticmolecule on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well suited to tackle these challenging systems. In addition to being a competitive density functional for sparse matter, the vdW-DF-cx construction presents a more robust general purpose functional that could be applied to a range of materials problems with a variety of competing interactions.
Berland, Kristian; Arter, Calvin A; Cooper, Valentino R; Lee, Kyuho; Lundqvist, Bengt I; Schröder, Elsebeth; Thonhauser, T; Hyldgaard, Per
2014-05-14
The theoretical description of sparse matter attracts much interest, in particular for those ground-state properties that can be described by density functional theory. One proposed approach, the van der Waals density functional (vdW-DF) method, rests on strong physical foundations and offers simple yet accurate and robust functionals. A very recent functional within this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89, 035412 (2014)] stands out in its attempt to use an exchange energy derived from the same plasmon-based theory from which the nonlocal correlation energy was derived. Encouraged by its good performance for solids, layered materials, and aromatic molecules, we apply it to several systems that are characterized by competing interactions. These include the ferroelectric response in PbTiO3, the adsorption of small molecules within metal-organic frameworks, the graphite/diamond phase transition, and the adsorption of an aromatic-molecule on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well suited to tackle these challenging systems. In addition to being a competitive density functional for sparse matter, the vdW-DF-cx construction presents a more robust general-purpose functional that could be applied to a range of materials problems with a variety of competing interactions. PMID:24832347
Otsuka, Takao; Sumita, Masato; Izawa, Hironori; Morihashi, Kenji
2016-07-21
Electron transfer (ET) states of 1-methyl-3-(N-(1,8-naphthalimidyl)ethyl)imidazolium iodide are responsible for its photophysics. Investigation of an ET state based on constrained density functional theory (CDFT) revealed that nonradiative decay from the ET excited state is mediated by the interaction of the iodine atom with the 1,8-naphthalimide or the imidazolium group. PMID:27222312
Bettinger, Holger F; Tönshoff, Christina; Doerr, Markus; Sanchez-Garcia, Elsa
2016-01-12
While the optical spectra of the acene series up to pentacene provide textbook examples for the annulation principle, the spectra of the larger members are much less understood. The present work provides an investigation of the optically allowed excited states of the acene series from pentacene to nonacene, the largest acene observed experimentally, using the density functional based multireference configuration method (DFT/MRCI). For this purpose, the ten lowest energy states of the B2u and B3u irreducible representations were computed. In agreement with previous computational investigations, the electronic wave functions of the acenes acquire significant multireference character with increasing acene size. The HOMO → LUMO excitation is the major contributor to the (1)La state (p band, B2u) also for the larger acenes. The oscillator strength decreases with increasing length. The (1)Lb state (α band, B3u), so far difficult to assign for the larger acenes due to overlap with photoprecursor bands, becomes almost insensitive to acene length. The (1)Bb state (β band, B3u) also moves only moderately to lower energy with increasing acene size. Excited states of B3u symmetry that formally result from double excitations involving HOMO, HOMO-1, LUMO, and LUMO+1 decrease in energy much faster with system size. One of them (D1) has very small oscillator strength but becomes almost isoenergetic with the (1)La state for nonacene. The other (D2) also has low oscillator strength as long as it is higher in energy than (1)Bb. Once it is lower in energy than the (1)Bb state, both states interact strongly resulting in two states with large oscillator strengths. The emergence of two strongly absorbing states is in agreement with experimental observations. The DFT/MRCI computations reproduce experimental excitation energies very well for pentacene and hexacene (within 0.1 eV). For the larger acenes deviations are larger (up to 0.2 eV), but qualitative agreement is observed. PMID
NASA Astrophysics Data System (ADS)
Kristyan, Sandor
2013-11-01
Using orbital-free framework, a simple numerical optimization of the density functional for ground state electronic energy is described for any type of functional approximation, demonstrated via the example of linear combinations of homogeneous functionals of the density. The numerical recipe is given and analyzed for solution: Originating from the linear dependence of nuclear-electron attraction functional on one-electron density ( V ne[ ρ 0( r 1)] = -ΣA = 1,…,MZA∫ ρ 0( r 1)rA1 -1d r 1), and a quadratic LCAO approximation for ρ 0, the optimization can be done with iterative use of lin-solver. This quadratic approximation, as simplest educated choice for ρ 0, is compared and analyzed algebraically to the HF-SCF one in the Appendices. We call the attention that the introduction of a self-consistent field optimization of non-linear density functional is a new element in this part of the related, general theory.
Vargas, Jorge; Springborg, Michael; Kirtman, Bernard
2014-02-07
The response to an electrostatic field is determined through simple model calculations, within both the restricted Hartree-Fock and density functional theory methods, for long, finite as well as infinite, periodic chains. The permanent dipole moment, μ{sub 0}, the polarizability, α, and the hyperpolarizabilities β and γ, calculated using a finite-field approach, are extensively analyzed. Our simple model allows for treatment of large systems and for separation of the properties into atomic and unit-cell contributions. That part of the response properties attributable to the terminations of the finite system change into delocalized current contributions in the corresponding infinite periodic system. Special emphasis is placed on analyzing the reasons behind the dramatic overestimation of the response properties found with density functional theory methods presently in common use.
Unexpected properties of a density functional
Karwowski, J.; Stanke, M.
2005-02-01
An observation on a pathological behavior of an exact density functional derived from either relativistic (Dirac) or nonrelativistic (Levy-Leblond) quantum-mechanical equation is reported. As expected, in the case of a one-electron atom the variational minimum of this functional is equal to the exact ground-state energy. However, apart from the correct density, this minimum is reached also by an infinite set of densities which do not correspond to the exact wave function. This paradoxical property of the functional is related to the multicomponent structure of both Dirac and Levy-Leblond wave functions. In particular, imposing the correct boundary conditions upon the trial densities removes only a part of the fake solutions. The results of this study demonstrate that in density-functional theories derived from models based on multicomponent wave functions, one should not expect any simple relation between the accuracy of the energy and the correctness of the corresponding density.
Mouesca, Jean-Marie
2014-01-01
The goal of this "how to" chapter is to present in a way as simple and practical as possible some of the concepts, key issues, and practices behind the so-called broken symmetry (BS) state which is widely used within the density functional theory (DFT) (for a very nice but thoughtful introduction to DFT (without equations!), read Perdew et al. (J Chem Theory Comput 5:902-908, 2009)) community to compute energetic as well as spectroscopic properties pertaining to (poly-)radicals, bioinorganic clusters (especially those containing transition metal ions), etc. Such properties encompass exchange coupling constants J (molecular magnetism) but also (among other things) g-tensors and hyperfine coupling tensors A (from electron paramagnetic resonance), isomer shifts δ and quadrupolar tensors ΔE Q (from Mössbauer), etc.Hopefully, this chapter will appeal to those DFT practitioners who would like to understand the basics behind the BS state and help them "demystify" some of the issues involved with them. More technical issues will only be alluded to, and appropriate references will be given for those interested to go beyond this mere introduction. This chapter is however not a review of the field. Consequently, it will be primarily based on my own experience. The goal here (in the spirit of a "how to" chapter) is to accompany the readers' thoughts in a progressive way along increasingly complex issues rather than encumbering the same thoughts with too complicate mathematical details (the few derivations which are given will therefore be explicit). Moreover, I will emphasize in this chapter the interplay between the computation of BS states on the one hand, and the derivation of phenomenological models on the other hand, whose parameters can be supplied from appropriate BS states. Finally, this chapter is dedicated to Louis Noodleman (Scripps Research Institute, CA, USA), pioneer (Noodleman, J Chem Phys 74:5737-5743, 1981; Noodleman, Chem Phys 109:131-143, 1986) and
NASA Astrophysics Data System (ADS)
Zhang, Qian; Liang, Xiao; Chen, Bor-Yann; Chang, Chang-Tang
2015-12-01
This study tended to decipher the mechanism of photo degradation of azo dyes, which bond was favorable to be broken for application of wastewater decolorization. That is, from chemical structure perspective, the critical substituents to affect electron donor/acceptor for dye degradation would be identified in this research. The model reactive blacks (RB5), reactive blue 171 (RB171) and reactive red 198 (RR198) were degraded by graphene loaded TiO2, indicating how the electron withdrawing and releasing groups affect azo dye degradability. The byproducts and intermediate products were analyzed by ultraviolet-visible spectroscopy (UV-vis), gas chromatography-mass spectrometry (GC-MS) and ion chromatography (IC). Furthermore, the radicals involved in the reaction were found by electron paramagnetic resonance (ESR) to confirm the main oxidized species of hydroxyl radicals rather than the light generated positive holes. The finding revealed that the breakages of the bonds were due to the electron density changes around the bonds. This principle can be applicable not only for RB5 degradation, but also for reactive blue 171 (RB171), reactive red 198 (RR198) and some other textile dyes.
Density-dependent covariant energy density functionals
Lalazissis, G. A.
2012-10-20
Relativistic nuclear energy density functionals are applied to the description of a variety of nuclear structure phenomena at and away fromstability line. Isoscalar monopole, isovector dipole and isoscalar quadrupole giant resonances are calculated using fully self-consistent relativistic quasiparticle randomphase approximation, based on the relativistic Hartree-Bogoliubovmodel. The impact of pairing correlations on the fission barriers in heavy and superheavy nuclei is examined. The role of pion in constructing desnity functionals is also investigated.
Density functionals from deep learning
NASA Astrophysics Data System (ADS)
McMahon, Jeffrey
Density-functional theory is a formally exact description of a many-body quantum system in terms of its density; in practice, however, approximations to the universal density functional (DF) are necessary. Machine learning has recently been proposed as a novel approach to discover such a DF (or components of it). Conventional machine learning algorithms, however, are limited in their ability to process data in their raw form, leading to invariance and/or sensitivity issues. In this presentation, an alternative approach based on deep learning will be demonstrated. Deep learning allows computational models that are capable of discovering intricate structure in large and/or high-dimensional data sets with multiple levels of abstraction, and do not suffer from the aforementioned issues. Results from the application of this approach to the prediction of the kinetic-energy DF of noninteracting electrons will be presented. Using theoretical results from computer science, a connection between the underlying model and the theorems of Hohenberg and Kohn will also be suggested.
Mazziotti, David A.
2010-06-15
An improved parametrization of the two-electron reduced density matrix (2-RDM) [D. A. Mazziotti, Phys. Rev. Lett. 101, 253002 (2008)] was recently shown to yield energies and properties that are markedly better than those calculated by traditional ab initio methods of similar computational scaling. In this paper a family of such energy functionals, generalizing the ones obtained previously, is derived through the use of (i) p-particle contraction relations based on the contraction of the cumulant expansions of p-particle RDMs and (ii) Cauchy-Schwarz relations that arise from an important set of N-representability constraints known as the two-positivity conditions. The 2-RDMs are explicitly parameterized in terms of the first-order part of the cumulant 2-RDM and, for the inclusion of single excitations, a second-order part of the 1-RDM. In contrast to earlier formulations based on the coefficients from configuration interaction with single and double excitations (CISD), the cumulant-based parametric 2-RDM methods, from the properties of cumulants, are rigorously size extensive. We also show that writing the energy functionals in terms of correlated 1-RDMs and cumulant 2-RDMs reduces the computational cost of the parametric 2-RDM methods to that of CISD. Applications are made to ground-state energies of several molecules, equilibrium bond distances, and frequencies of HF, F{sub 2}, and CO, the relative energy of the cis and trans isomers of HO{sub 3}{sup -}, and the HCN-HNC isomerization reaction. For bond breaking in hydrogen fluoride the improved and more efficient parametric 2-RDM methods yield energies with similar accuracies at both equilibrium and nonequilibrium geometries in 6-31G** and polarized valence quadruple-{zeta} basis sets. Computed 2-RDMs very nearly satisfy well-known N-representability conditions.
Arora, Priya; Moudgil, R. K.; Bhukal, Nisha
2015-05-15
Static density-density correlation function has been calculated for a spin-polarized two-dimensional quantum electron fluid by including the first-order exchange and self-energy corrections to the random-phase approximation (RPA). This is achieved by determining these corrections to the RPA linear density-density response function, obtained by solving the equation of motion for the single-particle Green’s function. Resulting infinite hierarchy of equations (involving higher-order Green’s functions) is truncated by factorizing the two-particle Green’s function as a product of the single-particle Green’s function and one-particle distribution function. Numerical results of correlation function are compared directly against the quantum Monte Carlo simulation data due to Tanatar and Ceperley for different coupling parameter (r{sub s}) values. We find almost exact agreement for r{sub s}=1, with a noticeable improvement over the RPA. Its quality, however, deteriorates with increasing r{sub s}, but correction to RPA is quite significant.
NASA Astrophysics Data System (ADS)
Pamuk, Betül; Baima, Jacopo; Dovesi, Roberto; Calandra, Matteo; Mauri, Francesco
2016-07-01
We investigate the capability of density functional theory (DFT) to appropriately describe the spin susceptibility, χs, and the intervalley electron-phonon coupling in LixZrNCl . At low doping, LixZrNCl behaves as a two-dimensional two-valley electron gas, with parabolic bands. In such a system, χs increases with decreasing doping because of the electron-electron interaction. We show that DFT with local functionals (LDA/GGA) is not capable of reproducing this behavior. The use of exact exchange in Hartree-Fock (HF) or in DFT hybrid functionals enhances χs. HF, B3LYP, and PBE0 approaches overestimate χs, whereas the range-separated HSE06 functional leads to results similar to those obtained in the random phase approximation (RPA) applied to a two-valley two-spin electron gas. Within HF, LixZrNCl is even unstable towards a ferromagnetic state for x <0.16 . The intervalley phonons induce an imbalance in the valley occupation that can be viewed as the effect of a pseudomagnetic field. Thus, similarly to what happens for χs, the electron-phonon coupling of intervalley phonons is enhanced by the electron-electron interaction. Only hybrid DFT functionals capture such an enhancement and the HSE06 functional reproduces the RPA results presented in M. Calandra et al. [Phys. Rev. Lett. 114, 077001 (2015), 10.1103/PhysRevLett.114.077001]. These results imply that the description of the susceptibility and electron-phonon coupling with a range-separated hybrid functional would be important also in other two-dimensional weakly doped semiconductors, such as transition-metal dichalcogenides and graphene.
Rüger, Robert; van Lenthe, Erik; Heine, Thomas; Visscher, Lucas
2016-05-14
We propose a new method of calculating electronically excited states that combines a density functional theory based ground state calculation with a linear response treatment that employs approximations used in the time-dependent density functional based tight binding (TD-DFTB) approach. The new method termed time-dependent density functional theory TD-DFT+TB does not rely on the DFTB parametrization and is therefore applicable to systems involving all combinations of elements. We show that the new method yields UV/Vis absorption spectra that are in excellent agreement with computationally much more expensive TD-DFT calculations. Errors in vertical excitation energies are reduced by a factor of two compared to TD-DFTB. PMID:27179467
NASA Astrophysics Data System (ADS)
Rüger, Robert; van Lenthe, Erik; Heine, Thomas; Visscher, Lucas
2016-05-01
We propose a new method of calculating electronically excited states that combines a density functional theory based ground state calculation with a linear response treatment that employs approximations used in the time-dependent density functional based tight binding (TD-DFTB) approach. The new method termed time-dependent density functional theory TD-DFT+TB does not rely on the DFTB parametrization and is therefore applicable to systems involving all combinations of elements. We show that the new method yields UV/Vis absorption spectra that are in excellent agreement with computationally much more expensive TD-DFT calculations. Errors in vertical excitation energies are reduced by a factor of two compared to TD-DFTB.
Arcelus, Oier; Li, Chunmei; Rojo, Teófilo; Carrasco, Javier
2015-06-01
Clarifying the electronic structure of sodium superoxide (NaO2) is a key step in understanding the electrochemical behavior of Na-O2 batteries. Here we report a density functional theory study to explore the effect of atomic structure and morphology on the electronic properties of different model systems: NaO2 bulk, (100) surface, and small (NaO2)n clusters (n = 3-8). We found that a correct description of the open-shell 2p electrons of O2(-) requires the use of a hybrid functional, which reveals a clear insulating nature of all of the investigated systems. This sheds light onto the capacity limitations of pure NaO2 as a discharge product and highlights the need for developing new strategies to enhance its electron transport in the optimization of Na-O2 cells. PMID:26266497
NASA Astrophysics Data System (ADS)
Bhandary, Sumanta; Schüler, Malte; Thunström, Patrik; di Marco, Igor; Brena, Barbara; Eriksson, Olle; Wehling, Tim; Sanyal, Biplab
2016-04-01
A proper theoretical description of the electronic structure of the 3 d orbitals in the metal centers of functional metalorganics is a challenging problem. We apply density functional theory and an exact diagonalization method in a many-body approach to study the ground-state electronic configuration of an iron porphyrin (FeP) molecule. Our study reveals that the consideration of multiple Slater determinants is important, and FeP is a potential candidate for realizing a spin crossover due to a subtle balance of crystal-field effects, on-site Coulomb repulsion, and hybridization between the Fe-d orbitals and ligand N-p states. The mechanism of switching between two close-lying electronic configurations of Fe-d orbitals is shown. We discuss the generality of the suggested approach and the possibility to properly describe the electronic structure and related low-energy physics of the whole class of correlated metal-centered organometallic molecules.
NASA Astrophysics Data System (ADS)
Perron, H.; Mellier, T.; Domain, C.; Roques, J.; Simoni, E.; Drot, R.; Catalette, H.
2007-08-01
Periodic density functional theory (DFT) calculations using plane-wave basis sets were performed in order to study the bulk of nickel ferrite NiFe2O4. The local spin density approximation (LSDA) and the generalized gradient approximation (GGA) formalism were used, and it appeared that the LSDA failed to describe the magnetic structure of this compound. However, the GGA formalism gave reliable results in good agreement with experimental data for the lattice parameters, the electronic properties and the bulk modulus. In addition, the calculated density of states of the metallic species d block as well as their local magnetic moments were correlated to the crystal-field theory. Then, a charge deformation map was computed and, as expected from the electronegativity scale, the electron excess is localized around oxygen atoms along the bond axes. The formation energies of metallic vacancies are in good agreement with the inverse spinel structure experimentally observed.
NASA Astrophysics Data System (ADS)
Mallikarjun Sharada, Shaama; Stück, David; Sundstrom, Eric J.; Bell, Alexis T.; Head-Gordon, Martin
2015-07-01
Wavefunction stability analysis is commonly applied to converged self-consistent field (SCF) solutions to verify whether the electronic energy is a local minimum with respect to second-order variations in the orbitals. By iterative diagonalisation, the procedure calculates the lowest eigenvalue of the stability matrix or electronic Hessian. However, analytical expressions for the electronic Hessian are unavailable for most advanced post-Hartree-Fock (HF) wave function methods and even some Kohn-Sham (KS) density functionals. To address such cases, we formulate the Hessian-vector product within the iterative diagonalisation procedure as a finite difference of the electronic gradient with respect to orbital perturbations in the direction of the vector. As a model application, following the lowest eigenvalue of the orbital-optimised second-order Møller-Plesset perturbation theory (OOMP2) Hessian during H2 dissociation reveals the surprising stability of the spin-restricted solution at all separations, with a second independent unrestricted solution. We show that a single stable solution can be recovered by using the regularised OOMP2 method (δ-OOMP2), which contains a level shift. Internal and external stability analyses are also performed for SCF solutions of a recently developed range-separated hybrid density functional, ωB97X-V, for which the analytical Hessian is not yet available due to the complexity of its long-range non-local VV10 correlation functional.
NASA Astrophysics Data System (ADS)
Shabbir, Ahmed; Muhammad, Zafar; M, Shakil; M, A. Choudhary
2016-03-01
The structural, electronic, mechanical, and thermal properties of Pt, Pd, Rh, Ir, Os metals and their alloys PtPdX (X = Ir, Os and Rh) are studied systematically using ab initio density functional theory. The groundstate properties such as lattice constant and bulk modulus are calculated to find the equilibrium atomic position for stable alloys. The electronic band structure and density of states are calculated to study the electronic behavior of metals on making their alloys. The electronic properties substantiate the metallic behavior for all studied materials. The firstprinciples density functional perturbation theory as implemented in quasi-harmonic approximation is used for the calculations of thermal properties. We have calculated the thermal properties such as the Debye temperature, vibrational energy, entropy and constant-volume specific heat. The calculated properties are compared with the previously reported experimental and theoretical data for metals and are found to be in good agreement. Calculated results for alloys could not be compared because there is no data available in the literature with such alloy composition.
Filatov, Michael
2014-09-28
Electron transfer in the ground and excited states of a model donor–acceptor (D–A) system is investigated using the single-reference and multi-reference density functional theory (DFT) methods. To analyze the results of the calculations, a simple two-site multi-reference model was derived that predicts a stepwise electron transfer in the S{sub 0} state and a wave-like dependence of the S{sub 1} electron transfer on the external stimulus. The standard single-reference Kohn-Sham (KS) DFT approach and the time-dependent DFT (TDDFT) method failed to describe the correct dependence of the S{sub 0} and S{sub 1} electron transfer on the external electric field applied along the donor–acceptor system. The multi-reference DFT approach, the spin-restricted ensemble-referenced KS (REKS) method, was able to successfully reproduce the correct behavior of the S{sub 0} and S{sub 1} electron transfer on the applied field. The REKS method was benchmarked against experimentally measured gas phase charge transfer excitations in a series of organic donor–acceptor complexes and displayed its ability to describe this type of electronic transitions with a very high accuracy, mean absolute error of 0.05 eV with the use of the standard range separated density functionals. On the basis of the calculations undertaken in this work, it is suggested that the non-adiabatic coupling between the S{sub 0} and S{sub 1} states may interfere with the electron transfer in a weakly coupled donor–acceptor system. It is also suggested that the electronic excitation of a D{sup +}–A{sup −} system may play a dual role by assisting the further electron transfer at certain magnitudes of the applied electric field and causing the backward transfer at lower electric field strengths.
NASA Astrophysics Data System (ADS)
Li, Shi-Chang; Guo, Yong; Gao, Tao; Ao, Bing-Yun
2015-09-01
The structural, electronic, and optical properties of NpO2 and PuO2 have been investigated by means of the hybrid density functional theory (HDFT) using the full-potential linearized augmented plane (FP-LAPW) wave plus local orbitals (lo) method. The weight of exact Hartree-Fock (HF) exchange, α = 0.25, 0.35 and 0.40, are chosen for the hybrid density functional calculation. The obtained energy band gaps of NpO2 and PuO2 are 2.75 eV and 2.80 eV within α = 0.35 scheme, respectively, which are in excellent agreement with the recent experimental data. The calculated charge density and charge density differences in the (110) plane suggest the chemical bonds for the two actinide dioxides have main ionic character. Furthermore, the dielectric function and related optical parameters of the two compounds are firstly calculated using the HDFT method. In particular, the obtained refractive index n for PuO2 is consistent well with the experimental value in the wavelength range of 400 to 900 nm. We also predicted the effective number of electrons (neff) contributing in the inter-band transitions reach a saturation value above 32 eV per unit cell for NpO2 and PuO2.
Kahk, J M; Poll, C G; Oropeza, F E; Ablett, J M; Céolin, D; Rueff, J-P; Agrestini, S; Utsumi, Y; Tsuei, K D; Liao, Y F; Borgatti, F; Panaccione, G; Regoutz, A; Egdell, R G; Morgan, B J; Scanlon, D O; Payne, D J
2014-03-21
The electronic structure of IrO2 has been investigated using hard x-ray photoelectron spectroscopy and density-functional theory. Excellent agreement is observed between theory and experiment. We show that the electronic structure of IrO2 involves crystal field splitting of the iridium 5d orbitals in a distorted octahedral field. The behavior of IrO2 closely follows the theoretical predictions of Goodenough for conductive rutile-structured oxides [J. B. Goodenough, J. Solid State Chem. 3, 490 (1971). PMID:24702416
NASA Astrophysics Data System (ADS)
da Silva Filho, J. G.; Freire, V. N.; Caetano, E. W. S.; Ladeira, L. O.; Fulco, U. L.; Albuquerque, E. L.
2013-11-01
In this letter, we study the electronic structure and optical properties of the active medicinal component γ-aminobutyric acid (GABA) and its cocrystals with oxalic (OXA) and benzoic (BZA) acid by means of the density functional theory formalism. It is shown that the cocrystallization strongly weakens the zwitterionic character of the GABA molecule leading to striking differences among the electronic band structures and optical absorption spectra of the GABA crystal and GABA:OXA, GABA:BZA cocrystals, originating from distinct sets of hydrogen bonds. Calculated band widths and Δ-sol band gap estimates indicate that both GABA and GABA:OXA, GABA:BZA cocrystals are indirect gap insulators.
NASA Astrophysics Data System (ADS)
Reshak, A. H.; Kamarudin, H.; Alahmed, Z. A.; Auluck, S.; Chyský, Jan
2014-06-01
A comprehensive theoretical density functional investigation of the electronic crystal structure, chemical bonding, and the electron charge densities of 9-Methyl-3-Thiophen-2-YI-Thieno [3, 2-e] [1, 2, 4] Thriazolo [4,3-c] Pyrimidine-8-Carboxylic Acid Ethyl Ester (C15H12N4O2S2) is performed. The density of states at Fermi level equal to 5.50 (3.45) states/Ry cell, and the calculated bare electronic specific heat coefficient is found to be 0.95 (0.59) mJ/mole-K2 for the local density approximation (Engel-Vosko generalized gradient approximation). The electronic charge density space distribution contours in (1 0 0) and (1 1 0) planes were calculated. We find that there are two independent molecules (A and B) in the asymmetric unit exhibit intramolecular C-H…O, C-H…N interactions. This intramolecular interaction is different in molecules A and B, where A molecule show C-H…O interaction while B molecule exhibit C-H…N interaction. We should emphasis that there is π-π interaction between the pyrimidine rings of the two neighbors B molecules gives extra strengths and stabilizations to the superamolecular structure. The calculated distance between the two neighbors pyrimidine rings found to be 3.345 Å, in good agreement with the measured one (3.424(1) Å).
Reshak, A.H.; Khan, Saleem Ayaz
2013-11-15
Graphical abstract: - Highlights: • FPLAPW method is used for calculating the electronic and optical properties of CdGa{sub 2}X{sub 4}. • Electronic and optical properties were calculated using LDA, GGA, EVGGA and mBJ. • Band gap conformed that CdGa{sub 2}X{sub 4} are semiconductors fit for UV and visible light. • The ECD shows that change in the bond length and bond nature affect the band gap. • The dielectric tensor components and its derivatives show considerable anisotropy. - Abstract: A density functional theory (DFT) based on full potential linear augmented plane wave (FPLAPW) was used for calculating the electronic structure, charge density and optical properties of CdGa{sub 2}X{sub 4} (X = S, Se) compounds. Local density approximation (LDA), generalized gradient approximation (GGA), Engle Vasko generalized gradient approximation (EVGGA) and recently modified Becke–Johnson (mBJ) were applied to calculate the band structure, total and partial density of states. The investigation of band structures and density of states of CdGa{sub 2}X{sub 4} (X = S, Se) elucidate that mBJ potential show close agreement to the experimental results. The mBJ potential was selected for further explanation of optical properties of CdGa{sub 2}X{sub 4} (X = S, Se). The study of electronic charge density contours shows that change in the bond lengths and bond nature affect the band gap of the compounds. The two non-zero dielectric tensor components and its derivatives show considerable anisotropy between the perpendicular and parallel components. The present work provide accurate information about the combination (hybridization) of orbital, formation of bands and dispersion of non-zero tensor components of CdGa{sub 2}X{sub 4} (X = S, Se)
NASA Astrophysics Data System (ADS)
Areshkin, Denis A.; Nikolić, Branislav K.
2010-04-01
The recent fabrication of graphene nanoribbon (GNR) field-effect transistors poses a challenge for first-principles modeling of carbon nanoelectronics due to many thousand atoms present in the device. The state of the art quantum transport algorithms, based on the nonequilibrium Green function formalism combined with the density-functional theory (NEGF-DFT), were originally developed to calculate self-consistent electron density in equilibrium and at finite bias voltage (as a prerequisite to obtain conductance or current-voltage characteristics, respectively) for small molecules attached to metallic electrodes where only a few hundred atoms are typically simulated. Here we introduce combination of two numerically efficient algorithms which make it possible to extend the NEGF-DFT framework to device simulations involving large number of atoms. Our first algorithm offers an alternative to the usual evaluation of the equilibrium part of electron density via numerical contour integration of the retarded Green function in the upper complex half-plane. It is based on the replacement of the Fermi function f(E) with an analytic function f˜(E) coinciding with f(E) inside the integration range along the real axis, but decaying exponentially in the upper complex half-plane. Although f˜(E) has infinite number of poles, whose positions and residues are determined analytically, only a finite number of those poles have non-negligible residues. We also discuss how this algorithm can be extended to compute the nonequilibrium contribution to electron density, thereby evading cumbersome real-axis integration (within the bias voltage window) of NEGFs which is very difficult to converge for systems with large number of atoms while maintaining current conservation. Our second algorithm combines the recursive formulas with the geometrical partitioning of an arbitrary multiterminal device into nonuniform segments in order to reduce the computational complexity of the retarded Green
Garza, Alejandro J.; Jiménez-Hoyos, Carlos A.; Scuseria, Gustavo E.
2014-06-28
Several schemes to avoid the double counting of correlations in methods that merge multireference wavefunctions with density functional theory (DFT) are studied and here adapted to a combination of spin-projected Hartree-Fock (SUHF) and DFT. The advantages and limitations of the new method, denoted SUHF+f{sub c}DFT, are explored through calculations on benchmark sets in which the accounting of correlations is challenging for pure SUHF or DFT. It is shown that SUHF+f{sub c}DFT can greatly improve the description of certain molecular properties (e.g., singlet-triplet energy gaps) which are not improved by simple addition of DFT dynamical correlation to SUHF. However, SUHF+f{sub c}DFT is also shown to have difficulties dissociating certain types of bonds and describing highly charged ions with static correlation. Possible improvements to the current SUHF+f{sub c}DFT scheme are discussed in light of these results.
Garza, Alejandro J; Jiménez-Hoyos, Carlos A; Scuseria, Gustavo E
2014-06-28
Several schemes to avoid the double counting of correlations in methods that merge multireference wavefunctions with density functional theory (DFT) are studied and here adapted to a combination of spin-projected Hartree-Fock (SUHF) and DFT. The advantages and limitations of the new method, denoted SUHF+fcDFT, are explored through calculations on benchmark sets in which the accounting of correlations is challenging for pure SUHF or DFT. It is shown that SUHF+fcDFT can greatly improve the description of certain molecular properties (e.g., singlet-triplet energy gaps) which are not improved by simple addition of DFT dynamical correlation to SUHF. However, SUHF+fcDFT is also shown to have difficulties dissociating certain types of bonds and describing highly charged ions with static correlation. Possible improvements to the current SUHF+fcDFT scheme are discussed in light of these results. PMID:24985613
NASA Astrophysics Data System (ADS)
van Sickle, Karina; Culberson, Lori Marie; Holzmacher, Jeremy Levin; Cafiero, Mauricio
In an effort to evaluate and design fast, accurate density functional theory (DFT) methods for calculating electrostatic and dispersion interactions between proteins and ligands, we have set up a model system examining interactions between mono-substituted benzene and indole in seven different stable conformations. We first optimized the geometries of the monomers at the B3LYP/6-31G level, and then scanned the potential curves with MP2, HF, B3LYP, SVWN, and HCTH407 [all at the 6-311++G(d,p) level] to find the optimum separation. We used the approximate counterpoise method to calculate the basis set superposition error-corrected interaction energies at the optimum geometries. We then applied these methods to the interactions between aromatic active site residues of horseradish peroxidase C with indole-3-propionic acid at two different binding sites.
Magnetic fields and density functional theory
Salsbury Jr., Freddie
1999-02-01
A major focus of this dissertation is the development of functionals for the magnetic susceptibility and the chemical shielding within the context of magnetic field density functional theory (BDFT). These functionals depend on the electron density in the absence of the field, which is unlike any other treatment of these responses. There have been several advances made within this theory. The first of which is the development of local density functionals for chemical shieldings and magnetic susceptibilities. There are the first such functionals ever proposed. These parameters have been studied by constructing functionals for the current density and then using the Biot-Savart equations to obtain the responses. In order to examine the advantages and disadvantages of the local functionals, they were tested numerically on some small molecules.
Arantes, J. T.; Lima, M. P.; Fazzio, A.; Xiang, H.; Wei, S. H.; Dalpian, G. M.
2009-04-01
The structural and electronic properties of perylene diimide liquid crystal PPEEB are studied using ab initio methods based on the density functional theory (DFT). Using available experimental crystallographic data as a guide, we propose a detailed structural model for the packing of solid PPEEB. We find that due to the localized nature of the band edge wave function, theoretical approaches beyond the standard method, such as hybrid functional (PBE0), are required to correctly characterize the band structure of this material. Moreover, unlike previous assumptions, we observe the formation of hydrogen bonds between the side chains of different molecules, which leads to a dispersion of the energy levels. This result indicates that the side chains of the molecular crystal not only are responsible for its structural conformation but also can be used for tuning the electronic and optical properties of these materials.
Nonlocal kinetic-energy-density functionals
Garcia-Gonzalez, P.; Alvarellos, J.E.; Chacon, E. |
1996-04-01
In this paper we present nonlocal kinetic-energy functionals {ital T}[{ital n}] within the average density approximation (ADA) framework, which do not require any extra input when applied to any electron system and recover the exact kinetic energy and the linear response function of a homogeneous system. In contrast with previous ADA functionals, these present good behavior of the long-range tail of the exact weight function. The averaging procedure for the kinetic functional (averaging the Fermi momentum of the electron gas, instead of averaging the electron density) leads to a functional without numerical difficulties in the calculation of extended systems, and it gives excellent results when applied to atoms and jellium surfaces. {copyright} {ital 1996 The American Physical Society.}
NASA Astrophysics Data System (ADS)
Rajesh, Chinagandham; Majumder, Chiranjib
2007-06-01
The geometric and electronic structures of the Pbn+ clusters (n=2-15) have been investigated and compared with neutral clusters. The search for several low-lying isomers was carried out under the framework of the density functional theory formalism using the generalized gradient approximation for the exchange correlation energy. The wave functions were expanded using a plane wave basis set and the electron-ion interactions have been described by the projector augmented wave method. The ground state geometries of the singly positively charged Pbn+ clusters showed compact growth pattern as those observed for neutrals with small local distortions. Based on the total energy of the lowest energy isomers, a systematic analysis was carried out to obtain the physicochemical properties, viz., binding energy, second order difference in energy, and fragmentation behavior. It is found that n =4, 7, 10, and 13 clusters are more stable than their neighbors, reflecting good agreement with experimental observation. The chemical stability of these clusters was analyzed by evaluating their energy gap between the highest occupied and lowest unoccupied molecular orbitals and adiabatic ionization potentials. The results revealed that, although Pb13 showed higher stability from the total energy analysis, its energy gap and ionization potential do not follow the trend. Albeit of higher stability in terms of binding energy, the lower ionization potential of Pb13 is interesting which has been explained based on its electronic structure through the density of states and electron shell filling model of spherical clusters.
NASA Astrophysics Data System (ADS)
Takada, Yasutami
2006-03-01
It is a well-known fact that the electronic compressibility κ diverges in the 3D electron gas as the density parameter rs approaches 5.25. A recent investigation clarifies that this divergence is due to the excitonic effect in the electron-hole pair excitation, in particular, to its zero-energy excitation [1]. For rs>5.25, κ becomes negative, leading to the negative static dielectric function ɛ(q,0) for at least small q owing to the compressibility sum rule. Then we can expect that two positive test charges do not repel but attract to each other in such a system. Keeping this situation in mind, we have calculated the ion-pair distribution function g(R) in the expanded Rb liquid metal by using the Monte Carlo method and found interesting features in g(R) characteristic to the negative ɛ(q,0) [2]. Such features have been observed by the recent measurement of g(R) in the supercritical fluid Rb metal with continuously increasing rs from 5.25 [3]. This confirms the situation of κ<0 in the low-density 3D electron gas for the first time. [1] YT, J. Superconductivity 18, No.3 (2005). [2] H. Maebashi and YT, to be submitted. [3] K. Matsuda and K. Tamura, private communication.
NASA Astrophysics Data System (ADS)
Kawaguchi, Yoshizo; Sasaki, Fumio; Mochizuki, Hiroyuki; Ishitsuka, Tomoaki; Tomie, Toshihisa; Ootsuka, Teruhisa; Watanabe, Shuji; Shimoi, Yukihiro; Yamao, Takeshi; Hotta, Shu
2013-02-01
We have investigated electronic states in the valence electron bands for the thin films of three thiophene/phenylene co-oligomer (TPCO) compounds, 2,5-bis(4-biphenylyl)thiophene (BP1T), 1,4-bis(5-phenylthiophen-2-yl)benzene (AC5), and 1,4-bis{5-[4-(trifluoromethyl)phenyl]thiophen-2-yl}benzene (AC5-CF3), by using extreme-UV excited photoelectron spectroscopy (EUPS). By comparing both EUPS spectra and secondary electron spectra between AC5 and AC5-CF3, we confirm that CF3 substitution to AC5 deepens valence states by 2 eV, and increases the ionization energy by 3 eV. From the cut-off positions of secondary electron spectra, the work functions of AC5, AC5-CF3, and BP1T are evaluated to be 3.8 eV, 4.8 eV, and 4.0 eV, respectively. We calculate molecular orbital (MO) energy levels by the density functional theory and compare results of calculations with those of experiments. Densities of states obtained by broadening MO levels well explain the overall features of experimental EUPS spectra of three TPCOs.
NASA Astrophysics Data System (ADS)
Reshak, A. H.; Abu-Jafar, M. S.; Al-Douri, Y.
2016-06-01
The first principles study of the (001) two symmetric n-type interfaces between two insulating perovskites, the nonpolar SrTiO3 (STO), and the polar LaAlO3 (LAO) was performed. We have analyzed the formation of metallic interface states between the STO and LAO heterointerfaces by using the all-electron full-potential linearized augmented plane-wave approach based on the density functional theory, within the local density approximation, the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA), and the Engel-Vosko GGA (EVGGA) formalism. It has been found that some bands cross the Fermi energy level (EF), forming a metallic nature of two symmetric n-type 6.5STO/1.5LAO interfaces with density of states at EF, N(EF) of about 3.56 (state/eV/unit cell), and bare electronic specific heat coefficient (γ) of about 0.62 mJ/(mol cell K2). The electronic band stature and the partial density of states in the vicinity of EF are mainly originated from Ti1,2,3,4-3dxy orbitals. These bands are responsible for the metallic behavior and the forming of the Fermi surface of the two symmetric n-type 6.5STO/1.5LAO interfaces. To obtain a clear map of the valence band electronic charge density distribution of the two symmetric n-type 6.5STO/1.5LAO interfaces, we have investigated the bond's nature and the interactions between the atoms. It reveals that the charge is attracted towards O atoms as it is clear that the O atoms are surrounded by uniform blue spheres which indicate the maximum charge accumulation.
Dai, Xing; Gao, Yang; Xin, Minsi; Wang, Zhigang; Zhou, Ruhong
2014-12-28
As a representative lanthanide endohedral metallofullerene, Gd@C{sub 82} has attracted a widespread attention among theorists and experimentalists ever since its first synthesis. Through comprehensive comparisons and discussions, as well as references to the latest high precision experiments, we evaluated the performance of different computational methods. Our results showed that the appropriate choice of the exchange-correlation functionals is the decisive factor to accurately predict both geometric and electronic structures for Gd@C{sub 82}. The electronic structure of the ground state and energy gap between the septet ground state and the nonet low-lying state obtained from pure density functional methods, such as PBE and PW91, are in good agreement with current experiment. Unlike pure functionals, the popularly used hybrid functionals in previous studies, such as B3LYP, could infer the qualitative correct ground state only when small basis set for C atoms is employed. Furthermore, we also highlighted that other geometric structures of Gd@C{sub 82} with the Gd staying at different positions are either not stable or with higher energies. This work should provide some useful references for various theoretical methodologies in further density functional studies on Gd@C{sub 82} and its derivatives in the future.
Yokogawa, D
2016-09-01
Theoretical approach to design bright bio-imaging molecules is one of the most progressing ones. However, because of the system size and computational accuracy, the number of theoretical studies is limited to our knowledge. To overcome the difficulties, we developed a new method based on reference interaction site model self-consistent field explicitly including spatial electron density distribution and time-dependent density functional theory. We applied it to the calculation of indole and 5-cyanoindole at ground and excited states in gas and solution phases. The changes in the optimized geometries were clearly explained with resonance structures and the Stokes shift was correctly reproduced. PMID:27608983
Metastable atom probe for measuring electron beam density profiles
NASA Technical Reports Server (NTRS)
Lockhart, J. M.; Zorn, J. C.
1972-01-01
Metastable atom probe was developed for measuring current density in electron beam as function of two arbitrary coordinates, with spatial resolution better than 0.5 mm. Probe shows effects of space charge, magnetic fields, and other factors which influence electron current density, but operates with such low beam densities that introduced perturbation is very small.
Electronic structure and surface properties of PrMnO3 (001): A density functional theory study
NASA Astrophysics Data System (ADS)
Zhou, Yongjun; Lü, Zhe; Wei, Bo; Wang, Zhihong; Zhu, Xingbao
2015-01-01
In this paper, the surface properties of the (001) surface in PrMnO3 are investigated using the projector augmented plane wave (PAW) methods within the spin-polarization generalized gradient approximation (GGA+U), where U is on-site Coulomb interaction correction. The result of the total density of states shows a half-metallic ground state for PrMnO3 in GGA+U treatment. The optimized structure parameters of both cubic and orthorhombic bulk phases are obtained. The electronic properties of the cubic PrMnO3 (001) surface with PrO- and MnO2-terminations are discussed. Based on the results of the calculated surface energies, we predict the surface energies of three low-index surfaces follows the sequence of (001)<(111)<(110). The rumpling for the PrO-terminated surface is much larger than that of the MnO2-terminated surface. Both the PrO- and MnO2-terminated surfaces display a reduction of d12 interlayer distance and an expansion of d23. Bader effective charge of the ion is much smaller than its formal charge which is due to the partial Mn-O bond covalency. The oxygen-vacancy formation energy in bulk PrMnO3 is found to be smaller than other perovskites, e.g. LaMnO3 and SrTiO3. We find that the formation energy of oxygen-vacancy on MnO2-terminated surface is lower than the formation energy of oxygen-vacancy on PrO-terminated surface and Sr-doped PrMnO3 makes oxygen-vacancy formation more easily compared pure PrMnO3.
Density functional study of structural and electronic properties of maximum-spin n+1Aun-1Ag clusters
NASA Astrophysics Data System (ADS)
Jiang, Zhen-Yi; Hou, Yu-Qin; Lee, Kuo-Hsing; Chu, San-Yan
The structures and relative stabilities of high-spin n+1Aun-1Ag and nAun-1Ag+ (n = 2-8) clusters have been studied with density functional calculation. We predicted the existence of a number of previously unknown isomers. Our results revealed that all structures of high-spin neutral or cationic Aun-1Ag clusters can be understood as a substitution of an Au atom by an Ag atom in the high-spin neutral or cationic Aun clusters. The properties of mixed gold-silver clusters are strongly sized and structural dependence. The high-spin bimetallic clusters tend to be holding three-dimensional geometry rather than planar form represented in their low-spin situations. Silver atom prefers to occupy those peripheral positions until to n = 8 for high-spin clusters, which is different from its position occupied by light atom in the low-spin situations. Our theoretical calculations indicated that in various high-spin Aun-1Ag neutral and cationic species, 5Au3Ag, 3AuAg and 5Au4Ag+ hold high stability, which can be explained by valence bond theory.
Electronic Flux Density beyond the Born-Oppenheimer Approximation.
Schild, Axel; Agostini, Federica; Gross, E K U
2016-05-19
In the Born-Oppenheimer approximation, the electronic wave function is typically real-valued and hence the electronic flux density (current density) seems to vanish. This is unfortunate for chemistry, because it precludes the possibility to monitor the electronic motion associated with the nuclear motion during chemical rearrangements from a Born-Oppenheimer simulation of the process. We study an electronic flux density obtained from a correction to the electronic wave function. This correction is derived via nuclear velocity perturbation theory applied in the framework of the exact factorization of electrons and nuclei. To compute the correction, only the ground state potential energy surface and the electronic wave function are needed. For a model system, we demonstrate that this electronic flux density approximates the true one very well, for coherent tunneling dynamics as well as for over-the-barrier scattering, and already for mass ratios between electrons and nuclei that are much larger than the true mass ratios. PMID:26878256
]: a density functional theory investigation
NASA Astrophysics Data System (ADS)
Ulian, Gianfranco; Tosoni, Sergio; Valdrè, Giovanni
2014-09-01
In this work, we modelled the structure, the compressional behaviour and the physical properties of talc over a wide range of pressure using a quantum mechanical approach based on periodic boundary conditions. We adopted the density functional theory using the B3LYP-D* functional, which includes a correction for the dispersive forces and all-electron Gaussian-type orbitals basis sets. An atomic level description of the athermal pressure-induced structural modification of talc is provided. From the compression results, we obtained the athermal ( T = 0 K) bulk modulus ( K T0), its first derivative ( K') and the athermal volume at zero pressure ( V 0) by a third-order Birch-Murnaghan equation with parameters K T0 = 56.25 GPa, K' = 5.66 and V 0 = 450.34 Å3. The mechanical behaviour is highly anisotropic, as observed by the axial compressibility. The presented data are in very good agreement with recent experimental results obtained by single-crystal neutron and X-ray diffraction experiments.
Uzunova, Ellie L
2011-03-01
The trioxide clusters with stoichiometry MO3, and the structural isomers with side-on and end-on bonded oxygen atoms, are studied by DFT with the B1LYP functional. For the first half of the 3d elements row (Sc to Cr), pyramidal or distorted pyramidal structures dominate among the trioxide and oxoperoxide ground states, while the remaining elements form planar trioxides, oxoperoxides, oxosuperoxides, and ozonides. Low-lying trioxide clusters are formed by Ti, V, Cr, and Mn, among which the distorted pyramidal VO3 in the (2)A'' state, the pyramidal CrO3 in the (1)A1 state, and the planar MnO3 in the (2)A1' state are global minima. With the exception of the middle-row elements Mn, Fe, and Co, the magnetic moment of the ground-state clusters is formed with a major contribution from unpaired electrons located at the oxygen atoms. The stability of trioxides and oxoperoxides toward release of molecular oxygen is significantly higher for Sc, Ti, and V than for the remaining elements of the row. A trend of increasing the capability to dissociate one oxygen molecule is observed from Cr to Cu, with the exception of OFe(O2) being more reactive than OCo(O2). A gradual increase of reactivity from Ti to Cu is observed for the complete fragmentation reaction M + O + O2. PMID:21299242
NASA Astrophysics Data System (ADS)
Bouhemadou, A.; Al-Essa, S.; Allali, D.; Ghebouli, M. A.; Bin-Omran, S.
2013-06-01
Structural, electronic and optical properties of the ZnSc2S4 and CdSc2S4 cubic spinels have been investigated by means of the full-potential (linearized) augmented plane wave plus local orbitals based on density functional theory. The exchange-correlation potential is treated by the GGA-PBEsol [J.P. Perdew, A. Ruzsinszky, G.I. Csonka, O.A. Vydrov, G.E. Scuseria, L.A. Constantin, X. Zhou, K. Burke, Phys. Rev. Lett. 100 (2008) 136406] and the recently proposed modified Becke-Johnson potential approximation (mBJ) [F. Tran, P. Blaha, Phys. Rev. Lett. 102 (2009) 226401], which successfully corrects the band-gap problem found with GGA for a wide range of materials. The obtained structural parameters are in good agreement with the available experimental data. This gives support for the predict properties for ZnSc2S4 and CdSc2S4. The band structures reveal that both compounds are semiconductor with a direct gap. The obtained gap values show that mBJ is superior for estimating band gap energy. We have calculated the electron and hole effective masses in different directions. The density of states has been analyzed. Based on our electronic structure obtained using the mBJ method we have calculated various optical properties, including the complex dielectric function ɛ(ω), complex index of refraction n(ω), reflectivity coefficient R(ω), absorption coefficient α(ω) and electron energy-loss function L(ω) as functions of the photon energy. We find that the values of zero-frequency limit ɛ1(0) increase with decreasing the energy band gap in agreement with the Penn model. The origin of the peaks and structures in the optical spectra is determined in terms of the calculated energy band structures.
NASA Astrophysics Data System (ADS)
Yang, Yu; Liu, Haitao; Zhang, Ping
2016-05-01
The structural and electronic properties of small uranium oxide clusters UnOm (n=1-3, m=1-3n) are systematically studied within the screened hybrid density functional theory. It is found that the formation of U-O-U bondings and isolated U-O bonds are energetically more stable than U-U bondings. As a result, no uranium cores are observed. Through fragmentation studies, we find that the UnOm clusters with the m/n ratio between 2 and 2.5 are very stable, hinting that UO2+x hyperoxides are energetically stable. Electronically, we find that the O-2p states always distribute in the deep energy range, and the U-5f states always distribute at the two sides of the Fermi level. The U-6d states mainly hybridize with the U-5f states in U-rich clusters, while hybridizing with O-2p states in O-rich clusters. Our work is the first one on the screened hybrid density functional theory level studying the atomic and electronic properties of the actinide oxide clusters.
Yang, Yu; Liu, Haitao; Zhang, Ping
2016-05-14
The structural and electronic properties of small uranium oxide clusters UnOm (n=1-3, m=1-3n) are systematically studied within the screened hybrid density functional theory. It is found that the formation of U-O-U bondings and isolated U-O bonds are energetically more stable than U-U bondings. As a result, no uranium cores are observed. Through fragmentation studies, we find that the UnOm clusters with the m/n ratio between 2 and 2.5 are very stable, hinting that UO2+x hyperoxides are energetically stable. Electronically, we find that the O-2p states always distribute in the deep energy range, and the U-5f states always distribute at the two sides of the Fermi level. The U-6d states mainly hybridize with the U-5f states in U-rich clusters, while hybridizing with O-2p states in O-rich clusters. Our work is the first one on the screened hybrid density functional theory level studying the atomic and electronic properties of the actinide oxide clusters. PMID:27179481
NASA Astrophysics Data System (ADS)
Blake, Nick P.; Metiu, Horia
1999-04-01
Electrons that are confined to zeolite cavities are modeled using a simplified pseudopotential scheme to represent the interaction of the electrons with both the sodalite framework and the Na+ ions. By comparing theory with recent experimental studies of G centers in Na-doped NaBr-SOD it is demonstrated that restricted forms of density functional theory, where two electrons are forced to pair in the same Kohn-Sham orbital, fail to correctly predict the true nature of the singlet, (spin unpolarized), G center. Electron confinement leads to generalized gradient corrections to the exchange of 0.74 eV and self-interaction corrections (SIC) of 0.7 eV over calculations performed in the local spin density approximation (LSDA). Only the self-interaction corrected generalized gradient approximation and the unrestricted Hartree-Fock approximation are in accord with experiment for the relative stability of the triplet (spin polarized) state. The unrestricted Hartree-Fock method is used to show that G-center absorptions will be blueshifted with respect to absorptions due to the isolated F centers. Constructing a Hubbard Hamiltonian we show that the exchange coupling ranges in values from 2.3 meV(UHF) to 3.6 meV(SIC-LSDA) corresponding to Neel temperatures that range from 27 to 41 K in agreement with experiment.
Phenomenological Relativistic Energy Density Functionals
Lalazissis, G. A.; Kartzikos, S.; Niksic, T.; Paar, N.; Vretenar, D.; Ring, P.
2009-08-26
The framework of relativistic nuclear energy density functionals is applied to the description of a variety of nuclear structure phenomena, not only in spherical and deformed nuclei along the valley of beta-stability, but also in exotic systems with extreme isospin values and close to the particle drip-lines. Dynamical aspects of exotic nuclear structure is explored using the fully consistent quasiparticle random-phase approximation based on the relativistic Hartree-Bogoliubov model. Recent applications of energy density functionals with explicit density dependence of the meson-nucleon couplings are presented.
NASA Astrophysics Data System (ADS)
Li, Ke Jing; Shao, Qing Yi; Zhang, Juan; Yao, Xin Hua
2016-07-01
Using density functional theory (DFT), we have investigated the stability and electronic structure of lithium (Li) adsorbed in triplet form of (5.0) carbon nanotubes (CNTs) and (5.0) boron nitrogen nanotubes (BNNTs). We have mainly found that three (5.0) tubes are covalently connected. The triplet form is an energetically stable semiconductor. Li atom can be chemically adsorbed in the triplet form of nanotubes (NTs). Meanwhile, upon the adsorption of Li, the triplet form convert into metal. Hence, the triplet form can improve reactivity and sensitivity of NTs to Li significantly.
Verma, Prakash; Autschbach, Jochen
2013-02-12
Different approaches are compared for relativistic calculations of electronic g factors of molecules with light atoms, transition metal complexes, and selected complexes with actinides, using density functional theory (DFT) and Hartree-Fock (HF) theory. The comparison includes functionals with range-separated exchange. Within the variationally stable zeroth-order regular approximation (ZORA) relativistic framework, g factors are obtained with a linear response (LR) method where spin-orbit (SO) coupling is treated as a linear perturbation, a spin-polarized approach based on magnetic anisotropy (MA) that includes SO coupling variationally, and a quasi-restricted variational SO method previously devised by van Lenthe, van der Avoird, and Wormer (LWA). The MA and LWA approaches were implemented in the open-source NWChem quantum chemistry package. We address the importance of electron correlation (DFT vs HF), the importance of including spin polarization in the g tensor methodology, the question of whether the use of nonrelativistic spin density functionals is adequate for such calculations, and the importance of treating spin-orbit coupling beyond first-order. For selected systems, the extent of the DFT delocalization error is explicitly investigated via calculations of the energy as a function of fractional electron numbers. For a test set of small molecules with light main group atoms, all levels of calculation perform adequately as long as there is no energetic near-degeneracy among occupied and unoccupied orbitals. The interplay between different factors determining the accuracy of calculated g factors becomes more complex for systems with heavy elements such as third row transition metals and actinides. The MA approach is shown to perform acceptably well for a wide range of scenarios. PMID:26588748
Jonić, S; Vargas, J; Melero, R; Gómez-Blanco, J; Carazo, J M; Sorzano, C O S
2016-06-01
Cryo-electron microscopy (cryo-EM) of frozen-hydrated preparations of isolated macromolecular complexes is the method of choice to obtain the structure of complexes that cannot be easily studied by other experimental methods due to their flexibility or large size. An increasing number of macromolecular structures are currently being obtained at subnanometer resolution but the interpretation of structural details in such EM-derived maps is often difficult because of noise at these high-frequency signal components that reduces their contrast. In this paper, we show that the method for EM density-map approximation using Gaussian functions can be used for denoising of single-particle EM maps of high (typically subnanometer) resolution. We show its denoising performance using simulated and experimental EM density maps of several complexes. PMID:27085420
Imaginary time density-density correlations for two-dimensional electron gases at high density
Motta, M.; Galli, D. E.; Moroni, S.; Vitali, E.
2015-10-28
We evaluate imaginary time density-density correlation functions for two-dimensional homogeneous electron gases of up to 42 particles in the continuum using the phaseless auxiliary field quantum Monte Carlo method. We use periodic boundary conditions and up to 300 plane waves as basis set elements. We show that such methodology, once equipped with suitable numerical stabilization techniques necessary to deal with exponentials, products, and inversions of large matrices, gives access to the calculation of imaginary time correlation functions for medium-sized systems. We discuss the numerical stabilization techniques and the computational complexity of the methodology and we present the limitations related to the size of the systems on a quantitative basis. We perform the inverse Laplace transform of the obtained density-density correlation functions, assessing the ability of the phaseless auxiliary field quantum Monte Carlo method to evaluate dynamical properties of medium-sized homogeneous fermion systems.
Ju, J.; Döpp, A.; Cros, B.; Svensson, K.; Genoud, G.; Wojda, F.; Burza, M.; Persson, A.; Lundh, O.; Wahlström, C.-G.; Ferrari, H.
2013-08-15
Laser wakefield electron acceleration in the blow-out regime and the associated betatron X-ray radiation were investigated experimentally as a function of the plasma density in a configuration where the laser is guided. Dielectric capillary tubes were employed to assist the laser keeping self-focused over a long distance by collecting the laser energy around its central focal spot. With a 40 fs, 16 TW pulsed laser, electron bunches with tens of pC charge were measured to be accelerated to an energy up to 300 MeV, accompanied by X-ray emission with a peak brightness of the order of 10{sup 21} ph/s/mm{sup 2}/mrad{sup 2}/0.1%BW. Electron trapping and acceleration were studied using the emitted X-ray beam distribution to map the acceleration process; the number of betatron oscillations performed by the electrons was inferred from the correlation between measured X-ray fluence and beam charge. A study of the stability of electron and X-ray generation suggests that the fluctuation of X-ray emission can be reduced by stabilizing the beam charge. The experimental results are in good agreement with 3D particle-in-cell (PIC) simulation.
NASA Astrophysics Data System (ADS)
Nguyen, Chuong V.; Hieu, Nguyen N.; Nguyen, Duong T.
2015-11-01
Strain-dependent structural and electronic properties of MoS2 materials are investigated using first principles calculations. The structural and electronic band structures of the MoS2 with relaxed unit cells are optimized and calculated by the dispersion-corrected density functional theory (DFT-D2). Calculations within the local density approximation (LDA) and GGA using PAW potentials were also performed for specific cases for the purpose of comparison. The effect of strain on the band gap and the dependence of formation energy on strain of MoS2 are also studied and discussed using the DFT-D2 method. In bulk MoS2, the orbitals shift towards the higher/lower energy area when strain is applied along the z/ x direction, respectively. The energy splitting of Mo4 d states is in the range from 0 to 2 eV, which is due to the reduction of the electronic band gap of MoS2.
Demján, Tamás; Vörös, Márton; Palummo, Maurizia; Gali, Adam
2014-08-14
Diamondoids are small diamond nanoparticles (NPs) that are built up from diamond cages. Unlike usual semiconductor NPs, their atomic structure is exactly known, thus they are ideal test-beds for benchmarking quantum chemical calculations. Their usage in spintronics and bioimaging applications requires a detailed knowledge of their electronic structure and optical properties. In this paper, we apply density functional theory (DFT) based methods to understand the electronic and optical properties of a few selected pure and modified diamondoids for which accurate experimental data exist. In particular, we use many-body perturbation theory methods, in the G0W0 and G0W0+BSE approximations, and time-dependent DFT in the adiabatic local density approximation. We find large quasiparticle gap corrections that can exceed thrice the DFT gap. The electron-hole binding energy can be as large as 4 eV but it is considerably smaller than the GW corrections and thus G0W0+BSE optical gaps are about 50% larger than the Kohn-Sham (KS) DFT gaps. We find significant differences between KS time-dependent DFT and GW+BSE optical spectra on the selected diamondoids. The calculated G0W0 quasiparticle levels agree well with the corresponding experimental vertical ionization energies. We show that nuclei dynamics in the ionization process can be significant and its contribution may reach about 0.5 eV in the adiabatic ionization energies. PMID:25134572
NASA Astrophysics Data System (ADS)
Sousa, A. M.; Coutinho, W. S.; Lima, A. F.; Lalic, M. V.
2015-02-01
We have investigated the structural, bonding, and electronic properties of both ferroelectric (FE) and paraelectric (PE) phases of the hexagonal LuMnO3 compound using calculations based on density functional theory. The structural properties have been determined by employing the generalized gradient approximation with Perdew-Burke-Ernzerhof and Wu-Cohen parameterization. The bonding and electronic properties have been treated by recently developed modified Becke-Johnson exchange potential, which succeeded to open a band gap for both PE and FE phases, in agreement with experimental predictions. The Bader's topological analysis of electronic density showed that the character of the Lu-O axial bonds changes when the crystal exhibits the PE → FE structural transition. This fact is in agreement with experimental findings. The covalent character of the Lu-O bond significantly increases due to orbital hybridization between the Lu 5dz2 and O 2pz-states. This bonding mechanism causes the ferroelectricity in the hexagonal LuMnO3 compound.
Sousa, A. M.; Coutinho, W. S.; Lima, A. F.; Lalic, M. V.
2015-02-21
We have investigated the structural, bonding, and electronic properties of both ferroelectric (FE) and paraelectric (PE) phases of the hexagonal LuMnO{sub 3} compound using calculations based on density functional theory. The structural properties have been determined by employing the generalized gradient approximation with Perdew-Burke-Ernzerhof and Wu-Cohen parameterization. The bonding and electronic properties have been treated by recently developed modified Becke-Johnson exchange potential, which succeeded to open a band gap for both PE and FE phases, in agreement with experimental predictions. The Bader’s topological analysis of electronic density showed that the character of the Lu–O axial bonds changes when the crystal exhibits the PE → FE structural transition. This fact is in agreement with experimental findings. The covalent character of the Lu–O bond significantly increases due to orbital hybridization between the Lu 5d{sub z}{sup 2} and O 2p{sub z}-states. This bonding mechanism causes the ferroelectricity in the hexagonal LuMnO{sub 3} compound.
Electron (charge) density studies of cellulose models
Technology Transfer Automated Retrieval System (TEKTRAN)
Introductory material first describes electron density approaches and demonstrates visualization of electron lone pairs and bonding as concentrations of electron density. Then it focuses on the application of Bader’s Quantum Theory of Atoms-in-Molecules (AIM) to cellulose models. The purpose of the ...
Arghavani Nia, Borhan; Sedighi, Matin; Shahrokhi, Masoud; Moradian, Rostam
2013-11-15
A density functional theory study of structural, electronical and optical properties of Ca{sub 3}Sb{sub 2} compound in hexagonal and cubic phases is presented. In the exchange–correlation potential, generalized gradient approximation (PBE-GGA) has been used to calculate lattice parameters, bulk modulus, cohesive energy, dielectric function and energy loss spectra. The electronic band structure of this compound has been calculated using the above two approximations as well as another form of PBE-GGA, proposed by Engle and Vosko (EV-GGA). It is found that the hexagonal phase of Ca{sub 3}Sb{sub 2} has an indirect gap in the Γ→N direction; while in the cubic phase there is a direct-gap at the Γ point in the PBE-GGA and EV-GGA. Effects of applying pressure on the band structure of the system studied and optical properties of these systems were calculated. - Graphical abstract: A density functional theory study of structural, electronic and optical properties of Ca{sub 3}Sb{sub 2} compound in hexagonal and cubic phases is presented. Display Omitted - Highlights: • Physical properties of Ca{sub 3}Sb{sub 2} in hexagonal and cubic phases are investigated. • It is found that the hexagonal phase is an indirect gap semiconductor. • Ca{sub 3}Sb{sub 2} is a direct-gap semiconductor at the Γ point in the cubic phase. • By increasing pressure the semiconducting band gap and anti-symmetry gap are decreased.
NASA Astrophysics Data System (ADS)
Toyoda, Kenji; Hamada, Ikutaro; Lee, Kyuho; Yanagisawa, Susumu; Morikawa, Yoshitada
2010-04-01
In order to clarify factors determining the interface dipole, we have studied the electronic structures of pentacene adsorbed on Cu(111), Ag(111), and Au(111) by using first-principles density functional theoretical calculations. In the structural optimization, a semiempirical van der Waals (vdW) approach [S. Grimme, J. Comput. Chem. 27, 1787 (2006)] is employed to include long-range vdW interactions and is shown to reproduce pentacene-metal distances quite accurately. The pentacene-metal distances for Cu, Ag, and Au are evaluated to be 0.24, 0.29, and 0.32 nm, respectively, and work function changes calculated by using the theoretically optimized adsorption geometries are in good agreement with the experimental values, indicating the validity of the present approach in the prediction of the interface dipole at metal/organic interfaces. We examined systematically how the geometric factors, especially the pentacene-substrate distance ( Z C ) , and the electronic properties of the metal substrates contribute to the interface dipole. We found that at Z C ≥ 0.35 nm , the work function changes ( Δ ϕ 's) do not depend on the substrate work function ( ϕ m ) , indicating that the interface level alignment is nearly in the Schottky limit, whereas at Z C ≤ 0.25 nm , Δ ϕ 's vary nearly linearly with ϕ m , and the interface level alignment is in the Bardeen limit. Our results indicate the importance of both the geometric and the electronic factors in predicting the interface dipoles. The calculated electronic structure shows that on Au, the long-range vdW interaction dominates the pentacene-substrate interaction, whereas on Cu and Ag, the chemical hybridization contributes to the interaction.
NASA Astrophysics Data System (ADS)
Fan, S. W.; Song, T.; Huang, X. N.; Yang, L.; Ding, L. J.; Pan, L. Q.
2016-09-01
Utilizing the full potential linearized augment plane wave method, the electronic structures and magnetism for carbon doped CdSe are investigated. Calculations show carbon substituting selenium could induce CdSe to be a diluted magnetic semiconductor. Single carbon dopant could induce 2.00 μB magnetic moment. Electronic structures show the long-range ferromagnetic coupling mainly originates from the p-d exchange-like p-p coupling interaction. Positive chemical pair interactions indicate carbon dopants would form homogeneous distribution in CdSe host. The formation energy implies the non-equilibrium fabricated technology is necessary during the samples fabricated.
NASA Astrophysics Data System (ADS)
Odkhuu, Dorj; Seok Yun, Won; Cheol Hong, Soon
2012-03-01
To understand the negligible magnetostriction of Fe-Si alloys used as cores in electric transformers or motors, density-functional calculations were performed on the tetragonal magnetostriction coefficient, λ001, of Fe1-xSix (x ≤ 0.25) alloys as a function of x using the highly precise all-electron full-potential linearized augmented plane-wave method. It was found that the calculated magnetostriction coefficients closely reproduced experimental trends and the negligible magnetostriction near x = 0.111 originated from the mixed phases of A2, B2, and D03; each different atomic type of Fe of Fe0.889Si0.111 has a small and opposite contribution to the magnetostriction.
Ohl, A; Boer, S De
2014-06-01
Purpose: To investigate the differences in relative electron density for different energy (kVp) settings and the effect that these differences have on dose calculations. Methods: A Nuclear Associates 76-430 Mini CT QC Phantom with materials of known relative electron densities was imaged by one multi-slice (16) and one single-slice computed tomography (CT) scanner. The Hounsfield unit (HU) was recorded for each material with energies ranging from 80 to 140 kVp and a representative relative electron density (RED) curve was created. A 5 cm thick inhomogeneity was created in the treatment planning system (TPS) image at a depth of 5 cm. The inhomogeneity was assigned HU for various materials for each kVp calibration curve. The dose was then calculated with the analytical anisotropic algorithm (AAA) at points within and below the inhomogeneity and compared using the 80 kVp beam as a baseline. Results: The differences in RED values as a function of kVp showed the largest variations of 580 and 547 HU for the Aluminum and Bone materials; the smallest differences of 0.6 and 3.0 HU were observed for the air and lung inhomogeneities. The corresponding dose calculations for the different RED values assigned to the 5 cm thick slab revealed the largest differences inside the aluminum and bone inhomogeneities of 2.2 to 6.4% and 4.3 to 7.0% respectively. The dose differences beyond these two inhomogeneities were between 0.4 to 1.6% for aluminum and 1.9 to 2.2 % for bone. For materials with lower HU the calculated dose differences were less than 1.0%. Conclusion: For high CT number materials the dose differences in the phantom calculation as high as 7.0% are significant. This result may indicate that implementing energy specific RED curves can increase dose calculation accuracy.
NASA Astrophysics Data System (ADS)
Cruz, Salvador A.
An assessment of the use of statistical atomic models for the study of many-electron atom confinement is presented. The Thomas-Fermi-Dirac-[lambda]-Weizsäcker TFD[lambda]W functional formalism based on known properties of the orbital electron density is shown to be an appropriate tool for the description of the ground-state energy evolution of many-electron atoms spatially limited by closed and open boundaries. A brief review of the strategy followed in the TFD[lambda]W method for the study of atoms enclosed in hard and soft spherical cavities is presented along with more refined quantitative calculations as compared with previous results. Also, detailed quantitative results are shown-for the first time-in the case of confinement by a hard prolate spheroidal box for nuclear positions located at one of the foci and for an atom located at a distance D from a hard plane. A discussion is presented on the physical consequences of different confinement geometries and the adequacy of the TFD[lambda]W formalism to explore many-electron atom confinement by open and closed boundaries.
Momentum-space properties from coordinate-space electron density
Harbola, Manoj K.; Zope, Rajendra R.; Kshirsagar, Anjali; Pathak, Rajeev K.
2005-05-22
Electron density and electron momentum density, while independently tractable experimentally, bear no direct connection without going through the many-electron wave function. However, invoking a variant of the constrained-search formulation of density-functional theory, we develop a general scheme (valid for arbitrary external potentials) yielding decent momentum-space properties, starting exclusively from the coordinate-space electron density. A numerical illustration of the scheme is provided for the closed-shell atomic systems He, Be, and Ne in their ground state and for 1s{sup 1} 2s{sup 1} singlet electronic excited state for helium by calculating the Compton profiles and the
expectation values derived from given coordinate-space electron densities.
Rustad, James R.; Dixon, David A.; Rosso, Kevin M.; Felmy, Andrew R.
1999-04-07
Metal ion hydrolysis is fundamental in aqueous chemistry because of the influence of coordinating hydroxide ions on reaction rates; examples include enhanced labilization of coordinating water molecules in hydrolyzed complexes1 and stabilization of oxidized products in electron-transfer reactions involving hydrolyzed reductants.2 Moreover, the role of metal hydrolysis reactions in defining a baseline for establishing trends in metal ligand binding has motivated efforts toward comprehensive integration of Mz+ xOHy stability constants.3-5
Rustad, J.R.; Dixon, D.A.; Rosso, K.M.; Felmy, A.R.
1999-04-07
Metal ion hydrolysis is fundamental in aqueous chemistry because of the influence of coordinating hydroxide ions on reaction rates; examples include enhanced labilization of coordinating water molecules in hydrolyzed complexes and stabilization of oxidized products in electron-transfer reactions involving hydrolyzed reductants. Moreover, the role of metal hydrolysis reactions in defining a baseline for establishing trends in metal-ligand binding has motivated efforts toward comprehensive integration of M{sup z+}{sub x}OH{sub y} stability constants.
NASA Astrophysics Data System (ADS)
Kosobutsky, A. V.; Basalaev, Yu. M.
2014-12-01
Using first-principles theoretical techniques within density functional theory and many-body perturbation theory we investigated the structural and electronic properties of two LiInSe2 crystal modifications, orthorhombic (β-NaFeO2-type) and tetragonal (CuFeS2-type), focusing on the interband transitions and band gaps. It is found that the Tran-Blaha (TB09) functional predicts LiInSe2 to be a direct-gap semiconductor with a significantly larger band gap as compared with that from common local-density and gradient-corrected functionals. The most accurate values of the fundamental energy gaps are calculated within quasiparticle GW approximation and found to be 2.95 eV for the orthorhombic phase and 2.85 eV for the tetragonal one, with equal pressure coefficients of 63 meV/GPa. Our theoretical results eliminate the uncertainty in the band gap of LiInSe2. Moreover, the data obtained define the upper limit of the band gap of solid solutions (Cu,Li)InSe2 and (Ag,Li)InSe2, which can be of interest for applications in optoelectronics.
Zeng, Z. H.; Da Silva, J. L. F.; Deng, H. Q.; Li, W. X.
2009-01-01
In this work, we report a first-principles investigation of the energetics, structures, electronic properties, and core-level shifts of NO adsorption on the Pt(111) surface. Our calculations are based on density functional theory within the framework of the ultrasoft pseudopotential plane-wave and the all-electron projected augmented-wave methods. We found that at 0.25, 0.50, and 0.75 monolayer, NO adsorbs preferentially in the fcc, fcc+top, and fcc+top+hcp sites, respectively. The geometric parameters, adsorption energies, vibrational frequencies, and work-function changes are in good agreement with the experimental data. The interaction between NO and Pt(111) was found to follow a donation-back-donation process, in which the NO {sigma} states donate electrons to the substrate Pt d states, while the substrate Pt d states back donate to the NO {pi} states. Though there is an overall net charge transfer from the substrate to the NO adsorbate regardless of the adsorption sites and coverages, the spatial redistribution of the transferred electron is site dependent. The charge accumulation for NO in the top sites occurs closer to the surface than NO in the hollow sites, which results in the reduction of the Pt(111) surface work function for the top NO but an increase for the hollow NO. The core-level shifts of the topmost surface Pt atoms coordinated with top and hollow NO molecules at different coverages are in excellent agreement with experiments. In contrast, the N 1s core-level shifts between top and hollow NO ({approx}0.7 eV) deviated significantly from the zero shift found in experiments. Our analysis indicates that the difference may come from the thermal vibration and rotation of adsorbed NO on the Pt(111) surface.
Kuiken, Benjamin E. Van; Valiev, Marat; Daifuku, Stephanie L.; Bannan, Caitlin; Strader, Matthew L.; Cho, Hana; Huse, Nils; Schoenlein, Robert W.; Govind, Niranjan; Khalil, Munira
2013-04-26
Ruthenium L3-edge X-ray absorption (XA) spectroscopy probes unoccupied 4d orbitals of the metal atom and is increasingly being used to investigate the local electronic structure in ground and excited electronic states of Ru complexes. The simultaneous development of computational tools for simulating Ru L3-edge spectra is crucial for interpreting the spectral features at a molecular level. This study demonstrates that time-dependent density functional theory (TDDFT) is a viable and predictive tool for simulating ruthenium L3-edge XA spectroscopy. We systematically investigate the effects of exchange correlation functional and implicit and explicit solvent interactions on a series of RuII and RuIII complexes in their ground and electronic excited states. The TDDFT simulations reproduce all of the experimentally observed features in Ru L3-edge XA spectra within the experimental resolution (0.4 eV). Our simulations identify ligand-specific charge transfer features in complicated Ru L3-edge spectra of [Ru(CN)6]4- and RuII polypyridyl complexes illustrating the advantage of using TDDFT in complex systems. We conclude that the B3LYP functional most accurately predicts the transition energies of charge transfer features in these systems. We use our TDDFT approach to simulate experimental Ru L3-edge XA spectra of transition metal mixed-valence dimers of the form [(NC)5MII-CN-RuIII(NH3)5] (where M = Fe or Ru) dissolved in water. Our study determines the spectral signatures of electron delocalization in Ru L3-edge XA spectra. We find that the inclusion of explicit solvent molecules is necessary for reproducing the spectral features and the experimentally determined valencies in these mixed-valence complexes. This study validates the use of TDDFT for simulating Ru 2p excitations using popular quantum chemistry codes and providing a powerful interpretive tool for equilibrium and ultrafast Ru L3-edge XA spectroscopy.
Periodic subsystem density-functional theory
Genova, Alessandro; Pavanello, Michele; Ceresoli, Davide
2014-11-07
By partitioning the electron density into subsystem contributions, the Frozen Density Embedding (FDE) formulation of subsystem Density Functional Theory (DFT) has recently emerged as a powerful tool for reducing the computational scaling of Kohn–Sham DFT. To date, however, FDE has been employed to molecular systems only. Periodic systems, such as metals, semiconductors, and other crystalline solids have been outside the applicability of FDE, mostly because of the lack of a periodic FDE implementation. To fill this gap, in this work we aim at extending FDE to treat subsystems of molecular and periodic character. This goal is achieved by a dual approach. On one side, the development of a theoretical framework for periodic subsystem DFT. On the other, the realization of the method into a parallel computer code. We find that periodic FDE is capable of reproducing total electron densities and (to a lesser extent) also interaction energies of molecular systems weakly interacting with metallic surfaces. In the pilot calculations considered, we find that FDE fails in those cases where there is appreciable density overlap between the subsystems. Conversely, we find FDE to be in semiquantitative agreement with Kohn–Sham DFT when the inter-subsystem density overlap is low. We also conclude that to make FDE a suitable method for describing molecular adsorption at surfaces, kinetic energy density functionals that go beyond the GGA level must be employed.
Connection formulas for thermal density functional theory
NASA Astrophysics Data System (ADS)
Pribram-Jones, A.; Burke, K.
2016-05-01
The adiabatic connection formula of ground-state density functional theory relates the correlation energy to a coupling-constant integral over a purely potential contribution, and is widely used to understand and improve approximations. The corresponding formula for thermal density functional theory is cast as an integral over temperatures instead, ranging upward from the system's physical temperature. We also show how to relate different correlation components to each other, either in terms of temperature or coupling-constant integrations. We illustrate our results on the uniform electron gas.
Yuan, H. K.; Kuang, A. L.; Tian, C. L.; Chen, H.
2014-03-15
The structural evolutions and electronic properties of bimetallic Au{sub n–x}Pt{sub x} (n = 2–14; x ⩽ n) clusters are investigated by using the density functional theory (DFT) with the generalized gradient approximation (GGA). The monatomic doping Au{sub n–1}Pt clusters are emphasized and compared with the corresponding pristine Au{sub n} clusters. The results reveal that the planar configurations are favored for both Au{sub n–1}Pt and Au{sub n} clusters with size up to n = 13, and the former often employ the substitution patterns based on the structures of the latter. The most stable clusters are Au{sub 6} and Au{sub 6}Pt, which adopt regular planar triangle (D{sub 3h}) and hexagon-ring (D{sub 6h}) structures and can be regarded as the preferential building units in designing large clusters. For Pt-rich bimetallic clusters, their structures can be obtained from the substitution of Pt atoms by Au atoms from the Pt{sub n} structures, where Pt atoms assemble together and occupy the center yet Au atoms prefer the apex positions showing a segregation effect. With respect to pristine Au clusters, Au{sub n}Pt clusters exhibit somewhat weaker and less pronounced odd-even oscillations in the highest occupied and lowest unoccupied molecular-orbital gaps (HOMO-LUMO gap), electron affinity (EA), and ionization potential (IP) due to the partially released electron pairing effect. The analyses of electronic structure indicate that Pt atoms in AuPt clusters would delocalize their one 6s and one 5d electrons to contribute the electronic shell closure. The sp-d hybridizations as well as the d-d interactions between the host Au and dopant Pt atoms result in the enhanced stabilities of AuPt clusters.
NASA Astrophysics Data System (ADS)
Yang, Rong; Tang, Bin; Gao, Tao; Ao, BingYun
2016-05-01
We perform first principles calculations to investigate the structural, magnetic, electronic and optical properties of PuC and PuC0.75. Furthermore, we examine the influence of carbon non-stoichiometry on plutonium monocarbide. For the treatment of strongly correlated electrons, the hybrid density functionals like PBE0, Fock-0.25 are used and we compare the results with the generalized gradient approximation (GGA), local density approximation (LDA), LDA + U and experimental ones. The optimized lattice constant a0 = 4.961 Å for PuC in the Fock-0.25 scheme is the most close to the experimental data. The ground states of PuC and PuC0.75 are found to be anti-ferromagnetic. Our results indicate that additional removal of a C atom make lattice contract and new DOS peak appear in the near-Fermi region. We also compute and compare the optical properties of PuC and PuC0.75. The difference in optical properties between PuC and PuC0.75 should also be the influence of carbon vacancies.
Semiclassical origins of density functionals
NASA Astrophysics Data System (ADS)
Burke, Kieron
By careful numerical analysis of non-relativistic atomic correlation energies, we show that (a) the local density approximation becomes relatively exact for the correlation energy as the atomic number approaches infinity, (b) we find the leading correction, which is about 38.5 milliHartrees per atom, (c) show how this correction dominates for larger atoms and (d) how to construct a generalized gradient approximation that respects this limit (See KB, A. Cancio, T. Gould, S. Pittalis, arXiv:1409.4834). The relevance to density functional calculations will also be explained. Support provided by NSF CHE-1464795.
Koopmans' condition for density-functional theory
Dabo, Ismaila; Ferretti, Andrea; Poilvert, Nicolas; Marzari, Nicola; Li, Yanli; Cococcioni, Matteo
2010-09-15
In approximate Kohn-Sham density-functional theory, self-interaction manifests itself as the dependence of the energy of an orbital on its fractional occupation. This unphysical behavior translates into qualitative and quantitative errors that pervade many fundamental aspects of density-functional predictions. Here, we first examine self-interaction in terms of the discrepancy between total and partial electron removal energies, and then highlight the importance of imposing the generalized Koopmans' condition - that identifies orbital energies as opposite total electron removal energies - to resolve this discrepancy. In the process, we derive a correction to approximate functionals that, in the frozen-orbital approximation, eliminates the unphysical occupation dependence of orbital energies up to the third order in the single-particle densities. This non-Koopmans correction brings physical meaning to single-particle energies; when applied to common local or semilocal density functionals it provides results that are in excellent agreement with experimental data - with an accuracy comparable to that of GW many-body perturbation theory - while providing an explicit total energy functional that preserves or improves on the description of established structural properties.
Mitin, Alexander V; van Wüllen, Christoph
2006-02-14
A two-component quasirelativistic Hamiltonian based on spin-dependent effective core potentials is used to calculate ionization energies and electron affinities of the heavy halogen atom bromine through the superheavy element 117 (eka-astatine) as well as spectroscopic constants of the homonuclear dimers of these atoms. We describe a two-component Hartree-Fock and density-functional program that treats spin-orbit coupling self-consistently within the orbital optimization procedure. A comparison with results from high-order Douglas-Kroll calculations--for the superheavy systems also with zeroth-order regular approximation and four-component Dirac results--demonstrates the validity of the pseudopotential approximation. The density-functional (but not the Hartree-Fock) results show very satisfactory agreement with theoretical coupled cluster as well as experimental data where available, such that the theoretical results can serve as an estimate for the hitherto unknown properties of astatine, element 117, and their dimers. PMID:16483205
Shao, Jianping; Chen, Jingwen; Xie, Qing; Wang, Ying; Li, Xuehua; Hao, Ce
2010-07-15
Photochemical behaviour of polycyclic aromatic hydrocarbons (PAHs) is strongly dependent on the physical and chemical nature of the media in/on which they exist. To understand the media effects, the photolysis of phenanthrene (PHE) and benzo[a]pyrene (BaP) in several solvents was investigated. Distinct photolysis rate constants for PHE and BaP in the different solvents were observed. Some theoretical parameters reflecting the solvent properties were computed and employed to explain the solvent effects. Acetone competitively absorbed light with PHE and BaP, and the excited acetone molecules played different roles for the photodegradation of PHE and BaP. The photolysis rate constants of PHE and BaP in hexane, isopropanol, ethanol, methanol, acetonitrile and dichloromethane were observed to correlate with the electron-accepting potential of the solvent molecules. Absolute electronegativity of the solvents linearly correlated with the photolytic activity (log k) of the PAHs significantly. The results are important for better understanding the photodegradation mechanism of PAHs in different media. PMID:20303660
Zobač, Vladimír; Lewis, James P; Abad, Enrique; Mendieta-Moreno, Jesús I; Hapala, Prokop; Jelínek, Pavel; Ortega, José
2015-05-01
The computational simulation of photo-induced processes in large molecular systems is a very challenging problem. Firstly, to properly simulate photo-induced reactions the potential energy surfaces corresponding to excited states must be appropriately accessed; secondly, understanding the mechanisms of these processes requires the exploration of complex configurational spaces and the localization of conical intersections; finally, photo-induced reactions are probability events, that require the simulation of hundreds of trajectories to obtain the statistical information for the analysis of the reaction profiles. Here, we present a detailed description of our implementation of a molecular dynamics with electronic transitions algorithm within the local-orbital density functional theory code FIREBALL, suitable for the computational study of these problems. As an example of the application of this approach, we also report results on the [2 + 2] cycloaddition of ethylene with maleic anhydride and on the [2 + 2] photo-induced polymerization reaction of two C60 molecules. We identify different deactivation channels of the initial electron excitation, depending on the time of the electronic transition from LUMO to HOMO, and the character of the HOMO after the transition. PMID:25791682
NASA Astrophysics Data System (ADS)
Zobač, Vladimír; Lewis, James P.; Abad, Enrique; Mendieta-Moreno, Jesús I.; Hapala, Prokop; Jelínek, Pavel; Ortega, José
2015-05-01
The computational simulation of photo-induced processes in large molecular systems is a very challenging problem. Firstly, to properly simulate photo-induced reactions the potential energy surfaces corresponding to excited states must be appropriately accessed; secondly, understanding the mechanisms of these processes requires the exploration of complex configurational spaces and the localization of conical intersections; finally, photo-induced reactions are probability events, that require the simulation of hundreds of trajectories to obtain the statistical information for the analysis of the reaction profiles. Here, we present a detailed description of our implementation of a molecular dynamics with electronic transitions algorithm within the local-orbital density functional theory code FIREBALL, suitable for the computational study of these problems. As an example of the application of this approach, we also report results on the [2 + 2] cycloaddition of ethylene with maleic anhydride and on the [2 + 2] photo-induced polymerization reaction of two C60 molecules. We identify different deactivation channels of the initial electron excitation, depending on the time of the electronic transition from LUMO to HOMO, and the character of the HOMO after the transition.
Yamaoka, Hitoshi; Schwier, Eike F.; Arita, Masashi; Shimada, Kenya; Tsujii, Naohito; Jarrige, Ignace; Jiang, Jian; Hayashi, Hirokazu; Iwasawa, Hideaki; Namatame, Hirofumi; et al
2015-03-30
The electronic structure of Ce₃Pd₂₀X₆ (X = Si, Ge) has been studied using detailed density functional theory (DFT) calculations and high-resolution photoelectron spectroscopy (PES) measurements. The orbital decomposition of the electronic structure by DFT calculations indicates that Ce atoms at the (8c) site surrounded by 16 Pd atoms have a more localized nature and a tendency to be magnetic. Ce atoms in the (4a) site surrounded by 12 Pd and 6 X atoms, on the other, show only a negligible magnetic moment. In the photoemission valence-band spectra we observe a strong f⁰ (Ce⁴⁺) component with a small fraction of f¹more » (Ce³⁺) component. The spectral weight of f¹ component near the Fermi level Ce₃Pd₂₀Si₆ is stronger than that for Ce₃Pd₂₀Ge₆ at the 4d-4f resonance, suggesting stronger c-f hybridization in the former. This may hint to the origin of the large electronic specific coefficient of Ce₃Pd₂₀Si₆ compared to Ce₃Pd₂₀Ge₆.« less
Mizutani, U; Inukai, M; Sato, H; Zijlstra, E S; Lin, Q
2014-05-16
There are three key electronic parameters in elucidating the physics behind the Hume–Rothery electron concentration rule: the square of the Fermi diameter (2kF)2, the square of the critical reciprocal lattice vector and the electron concentration parameter or the number of itinerant electrons per atom e/a. We have reliably determined these three parameters for 10 Rhombic Triacontahedron-type 2/1–2/1–2/1 (N = 680) and 1/1–1/1–1/1 (N = 160–162) approximants by making full use of the full-potential linearized augmented plane wave-Fourier band calculations based on all-electron density-functional theory. We revealed that the 2/1–2/1–2/1 approximants Al13Mg27Zn45 and Na27Au27Ga31 belong to two different sub-groups classified in terms of equal to 126 and 109 and could explain why they take different e/a values of 2.13 and 1.76, respectively. Among eight 1/1–1/1–1/1 approximants Al3Mg4Zn3, Al9Mg8Ag3, Al21Li13Cu6, Ga21Li13Cu6, Na26Au24Ga30, Na26Au37Ge18, Na26Au37Sn18 and Na26Cd40Pb6, the first two, the second two and the last four compounds were classified into three sub-groups with = 50, 46 and 42; and were claimed to obey the e/a = 2.30, 2.10–2.15 and 1.70–1.80 rules, respectively.
Electron density studies of methyl cellobioside
Technology Transfer Automated Retrieval System (TEKTRAN)
Experimental X-ray diffraction crystallography determines the variations in electron density that result from the periodic array of atoms in a crystal. Normally, the positions and type of atom are determined from the electron density based on an approximation that the atoms are spherical. However, t...
Park, Ji-Sang; Kang, Joongoo; Yang, Ji-Hui; Metzger, Wyatt; Wei, Su-Huai
2015-01-15
Using first-principles density functional calculations, we investigate the relative stability and electronic structure of the grain boundaries (GBs) in zinc-blende CdTe. Among the low-Σ-value symmetric tilt Σ3 (111), Σ3 (112), Σ5 (120), and Σ5 (130) GBs, we show that the Σ3 (111)GB is always the most stable due to the absence of dangling bonds and wrong bonds. The Σ5 (120) GBs, however, are shown to be more stable than the Σ3 (112) GBs, even though the former has a higher Σ value, and the latter is often used as a model system to study GB effects in zinc-blende semiconductors. Furthermore, we find that although containing wrong bonds, the Σ5 (120) GBs are electrically benign due to the short wrong bond lengths, and thus are not as harmful as the Σ3 (112) GBs also having wrong bonds but with longer bond lengths.
NASA Astrophysics Data System (ADS)
Zheng, T.; Stacchiola, D.; Saldin, D. K.; James, J.; Sholl, D. S.; Tysoe, W. T.
2005-01-01
The structure of formate species adsorbed on Pd(1 1 1) has been determined using low-energy electron diffraction (LEED). The presence of formate species on the Pd(1 1 1) surface was established using reflection-absorption infrared spectroscopy (RAIRS). The oxygen atoms of the formate species were found to be adsorbed over surface palladium atoms with the OCO plane perpendicular to the surface. The CO bond length was found to be 1.26 ± 0.05 Å, the palladium-oxygen distance was 2.16 ± 0.06 Å, and the OCO angle 130 ± 5°. The experimentally determined values were in excellent agreement with those calculated using density functional theory (DFT).
NASA Astrophysics Data System (ADS)
Adam, N.; Suprayoga, E.; Adiperdana, B.; Guo, H.; Tanida, H.; Mohd-Tajudin, S. S.; Kobayashi, R.; Sera, M.; Nishioka, T.; Matsumura, M.; Sulaiman, S.; Mohamed-Ibrahim, M. I.; Watanabe, I.
2014-12-01
Numerical investigations on muon sites in Ce-based Kondo semiconductors, Ce(Ru,Rh)2Al10 were carried out by using the Density Functional Theory. From the view point of simple electrostatic potential calculations, we found all the previously reported muon sites, suggested by different groups (Kambe S et al. 2010 J. Phys. Soc. Jpn. 79 053708 and Khalyavin D D et al., 2010 Phys. Rev. B 82 100405(R)), can be possibly chosen as muon stopping sites. We also investigated the changes in the potential of the Rh-doped case. We discovered that the electronic potential around the nearest Ru atom to the substituted Rh atom is affected and the potential becomes asymmetric around the nearest Ru ion. Although big changes in hyperfine fields at muon sites have been reported (Guo H et al. 2013 Phys. Rev. B 88 115206), the muon positions estimated from the potential calculations do not change much.
NASA Astrophysics Data System (ADS)
Boettger, J. C.
2000-09-01
A simple approximation is developed for the two-electron spin-orbit coupling terms generated by the Douglas-Kroll-Hess transformation, in the context of density-functional theory (DFT). For the special case of an isolated atom, the two-electron spin-orbit matrix element for each pair of basis functions of type l is replaced with the spin-orbit matrix element for a point charge -Q(l) placed at the origin; where Q(l)=0,2,10,28,... . Application of this screened-nuclear-spin-orbit (SNSO) approximation to linear combination of Gaussian-type orbital (LCGTO) DFT calculations on Ce, Ta, and Pu atoms yields spin-orbit splittings that agree with results from a numerical solution of the Dirac-Kohn-Sham equations to within about 6%. This is a marked improvement over the nuclear-only spin-orbit approximation, which systematically overestimates spin-orbit splittings; in some cases by as much as 100%. Crystalline LCGTO DFT calculations on the fcc phases of the light-actinide metals Th-->Pu, using a multiatom generalization of the SNSO approximation, yield atomic volumes that are in excellent agreement with results from full-potential linear-augmented-plane-wave calculations.
Demján, Tamás; Vörös, Márton; Palummo, Maurizia; Gali, Adam
2014-08-14
Diamondoids are small diamond nanoparticles (NPs) that are built up from diamond cages. Unlike usual semiconductor NPs, their atomic structure is exactly known, thus they are ideal test-beds for benchmarking quantum chemical calculations. Their usage in spintronics and bioimaging applications requires a detailed knowledge of their electronic structure and optical properties. In this paper, we apply density functional theory (DFT) based methods to understand the electronic and optical properties of a few selected pure and modified diamondoids for which accurate experimental data exist. In particular, we use many-body perturbation theory methods, in the G{sub 0}W{sub 0} and G{sub 0}W{sub 0}+BSE approximations, and time-dependent DFT in the adiabatic local density approximation. We find large quasiparticle gap corrections that can exceed thrice the DFT gap. The electron-hole binding energy can be as large as 4 eV but it is considerably smaller than the GW corrections and thus G{sub 0}W{sub 0}+BSE optical gaps are about 50% larger than the Kohn-Sham (KS) DFT gaps. We find significant differences between KS time-dependent DFT and GW+BSE optical spectra on the selected diamondoids. The calculated G{sub 0}W{sub 0} quasiparticle levels agree well with the corresponding experimental vertical ionization energies. We show that nuclei dynamics in the ionization process can be significant and its contribution may reach about 0.5 eV in the adiabatic ionization energies.
Generalized local-spin-density-functional theory
NASA Astrophysics Data System (ADS)
Guo, Yufei; Whitehead, M. A.
1991-01-01
An alternative density-functional theory, the generalized local-spin-density-functional (G-LSD) theory, is proposed based on the boundary conditions and sum rule of the Fermi-correlation factor in the Hartree-Fock (HF) limit. It avoids the physical restrictions to the boundary conditions and the sum rule used in the generalized exchange local-spin-density-functional (GX-LSD) theory completely, the homogeneous electron-density approximation in the Hartree-Fock-Slater (HFS) theory and in the Gáspár-Kohn-Sham (GKS) theory partly, and the time-consuming step to search for the optimal exchange parameter for each atom or ion in the Xα and Ξa theories. The alternative G-LSD theory generates the GX-LSD, HFS, GKS, Xα, and Ξa theories, when additional approximations or certain Fermi-hole shapes or high electron-density limit in a system are used. Theoretically, the G-LSD theory is more rigorous than the GX-LSD, HFS, GKS, and Ξa theories. Numerically, the statistical total energies for atoms in the G-LSD theory are in excellent agreement with the HF results, when the Gopinathan, Whitehead, and Bogdanovic [Phys. Rev. A 14, 1 (1976)] Fermi-hole parameters are used.
The problem of the universal density functional and the density matrix functional theory
Bobrov, V. B. Trigger, S. A.
2013-04-15
The analysis in this paper shows that the Hohenberg-Kohn theorem is the constellation of two statements: (i) the mathematically rigorous Hohenberg-Kohn lemma, which demonstrates that the same ground-state density cannot correspond to two different potentials of an external field, and (ii) the hypothesis of the existence of the universal density functional. Based on the obtained explicit expression for the nonrel-ativistic particle energy in a local external field, we prove that the energy of the system of more than two non-interacting electrons cannot be a functional of the inhomogeneous density. This result is generalized to the system of interacting electrons. It means that the Hohenberg-Kohn lemma cannot provide justification of the universal density functional for fermions. At the same time, statements of the density functional theory remain valid when considering any number of noninteracting ground-state bosons due to the Bose condensation effect. In the framework of the density matrix functional theory, the hypothesis of the existence of the universal density matrix functional corresponds to the cases of noninteracting particles and to interaction in the Hartree-Fock approximation.
Density Functionals with Broad Applicability in Chemistry
Zhao, Yan; Truhlar, Donald G.
2008-02-01
The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Although density functional theory is widely used in the computational chemistry community, the most popular density functional, B3LYP, has some serious shortcomings: (i) it is better for main-group chemistry than for transition metals; (ii) it systematically underestimates reaction barrier heights; (iii) it is inaccurate for interactions dominated by mediumrange correlation energy, such as van der Waals attraction, aromatic-aromatic stacking, and alkane isomerization energies. We have developed a variety of databases for testing and designing new density functionals. We used these data to design new density functionals, called M06-class (and, earlier, M05-class) functionals, for which we enforced some fundamental exact constraints such as the uniform-electron-gas limit and the absence of self-correlation energy. Our M06-class functionals depend on spin-up and spin-down electron densities (i.e., spin densities), spin density gradients, spin kinetic energy densities, and, for nonlocal (also called hybrid) functionals, Hartree-Fock exchange. We have developed four new functionals that overcome the above-mentioned difficulties: (a) M06, a hybrid meta functional, is a functional with good accuracy “across-theboard” for transition metals, main group thermochemistry, medium-range correlation energy, and barrier heights; (b) M06- 2X, another hybrid meta functional, is not good for transition metals but has excellent performance for main group chemistry, predicts accurate valence and Rydberg electronic excitation energies, and is an excellent functional for aromatic-aromatic stacking interactions; (c) M06-L is not as accurate as M06 for barrier heights but is the most accurate
Kazemiabnavi, Saeed; Dutta, Prashanta; Banerjee, Soumik
2015-05-01
The unique properties of ionic liquids such as a relatively wide electrochemical stability window and very low vapor pressure have made them promising candidates as electrolytes for improving the cyclic performance of lithium-air batteries. The local current density, which is an important parameter in determining the performance of lithium-air batteries, is a function of the rate constant of the electron transfer reactions at the surface of the cathode. In this study, a novel method based on Marcus theory is presented to investigate the effect of varying the length of the alkyl side chain of model imidazolium based cations and the operating temperature on the rates of electron transfer reactions at the cathode. The necessary free energies of all the species involved in the multi-step reduction of oxygen into the peroxide ion were calculated using density functional theory (DFT). Our results indicate that the magnitude of the Gibbs free energy for the reduction of oxygen into the superoxide ion and also for the reduction of superoxide into the peroxide ion increases with an increase in the static dielectric constant of the ionic liquid. This trend in turn corresponds to the decrease in the length of the alkyl side chain of the ionic liquid cation. Furthermore, the change in Gibbs free energy decreases with increase in the operating temperature. The inner-sphere reorganization energies were evaluated using Nelsen's four point method. The total reorganization energies of all reduction reactions increase with decrease in the length of the alkyl side chain and increase in the operating temperature. Finally, the rate constants of the electron transfer reaction involved in the reduction of oxygen were calculated. The logarithm of the reaction rate constants decreases with increase in the static dielectric constant and increases with increase in the operating temperature. Our results provide fundamental insight into the kinetics and thermodynamics of the electron transfer
Jankowski, K; Nowakowski, K; Grabowski, I; Wasilewski, J
2009-04-28
The problem of linking the dynamic electron correlation effects defined in traditional ab initio methods [or wave function theories (WFTs)] with the structure of the individual density functional theory (DFT) exchange and correlation functionals has been analyzed for the Ne atom, for which nondynamic correlation effects play a negligible role. A density-based approach directly hinged on difference radial-density (DRD) distributions defined with respect the Hartree-Fock radial density has been employed for analyzing the impact of dynamic correlation effects on the density. Attention has been paid to the elimination of basis-set incompleteness errors. The DRD distributions calculated by several ab initio methods have been compared to their DFT counterparts generated for representatives of several generations of broadly used exchange-correlation functionals and for the recently developed orbital-dependent OEP2 exchange-correlation functional [Bartlett et al., J. Chem. Phys. 122, 034104 (2005)]. For the local, generalized-gradient, and hybrid functionals it has been found that the dynamic correlation effects are to a large extend accounted for by densities resulting from exchange-only calculations. Additional calculations with self-interaction corrected exchange potentials indicate that this finding cannot be explained as an artifact caused by the self-interaction error. It has been demonstrated that the VWN5 and LYP correlation functionals do not represent any substantial dynamical correlation effects on the electron density, whereas these effects are well represented by the orbital-dependent OEP2 correlation functional. Critical comparison of the present results with their counterparts reported in literature has been made. Some attention has been paid to demonstrating the differences between the energy- and density-based perspectives. They indicate the usefulness of density-based criteria for developing new exchange-correlation functionals. PMID:19405556
NASA Astrophysics Data System (ADS)
Jankowski, K.; Nowakowski, K.; Grabowski, I.; Wasilewski, J.
2009-04-01
The problem of linking the dynamic electron correlation effects defined in traditional ab initio methods [or wave function theories (WFTs)] with the structure of the individual density functional theory (DFT) exchange and correlation functionals has been analyzed for the Ne atom, for which nondynamic correlation effects play a negligible role. A density-based approach directly hinged on difference radial-density (DRD) distributions defined with respect the Hartree-Fock radial density has been employed for analyzing the impact of dynamic correlation effects on the density. Attention has been paid to the elimination of basis-set incompleteness errors. The DRD distributions calculated by several ab initio methods have been compared to their DFT counterparts generated for representatives of several generations of broadly used exchange-correlation functionals and for the recently developed orbital-dependent OEP2 exchange-correlation functional [Bartlett et al., J. Chem. Phys. 122, 034104 (2005)]. For the local, generalized-gradient, and hybrid functionals it has been found that the dynamic correlation effects are to a large extend accounted for by densities resulting from exchange-only calculations. Additional calculations with self-interaction corrected exchange potentials indicate that this finding cannot be explained as an artifact caused by the self-interaction error. It has been demonstrated that the VWN5 and LYP correlation functionals do not represent any substantial dynamical correlation effects on the electron density, whereas these effects are well represented by the orbital-dependent OEP2 correlation functional. Critical comparison of the present results with their counterparts reported in literature has been made. Some attention has been paid to demonstrating the differences between the energy- and density-based perspectives. They indicate the usefulness of density-based criteria for developing new exchange-correlation functionals.
Meng, Da; Zheng, Bin; Lin, Guang; Sushko, Maria L.
2014-08-29
We have developed efficient numerical algorithms for the solution of 3D steady-state Poisson-Nernst-Planck equations (PNP) with excess chemical potentials described by the classical density functional theory (cDFT). The coupled PNP equations are discretized by finite difference scheme and solved iteratively by Gummel method with relaxation. The Nernst-Planck equations are transformed into Laplace equations through the Slotboom transformation. Algebraic multigrid method is then applied to efficiently solve the Poisson equation and the transformed Nernst-Planck equations. A novel strategy for calculating excess chemical potentials through fast Fourier transforms is proposed which reduces computational complexity from O(N2) to O(NlogN) where N is the number of grid points. Integrals involving Dirac delta function are evaluated directly by coordinate transformation which yields more accurate result compared to applying numerical quadrature to an approximated delta function. Numerical results for ion and electron transport in solid electrolyte for Li ion batteries are shown to be in good agreement with the experimental data and the results from previous studies.
Communication: Embedded fragment stochastic density functional theory
Neuhauser, Daniel; Baer, Roi; Rabani, Eran
2014-07-28
We develop a method in which the electronic densities of small fragments determined by Kohn-Sham density functional theory (DFT) are embedded using stochastic DFT to form the exact density of the full system. The new method preserves the scaling and the simplicity of the stochastic DFT but cures the slow convergence that occurs when weakly coupled subsystems are treated. It overcomes the spurious charge fluctuations that impair the applications of the original stochastic DFT approach. We demonstrate the new approach on a fullerene dimer and on clusters of water molecules and show that the density of states and the total energy can be accurately described with a relatively small number of stochastic orbitals.
NASA Astrophysics Data System (ADS)
Uǧur, Gökay; Candan, Abdullah
2014-10-01
First-principle calculations of structural, electronic, elastic and phonon properties of SnMg2O4, SnZn2O4 and SnCd2O4 compounds are presented, using the pseudo-potential plane waves approach based on density functional theory (DFT) within the generalized gradient approximation (GGA). The computed ground state structural parameters, i.e. lattice constants, internal free parameter and bulk modulus are in good agreement with the available theoretical results. Our calculated elastic constants are indicative of stability of SnX2O4 (X=Mg, Zn, Cd) compounds in the spinel structure. The partial density of states (PDOS) of these compounds is in good agreement with the earlier ab-initio calculations. The phonon dispersion relations were calculated using the direct method. Phonon dispersion results indicate that SnZn2O4 is dynamically stable, while SnMg2O4 and SnCd2O4 are unstable.
Uğur, Gökay; Candan, Abdullah
2014-10-06
First-principle calculations of structural, electronic, elastic and phonon properties of SnMg{sub 2}O{sub 4}, SnZn{sub 2}O{sub 4} and SnCd{sub 2}O{sub 4} compounds are presented, using the pseudo-potential plane waves approach based on density functional theory (DFT) within the generalized gradient approximation (GGA). The computed ground state structural parameters, i.e. lattice constants, internal free parameter and bulk modulus are in good agreement with the available theoretical results. Our calculated elastic constants are indicative of stability of SnX{sub 2}O{sub 4} (X=Mg, Zn, Cd) compounds in the spinel structure. The partial density of states (PDOS) of these compounds is in good agreement with the earlier ab-initio calculations. The phonon dispersion relations were calculated using the direct method. Phonon dispersion results indicate that SnZn{sub 2}O{sub 4} is dynamically stable, while SnMg{sub 2}O{sub 4} and SnCd{sub 2}O{sub 4} are unstable.
Van Kuiken, Benjamin E.; Valiev, Marat; Daifuku, Stephanie L.; Bannan, Caitlin; Strader, Matthew L.; Cho, Hana; Huse, N.; Schoenlein, R. W.; Govind, Niranjan; Khalil, Munira
2013-05-01
Ruthenium L2,3-edge X-ray absorption (XA) spectroscopy probes transitions from core 2p orbitals to the 4d levels of the atom and is a powerful tool for interrogating the local electronic and molecular structure around the metal atom. However, a molecular-level interpretation of the Ru L2,3-edge spectral lineshapes is often complicated by spin–orbit coupling (SOC) and multiplet effects. In this study, we develop spin-free time-dependent density functional theory (TDDFT) as a viable and predictive tool to simulate the Ru L3-edge spectra. We successfully simulate and analyze the ground state Ru L3-edge XA spectra of a series of RuII and RuIII complexes: [Ru(NH3)6]2+/3+, [Ru(CN)6]4-/3-, [RuCl6]4-/3-, and the ground (1A1) and photoexcited (3MLCT) transient states of [Ru(bpy)3]2+ and Ru(dcbpy)2(NCS)2 (termed N3). The TDDFT simulations reproduce all the experimentally observed features in Ru L3-edge XA spectra. The advantage of using TDDFT to assign complicated Ru L3-edge spectra is illustrated by its ability to identify ligand specific charge transfer features in complex molecules. We conclude that the B3LYP functional is the most reliable functional for accurately predicting the location of charge transfer features in these spectra. Experimental and simulated Ru L3-edge XA spectra are presented for the transition metal mixed-valence dimers [(NC)5MII-CN-RuIII(NH3)5]- (where M = Fe or Ru) dissolved in water. We explore the spectral signatures of electron delocalization in Ru L3-edge XA spectroscopy and our simulations reveal that the inclusion of explicit solvent molecules is crucial for reproducing the experimentally determined valencies, highlighting the importance of the role of the solvent in transition metal charge transfer chemistry.
NASA Astrophysics Data System (ADS)
Majumder, Chiranjib; Kulshreshtha, S. K.
2004-03-01
A systematic theoretical study of the equilibrium geometry and energetics of Sin and Sin-1Al clusters has been carried out using a combination of the density functional theory and molecular dynamics simulation under the local spin density (LSD) approximation. The lowest energy isomer thus obtained is further used to evaluate the total energy using ab initio quantum chemical technique at the second-order Møller-Plesset [MP2/6-31G(d)] level taking all electrons into account. Based on the comparative study between Sin and Sin-1Al clusters it is found that the ground-state geometries of the Sin-1Al clusters are almost similar to that of Sin clusters with the Al atom replacing one of the Si atoms with small local distortion. However, significant differences have been observed in their electronic structure and the fragmentation behavior. The average binding energies of the Sin and Sin-1Al clusters vary in a similar way with slightly higher values for the Sin clusters. Clusters of both these series with n=4,6,10 show higher stability as compared to its neighboring clusters. The dissociation energies calculated for Al and Si atoms suggest that the evaporation of an Al atom is easier than that of Si. However, a comparison of the dissociation energies of Si atoms from Sin and Sin-1Al clusters revealed that for n=4,6,10, the evaporation of Si atoms from the Sin-1Al cluster requires more energy than that of the corresponding Sin cluster implying an improvement in the bond strength between Si-Si bonds for these Sin-1Al clusters due to Al substitution in Sin. Finally, a good agreement of our results and the previously published results on Si clusters gives us the confidence to believe the good prediction of our results on the Sin-1Al clusters.
Reunchan, Pakpoom; Boonchun, Adisak; Umezawa, Naoto
2016-08-17
The electronic structures of highly active Ag-based oxide photocatalysts Ag3AsO4 and Ag3PO4 are studied by hybrid-density functional calculations. It is revealed that Ag3AsO4 and Ag3PO4 are indirect band gap semiconductors. The Hartree-Fock mixing parameters are fitted for experimental band gaps of Ag3AsO4 (1.88 eV) and Ag3PO4 (2.43 eV). The smaller electron effective mass and the lower valence band edge of Ag3AsO4 are likely to be responsible for the superior photocatalytic oxidation reaction to Ag3PO4. The comparable lattice constant and analogous crystal structure between the two materials allow the opportunities of fine-tuning the band gap of Ag3AsxP1-xO4 using a solid-solution approach. The development of Ag3AsxP1-xO4 should be promising for the discovery of novel visible-light sensitized photocatalysts. PMID:27502998
NASA Astrophysics Data System (ADS)
Poli, Emiliano; Teobaldi, Gilberto
2015-03-01
We report a linear-scaling Density Functional Theory (DFT) study of cation-vacancy related defects in single-walled aluminosilicate nanotubes (AlSi NTs), based on the structures derived from solid-state Nuclear Magnetic Resonance. Defect geometry optimization leads to water condensation and modifications to the AlSi NT hydrogen network around the defect sites, leaving no dangling bond. Electronic structure analysis indicates that defect-states are highly localized in real-space and energy, with appearance of shallow and deep occupied defect states above the valence band (VB) edge of the pristine-NT. Electrostatic alignment of the defect states suggests energetically favourable separation of photo-generated electrons and holes on different defects, which may promote defect-centred photochemistry. The peculiar energy alignment of the defect-states is found to be qualitative unaffected by protonation of the defect-sites. These results should be a useful complement to ongoing experimental research in the potential of (alumino)silicate-based nano-porous materials for photocatalysis.
Density functional theory: Foundations reviewed
NASA Astrophysics Data System (ADS)
Kryachko, Eugene S.; Ludeña, Eduardo V.
2014-11-01
Guided by the above motto (quotation), we review a broad range of issues lying at the foundations of Density Functional Theory, DFT, a theory which is currently omnipresent in our everyday computational study of atoms and molecules, solids and nano-materials, and which lies at the heart of modern many-body computational technologies. The key goal is to demonstrate that there are definitely the ways to improve DFT. We start by considering DFT in the larger context provided by reduced density matrix theory (RDMT) and natural orbital functional theory (NOFT), and examine the implications that N-representability conditions on the second-order reduced density matrix (2-RDM) have not only on RDMT and NOFT but, also, by extension, on the functionals of DFT. This examination is timely in view of the fact that necessary and sufficient N-representability conditions on the 2-RDM have recently been attained. In the second place, we review some problems appearing in the original formulation of the first Hohenberg-Kohn theorem which is still a subject of some controversy. In this vein we recall Lieb's comment on this proof and the extension to this proof given by Pino et al. (2009), and in this context examine the conditions that must be met in order that the one-to-one correspondence between ground-state densities and external potentials remains valid for finite subspaces (namely, the subspaces where all Kohn-Sham solutions are obtained in practical applications). We also consider the issue of whether the Kohn-Sham equations can be derived from basic principles or whether they are postulated. We examine this problem in relation to ab initio DFT. The possibility of postulating arbitrary Kohn-Sham-type equations, where the effective potential is by definition some arbitrary mixture of local and non-local terms, is discussed. We also deal with the issue of whether there exists a universal functional, or whether one should advocate instead the construction of problem
A half century of density functional theory
Zangwill, Andrew
2015-07-15
Today’s most popular method for calculating the electronic structure of atoms, molecules, liquids, solids, and plasmas began as a bold hypothesis: The electron density distribution completely characterizes the ground state of a many-electron system.
NASA Astrophysics Data System (ADS)
Yelgel, Celal
2016-02-01
The structural and electronic properties of multilayer graphene adsorbed on monolayer hexagonal boron nitride (h-BN)/Ni(111) interface system are investigated using the density functional theory with a recently developed non-local van der Waals density functional (rvv10). The most energetically favourable configuration for a monolayer h-BN/Ni(111) interface is found to be N atom atop the Ni atoms and B atom in fcc site with the interlayer distance of 2.04 Å and adsorption energy of 302 meV/BN. Our results show that increasing graphene layers on a monolayer h-BN/Ni(111) interface leads to a weakening of the interfacial interaction between the monolayer h-BN and Ni(111) surface. The adsorption energy of graphene layers on the h-BN/Ni(111) interface is found to be in the range of the 50-120 meV/C atom as the vertical distance from h-BN to the bottommost graphene layers decreases. With the adsorption of a multilayer graphene on the monolayer h-BN/Ni(111) interface system, the band gap of 0.12 eV and 0.25 eV opening in monolayer graphene and bilayer graphene near the K point is found with an upward shifting of the Fermi level. However, a stacking-sensitive band gap is opened in trilayer graphene. We obtain the band gap of 0.35 eV close to the K point with forming a Mexican hat band structure for ABC-stacked trilayer graphene.
NASA Astrophysics Data System (ADS)
Duan, Yuhua; Zhang, Bo; Sorescu, Dan C.; Johnson, J. Karl; Majzoub, Eric H.; Luebke, David R.
2012-08-01
The structural, electronic, phonon dispersion and thermodynamic properties of MHCO3 (M = Li, Na, K) solids were investigated using density functional theory. The calculated bulk properties for both their ambient and the high-pressure phases are in good agreement with available experimental measurements. Solid phase LiHCO3 has not yet been observed experimentally. We have predicted several possible crystal structures for LiHCO3 using crystallographic database searching and prototype electrostatic ground state modeling. Our total energy and phonon free energy (FPH) calculations predict that LiHCO3 will be stable under suitable conditions of temperature and partial pressures of CO2 and H2O. Our calculations indicate that the {{HCO}}_{3}^{-} groups in LiHCO3 and NaHCO3 form an infinite chain structure through O⋯H⋯O hydrogen bonds. In contrast, the {{HCO}}_{3}^{-} anions form dimers, ({{HCO}}_{3}^{-})_{2}, connected through double hydrogen bonds in all phases of KHCO3. Based on density functional perturbation theory, the Born effective charge tensor of each atom type was obtained for all phases of the bicarbonates. Their phonon dispersions with the longitudinal optical-transverse optical splitting were also investigated. Based on lattice phonon dynamics study, the infrared spectra and the thermodynamic properties of these bicarbonates were obtained. Over the temperature range 0-900 K, the FPH and the entropies (S) of MHCO3 (M =Li, Na, K) systems vary as FPH(LiHCO3) > FPH(NaHCO3) > FPH(KHCO3) and S(KHCO3) > S(NaHCO3) > S(LiHCO3), respectively, in agreement with the available experimental data. Analysis of the predicted thermodynamics of the CO2 capture reactions indicates that the carbonate/bicarbonate transition reactions for Na and K could be used for CO2 capture technology, in agreement with experiments.
Xiao, W; Wang, J N; Wang, J W; Huang, G J; Cheng, L; Jiang, L J; Wang, L G
2016-04-28
The quaternary compound semiconductor Cu2ZnSnS4 (CZTS) is a promising photovoltaic absorber material for thin-film solar cell applications. Density-functional theory calculations have been performed to investigate the structural and electronic properties of the CdS/CZTS heterointerfaces in CZTS-based cells. We find that CdS favors epitaxial growth on the Cu-Zn plane of CZTS along the direction of [100], which can eliminate the effects of the wrong bonds at the interfaces and enhance the energetic barrier for charge carrier recombination across the interfaces with an increased band gap. The band alignment is calculated for the epitaxial CZTS/CdS heterointerface by employing the HSE06 functional and the results show a type-II band alignment with VBO and CBO values of 0.95 eV and -0.05 eV, respectively. Also, the experimental phenomenon of Zn segregation at CdS/CZTS interfaces is corroborated. Zn segregation can enhance the stability of the heterointerfaces, but damage the solar cell performance by decreasing the band gap when the Zn concentration is sufficiently high. We show that besides the defects and undesired phases in CZTS, the heterointerfaces between the absorption layers (CZTS) and the buffer layer (CdS) can also be an important factor that affects the performance of CZTS cells. The present work provides a theoretical base for engineering the heterointerfaces and achieving better performance of CZTS-based solar cells. PMID:27067113
NASA Astrophysics Data System (ADS)
Ramos Murillo, Manuel Antonio
Many studies had been done in order to understand promotion effect and structure/function in unsupported catalyst. Results indicated that d-electrons play an important role promoting catalytical active sites at the edges of MoS2 catalytically structures. Sulfur removal from crude oil, occurs on the edge of molybdenum di-sulfide (MoS2) nano structures, due to promotion of MoS2 nano structures with nickel or cobalt in sulfur-terminated or molybdenum-terminated (1¯010) edge planes. The promotion leads to formation of so-called CoMoS phase (MoS2/Co9S8 ) first discovered by meaning of Mossbauer spectroscopy, much more CoMoS phase usually called unsupported catalytical particles, have been topic of great interest in scientific community, leading to several intriguing conclusions. Nevertheless atomistic conclusions cannot be concluded by meaning of a single theoretical work, in fact several technical paper were found in an attempt to explain promotion effect on MoS2 nano structures, with no one single conclusion found; meaning key information on about how Cobalt Molybdenum and Sulfur acted together, in terms of atomic position as described by observations made in these nano structures by electron microscopy techniques to create the so-called CoMoS phase. Few technical reports have approached and discussed this intrigue key problem by means of Density Functional Theory methods and are in agreement data obtained by other techniques. As for microstructure characterization, in the literature most of the articles just report the nano structure type indicated as fringes of MoS2 but no further important information like the localization of the promoters. It is also described, in the literature, how one reaction; which is the hydrodesulfurization (HDS) of large molecules usually dibenzothiophene, thiophenes and others contained in crude oil, occurred at edges of MoS2 nano structures when they are Co or Ni promoted, with almost zero activity in the (001) basal planes, as anyone
Pseudospectral time-dependent density functional theory
NASA Astrophysics Data System (ADS)
Ko, Chaehyuk; Malick, David K.; Braden, Dale A.; Friesner, Richard A.; Martínez, Todd J.
2008-03-01
Time-dependent density functional theory (TDDFT) is implemented within the Tamm-Dancoff approximation (TDA) using a pseudospectral approach to evaluate two-electron repulsion integrals. The pseudospectral approximation uses a split representation with both spectral basis functions and a physical space grid to achieve a reduction in the scaling behavior of electronic structure methods. We demonstrate here that exceptionally sparse grids may be used in the excitation energy calculation, following earlier work employing the pseudospectral approximation for determining correlation energies in wavefunction-based methods with similar conclusions. The pseudospectral TDA-TDDFT method is shown to be up to ten times faster than a conventional algorithm for hybrid functionals without sacrificing chemical accuracy.
Electron density measurements in highly electronegative plasmas
NASA Astrophysics Data System (ADS)
Rafalskyi, D.; Lafleur, T.; Aanesland, A.
2016-08-01
In this paper we present experimental measurements of the electron density in very electronegative ‘ion–ion’ Ar–SF6 plasmas where previous investigations using Langmuir probes have observed electronegativities of up to 5000. The electron density is measured using a short matched dipole probe technique that provides a tolerance better than ±2 · 1013 m‑3. The results demonstrate that the electron density in the low pressure plasma source (which contains a magnetic filter) can be reduced to around 2.7 · 1013 m‑3 with a corresponding plasma electronegativity of about 4000; close to that from fluid simulation predictions. The highest electronegativity, and lowest electron density, is achieved with a pure SF6 plasma, while adding only 6% SF6 to Ar allows the electronegativity to be increased from 0 to a few hundred with a corresponding decrease in the electron density by more than a thousand. The impedance probe based on a short matched dipole appears to be a practical diagnostic that can be used for independent measurements of the electron density in very electronegative plasmas, and opens up the possibility to further investigate and optimize electronegative plasma sources.
Morisato, Tsuguo; Ohno, Kaoru; Ohtsuki, Tsutomu; Hirose, Kentaro; Sluiter, Marcel; Kawazoe, Yoshiyuki
2008-09-15
Carrying out a first-principles calculation assuming linear relationship between the electron density at Be nucleus and the electron-capture (EC) decay rate, we explained why {sup 7}Be-C{sub 60} shows higher EC decay rate than {sup 7}Be crystal, which was originally found experimentally by Ohtsuki et al. [Phys. Rev. Lett. 93, 112501 (2004)]. From the results of the calculation, we found that there are inequivalent four stable (i.e., lower energy) Be sites inside C{sub 60} and that center of C{sub 60} (C{sub C60}) is the most favorable site. For C{sub C60}, the electron density at the Be nucleus is the highest. It is also much higher than that at the Be nucleus in a Be crystal. Also, we estimated the expected electron density at the Be nucleus at room temperature by taking statistical average of the electron densities at the four Be nucleus sites using the Boltzmann distribution. The results of the calculation show fairly good agreement with the experimental results. In this paper, we focus on the detail of calculation, which was not fully demonstrated in the paper by Ohtsuki.
Modification of ionospheric electron density by dust suspension
NASA Astrophysics Data System (ADS)
Srivastava, Sweta; Mishra, Rashmi; Singh Sodha, Mahendra
2016-05-01
On the basis of a dynamic analysis the effectiveness of dust suspension for the reduction and enhancement of electron density in the E-layer of the ionosphere has been investigated in this paper. The analysis is based on the modelling of the E-layer as the Chapman α layer (validated earlier); the electron/ion production function, arrived at by Chapman and effective electron temperature-dependent electron–ion recombination coefficients in agreement with observations have been used. The balance of the charge on the particles and the number/energy balance of the constituents have been taken into account. The following is the physics of the change in electron density in the ionosphere by the suspension of dust. First, the dust provides a source (emission) and sink (accretion) of electrons. Second, the dust emits photoelectrons with energies much higher than those of ambient electrons, which enhances the electron temperature, leading to a reduced electron–ion recombination coefficient, and thus to a higher electron density. An interplay of these processes and the natural processes of electron production/annihilation determines the electron density and temperature in the dust suspension in the ionosphere. The numerical results, corresponding to suspension of dust of silicate (high work function) and Cs coated bronze (low work function) in the E-layer at 105 \\text{km} are presented and discussed.
Nozaki, Hiroo; Fujii, Yosuke; Ichikawa, Kazuhide; Watanabe, Taku; Aihara, Yuichi; Tachibana, Akitomo
2016-07-01
We analyze the electronic structure of lithium ionic conductors, Li3PO4 and Li3PS4, using the electronic stress tensor density and kinetic energy density with special focus on the ionic bonds among them. We find that, as long as we examine the pattern of the eigenvalues of the electronic stress tensor density, we cannot distinguish between the ionic bonds and bonds among metalloid atoms. We then show that they can be distinguished by looking at the morphology of the electronic interface, the zero surface of the electronic kinetic energy density. © 2016 Wiley Periodicals, Inc. PMID:27232445
Absolute Measurement of Electron Cloud Density
Covo, M K; Molvik, A W; Cohen, R H; Friedman, A; Seidl, P A; Logan, G; Bieniosek, F; Baca, D; Vay, J; Orlando, E; Vujic, J L
2007-06-21
Beam interaction with background gas and walls produces ubiquitous clouds of stray electrons that frequently limit the performance of particle accelerator and storage rings. Counterintuitively we obtained the electron cloud accumulation by measuring the expelled ions that are originated from the beam-background gas interaction, rather than by measuring electrons that reach the walls. The kinetic ion energy measured with a retarding field analyzer (RFA) maps the depressed beam space-charge potential and provides the dynamic electron cloud density. Clearing electrode current measurements give the static electron cloud background that complements and corroborates with the RFA measurements, providing an absolute measurement of electron cloud density during a 5 {micro}s duration beam pulse in a drift region of the magnetic transport section of the High-Current Experiment (HCX) at LBNL.
Electron density profile description in the international reference ionosphere
NASA Technical Reports Server (NTRS)
Rawer, K.; Bilitza, D.
1989-01-01
Problems encountered during efforts to reformulate the IRI description of the electron density profile are examined. Consideration is given to Booker's (1979) proposal that the unique, analytic profile functions should cover the entire ionospheric height range. The IRI topside model is reviewed and the electron density profile of the middle and lower ionosphere are discussed. Rawer's (1983) procedure for combining the topside, middle, and lower ionospheric profiles into one analytic profile is reviewed.
Electron density profile description in the international reference ionosphere
NASA Astrophysics Data System (ADS)
Rawer, K.; Bilitza, D.
1989-10-01
Problems encountered during efforts to reformulate the IRI description of the electron density profile are examined. Consideration is given to Booker's (1979) proposal that the unique, analytic profile functions should cover the entire ionospheric height range. The IRI topside model is reviewed and the electron density profile of the middle and lower ionosphere are discussed. Rawer's (1983) procedure for combining the topside, middle, and lower ionospheric profiles into one analytic profile is reviewed.
Yan, Li-Li; Liu, Yi-Rong; Huang, Teng; Jiang, Shuai; Wen, Hui; Gai, Yan-Bo; Zhang, Wei-Jun E-mail: wjzhang@aiofm.ac.cn; Huang, Wei E-mail: wjzhang@aiofm.ac.cn
2013-12-28
The equilibrium geometric structures, relative stabilities, and electronic properties of Au{sub n}C{sup −} and Au{sub n+1}{sup −} (n = 1–10) clusters are systematically investigated using density functional theory with hyper-generalized gradient approximation. The optimized geometries show that one Au atom capped on Au{sub n−1}C{sup −} clusters is a dominant growth pattern for Au{sub n}C{sup −} clusters. In contrast to Au{sub n+1}{sup −} clusters, Au{sub n}C{sup −} clusters are most stable in a quasi-planar or three-dimensional structure because C doping induces the local non-planarity while the rest of the structure continues to grow in a planar mode, resulting in an overall non-2D configuration. The relative stability calculations show that the impurity C atom can significantly enhance the thermodynamic stability of pure gold clusters. Moreover, the effect of C atom on the Au{sub n}{sup −} host decreases with the increase of cluster size. The HOMO-LUMO gap curves show that the interaction of the C atom with Au{sub n}{sup −} clusters improves the chemical stability of pure gold clusters, except for Au{sub 3}{sup −} and Au{sub 4}{sup −} clusters. In addition, a natural population analysis shows that the charges in corresponding Au{sub n}C{sup −} clusters transfer from the Au{sub n}{sup −} host to the C atom. Meanwhile, a natural electronic configuration analysis also shows that the charges mainly transfer between the 2s and 2p orbitals within the C atom.
NASA Astrophysics Data System (ADS)
Zhou, Shi Wen; Liu, Jian; Peng, Ping; Chen, Wen Qin
2015-12-01
The electronic and optical properties of S- and/or Ce-(co)doped anatase titanium dioxide (TiO2) are investigated using density functional theory plus U (DFT+U) calculations. The optimized total energy suggests that TiO2 codoping by Ce and S favours the configuration of one substitutional Ce atom occupied on a Ti site with one substitutional S atom either on its nearest neighboring O or Ti site. The calculated results show that all doping configurations exhibit remarkable red-shift and excellent photocatalytic properties compared with pure TiO2. These reinforced features can mainly be ascribed to the appearance of S 3p states in the top of valence band (VB) and Ce 4f states in the bottom of conduction band (CB) as well as the contribution from the increasing octahedral dipole moments. The synergetic effects of cationic Ce and anionic S can extend optical absorption edge, which results in higher absorption coefficient in the visible light region than that of the anionic S monodoping and cationic Ce monodoping case; in the same time, decreasing the codoping concentration leads to reduced optical absorption. Additionally, Ce and S as cations incorporating into TiO2 lattices can induce stronger redox potential with a lower defect formation energy under O-rich condition compared with other doping systems.
NASA Astrophysics Data System (ADS)
Rai, D. P.; Sandeep; Shankar, A.; Pradhan Sakhya, Anup; Sinha, T. P.; Khenata, R.; Ghimire, M. P.; Thapa, R. K.
2016-07-01
The electronic and magnetic properties of Heusler compounds X2YZ and XYZ (X = Co, Ni, Pt, Fe; Y = Mn, Cr, Vi; Z = Al, Sb, Ga) are investigated by using the density functional theory with generalized gradient approximation (GGA), GGA plus U (LSDA+U), and modified Becke‑Johnson (mBJ) exchange potential. It is found that the half-metallic gaps are generally widened reasonably by LSDA+U and mBJ as compared to the conventional GGA. For the Co-based Heusler compounds the inclusion of U in GGA leads to a larger minority band gap while it is destroyed for Fe2VAl and NiMnSb. The magnetic properties of Co2VSi and Co2VSn are well defined within LSDA+U and mBJ with an exact integer value of magnetic moment. The band gaps of Fe2VAl and CoMnSb given by mBJ are in good agreement with the available experimental data of x-ray absorption spectroscopy. Except for the reasonably larger band gap, the mBJ band structure is almost same as that of GGA but is remarkably different from that of LSDA+U.
Mohammadi Hesari, Asghar; Shamlouei, Hamid Reza; Raoof Toosi, Ali
2016-08-01
The effect of alkali metal oxides M n O (M = Li, Na, K; n = 2, 3, 4) on the geometric, electronic, and linear and nonlinear optical properties of the Mg12O12 nanocage was investigated by density-functional-based methods. According to the computational results, these alkali metal oxides are adsorbed on the Mg12O12 nanocage because this adsorption reduces its energy gap. The static first hyperpolarizability (β 0) of the nanocage is dramatically increased in the presence of the alkali metal oxides, with the greatest increase seen in the presence of the superalkalis (i.e., M3O; M = Li, Na, and K). The highest first hyperpolarizability (β 0 ≈ 600,000 a.u.) was calculated for K3O@Mg12O12, which was considerably more than that for Mg12O12. The thermodynamic properties and relative stabilities of these inorganic compounds are discussed. Graphical Abstract Optimized structure and DOS spectrum of K3O(e@Mg12O12). PMID:27449668
Exact conditions on the temperature dependence of density functionals
NASA Astrophysics Data System (ADS)
Burke, K.; Smith, J. C.; Grabowski, P. E.; Pribram-Jones, A.
2016-05-01
Universal exact conditions guided the construction of most ground-state density functional approximations in use today. We derive the relation between the entropy and Mermin free energy density functionals for thermal density functional theory. Both the entropy and sum of kinetic and electron-electron repulsion functionals are shown to be monotonically increasing with temperature, while the Mermin functional is concave downwards. Analogous relations are found for both exchange and correlation. The importance of these conditions is illustrated in two extremes: the Hubbard dimer and the uniform gas.
Uzunova, Ellie L; Mikosch, Hans
2014-07-28
The iron sulfide dimers (FeS)2 and their persulfide isomers with S-S bonds are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. The disulfides are more stable than the persulfides as bare clusters and the persulfide ground state lies at 3.2 eV above the global minimum, while in the hexacarbonyl complexes this order is reversed: persulfides are more stable, but the energy gap between disulfides and persulfides becomes much smaller and the activation barrier for the transition persulfide → disulfide is 1.11 eV. Carbonylation also favors a non-planar Fe2S2 ring for both the disulfides and the persulfides and high electron density in the Fe2S2 core is induced. The diamagnetic ordering is preferred in the hexacarbonyls, unlike the bare clusters. The hexacarbonyls possess low-lying triplet excited states. In the persulfide, the lowest singlet-to-triplet state excitation occurs by electron transition from the iron centers to an orbital located predominantly at S2 via metal-to-ligand charge transfer. In the disulfide this excitation corresponds to ligand-to-metal charge transfer from the sulfur atoms to an orbital located at the iron centers and the Fe-Fe bond. Water splitting occurs on the hexacarbonyls, but not on the bare clusters. The singlet and triplet state reaction paths were examined and activation barriers were determined: 50 kJ mol(-1) for HO-H bond dissociation and 210 kJ mol(-1) for hydrogen evolution from the intermediate sulfoxyl-hydroxyl complexes Fe2S(OH)(SH)(CO)6 formed. The lowest singlet-singlet excitations in the hexacarbonyls, the water adsorption complexes and in the reaction intermediates, formed prior to dihydrogen release, fall in the visible light region. The energy barrier of 210 kJ mol(-1) for the release of one hydrogen molecule corresponds to one visible photon of 570 nm. The dissociation of a second water molecule, followed by H2 and O2 release via hydro-peroxide intermediate is a two-step process, with
NASA Astrophysics Data System (ADS)
Uzunova, Ellie L.; Mikosch, Hans
2014-07-01
The iron sulfide dimers (FeS)2 and their persulfide isomers with S-S bonds are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. The disulfides are more stable than the persulfides as bare clusters and the persulfide ground state lies at 3.2 eV above the global minimum, while in the hexacarbonyl complexes this order is reversed: persulfides are more stable, but the energy gap between disulfides and persulfides becomes much smaller and the activation barrier for the transition persulfide → disulfide is 1.11 eV. Carbonylation also favors a non-planar Fe2S2 ring for both the disulfides and the persulfides and high electron density in the Fe2S2 core is induced. The diamagnetic ordering is preferred in the hexacarbonyls, unlike the bare clusters. The hexacarbonyls possess low-lying triplet excited states. In the persulfide, the lowest singlet-to-triplet state excitation occurs by electron transition from the iron centers to an orbital located predominantly at S2 via metal-to-ligand charge transfer. In the disulfide this excitation corresponds to ligand-to-metal charge transfer from the sulfur atoms to an orbital located at the iron centers and the Fe-Fe bond. Water splitting occurs on the hexacarbonyls, but not on the bare clusters. The singlet and triplet state reaction paths were examined and activation barriers were determined: 50 kJ mol-1 for HO-H bond dissociation and 210 kJ mol-1 for hydrogen evolution from the intermediate sulfoxyl-hydroxyl complexes Fe2S(OH)(SH)(CO)6 formed. The lowest singlet-singlet excitations in the hexacarbonyls, the water adsorption complexes and in the reaction intermediates, formed prior to dihydrogen release, fall in the visible light region. The energy barrier of 210 kJ mol-1 for the release of one hydrogen molecule corresponds to one visible photon of 570 nm. The dissociation of a second water molecule, followed by H2 and O2 release via hydro-peroxide intermediate is a two-step process, with activation
Uzunova, Ellie L.; Mikosch, Hans
2014-07-28
The iron sulfide dimers (FeS){sub 2} and their persulfide isomers with S–S bonds are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. The disulfides are more stable than the persulfides as bare clusters and the persulfide ground state lies at 3.2 eV above the global minimum, while in the hexacarbonyl complexes this order is reversed: persulfides are more stable, but the energy gap between disulfides and persulfides becomes much smaller and the activation barrier for the transition persulfide → disulfide is 1.11 eV. Carbonylation also favors a non-planar Fe{sub 2}S{sub 2} ring for both the disulfides and the persulfides and high electron density in the Fe{sub 2}S{sub 2} core is induced. The diamagnetic ordering is preferred in the hexacarbonyls, unlike the bare clusters. The hexacarbonyls possess low-lying triplet excited states. In the persulfide, the lowest singlet-to-triplet state excitation occurs by electron transition from the iron centers to an orbital located predominantly at S{sub 2} via metal-to-ligand charge transfer. In the disulfide this excitation corresponds to ligand-to-metal charge transfer from the sulfur atoms to an orbital located at the iron centers and the Fe–Fe bond. Water splitting occurs on the hexacarbonyls, but not on the bare clusters. The singlet and triplet state reaction paths were examined and activation barriers were determined: 50 kJ mol{sup −1} for HO–H bond dissociation and 210 kJ mol{sup −1} for hydrogen evolution from the intermediate sulfoxyl-hydroxyl complexes Fe{sub 2}S(OH)(SH)(CO){sub 6} formed. The lowest singlet-singlet excitations in the hexacarbonyls, the water adsorption complexes and in the reaction intermediates, formed prior to dihydrogen release, fall in the visible light region. The energy barrier of 210 kJ mol{sup −1} for the release of one hydrogen molecule corresponds to one visible photon of 570 nm. The dissociation of a second water molecule, followed by H{sub 2
NASA Astrophysics Data System (ADS)
Vojvodic, A.; Ruberto, C.; Lundqvist, B. I.
2010-09-01
This study explores atomic and molecular adsorption on a number of early transition-metal carbides (TMCs) in NaCl structure by means of density-functional theory calculations. The investigated substrates are the TM-terminated TMC(111) surfaces, of interest because of the presence of different types of surface resonances (SRs) on them and because of their technological importance in growth processes. Also, TM compounds have shown potential in catalysis applications. Trend studies are conducted with respect to both period and group in the periodic table, choosing the substrates ScC, TiC, VC, ZrC, NbC, δ-MoC, TaC, and WC (in NaCl structure) and the adsorbates H, B, C, N, O, F, NH, NH2, and NH3. Trends in adsorption strength are explained in terms of surface electronic factors, by correlating the calculated adsorption-energy values with the calculated surface electronic structures. The results are rationalized by use of a concerted-coupling model (CCM), which has previously been applied successfully to the description of adsorption on TiC(111) and TiN(111) surfaces (Ruberto et al 2007 Solid State Commun. 141 48). First, the clean TMC(111) surfaces are characterized by calculating surface energies, surface relaxations, Bader charges, and surface-localized densities of states (DOSs). Detailed comparisons between surface and bulk DOSs reveal the existence of transition-metal localized SRs (TMSRs) in the pseudogap and of several C-localized SRs (CSRs) in the upper valence band on all considered TMC(111) surfaces. The spatial extent and the dangling bond nature of these SRs are supported by real-space analyses of the calculated Kohn-Sham wavefunctions. Then, atomic and molecular adsorption energies, geometries, and charge transfers are presented. An analysis of the adsorbate-induced changes in surface DOSs reveals a presence of both adsorbate-TMSR and adsorbate-CSRs interactions, of varying strengths depending on the surface and the adsorbate. These variations are
Adiabatic corrections to density functional theory energies and wave functions.
Mohallem, José R; Coura, Thiago de O; Diniz, Leonardo G; de Castro, Gustavo; Assafrão, Denise; Heine, Thomas
2008-09-25
The adiabatic finite-nuclear-mass-correction (FNMC) to the electronic energies and wave functions of atoms and molecules is formulated for density-functional theory and implemented in the deMon code. The approach is tested for a series of local and gradient corrected density functionals, using MP2 results and diagonal-Born-Oppenheimer corrections from the literature for comparison. In the evaluation of absolute energy corrections of nonorganic molecules the LDA PZ81 functional works surprisingly better than the others. For organic molecules the GGA BLYP functional has the best performance. FNMC with GGA functionals, mainly BLYP, show a good performance in the evaluation of relative corrections, except for nonorganic molecules containing H atoms. The PW86 functional stands out with the best evaluation of the barrier of linearity of H2O and the isotopic dipole moment of HDO. In general, DFT functionals display an accuracy superior than the common belief and because the corrections are based on a change of the electronic kinetic energy they are here ranked in a new appropriate way. The approach is applied to obtain the adiabatic correction for full atomization of alcanes C(n)H(2n+2), n = 4-10. The barrier of 1 mHartree is approached for adiabatic corrections, justifying its insertion into DFT. PMID:18537228
Jamshidi, Zahra; Kaveei, Elham; Mohammadpour, Mozhdeh
2015-08-13
The effects of the weak interactions of rare gas atoms on the UV-visible absorption spectra of gold dimer and tetramer clusters are investigated. The time-dependent density functional theory based on the two-component relativistic zeroth-order regular approximation that considered spin-orbit coupling is performed to estimate the absorption spectra of Au2,4-Rgn (Rg = Ne-Xe, and n = 1-6) complexes. Using spin-orbit, including the appropriate functional, shows a close correlation between experiment and our calculations. It is also demonstrated that the weak interactions between rare gas atoms and gold clusters affect the UV-vis spectra of Au2,4 clusters by shifting the electronic transition toward the blue. Moreover, we find that the order of change in peak position, Δν̃, is proportional to the strength of interactions: Δν̃Au2,4-Xe > Δν̃Au2,4-Kr > Δν̃Au2,4-Ar > Δν̃Au2,4-Ne. In addition, comparing the UV-visible spectra of Au2,4-Rgn complexes with those of isolated Au2 and Au4 clusters shows that for Au2,4-Rg2,4,6 complexes in which Rg atoms interacted symmetrically with gold clusters no additional peaks are observed compared to isolated clusters; however, for Au2,4-Rg1,3,5 complexes, extra peaks appear because of the decrease in symmetry. PMID:26186279
NASA Astrophysics Data System (ADS)
Pribram-Jones, Aurora
Warm dense matter (WDM) is a high energy phase between solids and plasmas, with characteristics of both. It is present in the centers of giant planets, within the earth's core, and on the path to ignition of inertial confinement fusion. The high temperatures and pressures of warm dense matter lead to complications in its simulation, as both classical and quantum effects must be included. One of the most successful simulation methods is density functional theory-molecular dynamics (DFT-MD). Despite great success in a diverse array of applications, DFT-MD remains computationally expensive and it neglects the explicit temperature dependence of electron-electron interactions known to exist within exact DFT. Finite-temperature density functional theory (FT DFT) is an extension of the wildly successful ground-state DFT formalism via thermal ensembles, broadening its quantum mechanical treatment of electrons to include systems at non-zero temperatures. Exact mathematical conditions have been used to predict the behavior of approximations in limiting conditions and to connect FT DFT to the ground-state theory. An introduction to FT DFT is given within the context of ensemble DFT and the larger field of DFT is discussed for context. Ensemble DFT is used to describe ensembles of ground-state and excited systems. Exact conditions in ensemble DFT and the performance of approximations depend on ensemble weights. Using an inversion method, exact Kohn-Sham ensemble potentials are found and compared to approximations. The symmetry eigenstate Hartree-exchange approximation is in good agreement with exact calculations because of its inclusion of an ensemble derivative discontinuity. Since ensemble weights in FT DFT are temperature-dependent Fermi weights, this insight may help develop approximations well-suited to both ground-state and FT DFT. A novel, highly efficient approach to free energy calculations, finite-temperature potential functional theory, is derived, which has the
NASA Astrophysics Data System (ADS)
Hou, Xiao-Fei; Yan, Li-Li; Huang, Teng; Hong, Yu; Miao, Shou-Kui; Peng, Xiu-Qiu; Liu, Yi-Rong; Huang, Wei
2016-06-01
The equilibrium geometric structures, relative stabilities, electronic stabilities, and electronic and magnetic properties of the AunC and Aun+1 (n = 1-9) clusters are systematically investigated using density functional theory (DFT) with hyper-generalized gradient approximation (GGA). The optimized geometries show that one Au atom added to the Aun-1C cluster is the dominant growth pattern for the AunC clusters. In contrast to the pure gold clusters, the AunC clusters are most stable in a quasi-planar or three-dimensional (3D) structure because the C dopant induces the local non-planarity, with exceptions of the Au6,8C clusters who have 2D structures. The analysis of the relative and electronic stabilities reveals that the Au4C and Au6 clusters are the most stable in the series of studied clusters, respectively. In addition, a natural bond orbital (NBO) analysis shows that the charges in the AunC clusters transfer from the Aun host to the C atom. Moreover, the Au and C atoms interact with each other mostly via covalent bond rather than ionic bond, which can be confirmed through the average ionic character of the Au-C bond. Meanwhile, the charges mainly transfer between 2s and 2p orbitals within the C atom, and among 5d, 6s, and 6p orbitals within the Au atom for the AunC clusters. As for the magnetic properties of the AunC clusters, the total magnetic moments are 1 μB for n = odd clusters, with the total magnetic moments mainly locating on the C atoms for Au1,3,9C and on the Aun host for Au5,7C clusters. However, the total magnetic moments of the AunC clusters are zero for n = even clusters. Simultaneously, the magnetic moments mainly locate on the 2p orbital within the C atom and on the 5d, 6s orbitals within the Au atom.
Orbital-optimized density cumulant functional theory
Sokolov, Alexander Yu. Schaefer, Henry F.
2013-11-28
In density cumulant functional theory (DCFT) the electronic energy is evaluated from the one-particle density matrix and two-particle density cumulant, circumventing the computation of the wavefunction. To achieve this, the one-particle density matrix is decomposed exactly into the mean-field (idempotent) and correlation components. While the latter can be entirely derived from the density cumulant, the former must be obtained by choosing a specific set of orbitals. In the original DCFT formulation [W. Kutzelnigg, J. Chem. Phys. 125, 171101 (2006)] the orbitals were determined by diagonalizing the effective Fock operator, which introduces partial orbital relaxation. Here we present a new orbital-optimized formulation of DCFT where the energy is variationally minimized with respect to orbital rotations. This introduces important energy contributions and significantly improves the description of the dynamic correlation. In addition, it greatly simplifies the computation of analytic gradients, for which expressions are also presented. We offer a perturbative analysis of the new orbital stationarity conditions and benchmark their performance for a variety of chemical systems.
Optimization of constrained density functional theory
NASA Astrophysics Data System (ADS)
O'Regan, David D.; Teobaldi, Gilberto
2016-07-01
Constrained density functional theory (cDFT) is a versatile electronic structure method that enables ground-state calculations to be performed subject to physical constraints. It thereby broadens their applicability and utility. Automated Lagrange multiplier optimization is necessary for multiple constraints to be applied efficiently in cDFT, for it to be used in tandem with geometry optimization, or with molecular dynamics. In order to facilitate this, we comprehensively develop the connection between cDFT energy derivatives and response functions, providing a rigorous assessment of the uniqueness and character of cDFT stationary points while accounting for electronic interactions and screening. In particular, we provide a nonperturbative proof that stable stationary points of linear density constraints occur only at energy maxima with respect to their Lagrange multipliers. We show that multiple solutions, hysteresis, and energy discontinuities may occur in cDFT. Expressions are derived, in terms of convenient by-products of cDFT optimization, for quantities such as the dielectric function and a condition number quantifying ill definition in multiple constraint cDFT.
Recent progress in density functional theory
NASA Astrophysics Data System (ADS)
Truhlar, Donald
2014-03-01
Ongoing work involves several areas of density functional theory: new methods for computing electronic excitation energies, including a new way to remove spin contamination in the spin-flip Tamm-Dancoff approximation and a configuration-interaction-corrected Tamm-Dancoff Approximation for treating conical intersections; new ways to treat open-shell states, including a reinterpreted broken-symmetry method and multi-configuration Kohn-Sham theory; a new exchange-correlation functional; new tests of density functional theory against databases for electronic transition energies and molecules and solids containing metal atoms; and applications. A selection of results will be presented. I am grateful to the following collaborators for contributions to the ongoing work: Boris Averkiev, Rebecca Carlson, Laura Fernandez, Laura Gagliardi, Chad Hoyer, Francesc Illas, Miho Isegawa, Shaohong Li, Giovanni Li Manni, Sijie Luo, Dongxia Ma, Remi Maurice, Rubén Means-Pañeda, Roberto Peverati, Nora Planas, Prasenjit Seal, Pragya Verma, Bo Wang, Xuefei Xu, Ke R. Yang, Haoyu Yu, Wenjing Zhang, and Jingjing Zheng. Supported in part by the AFOSR and U.S. DOE.
Electron density measurement by differential interferometry
Ding, W. X.; Brower, D. L.; Deng, B. H.; Yates, T.
2006-10-15
A novel differential interferometer is being developed to measure the electron density gradient and its fluctuations. Two separate laser beams with slight spatial offset and frequency difference are coupled into a single mixer making a heterodyne measurement of the phase difference which is <1% of the total phase change experienced by each beam separately. This measure of the differential phase is made at multiple spatial points and can be inverted directly to provide the local density distribution.
NASA Astrophysics Data System (ADS)
Ryu, K.; Lee, E.; Chae, J. S.; Parrot, M.; Pulinets, S.
2014-10-01
We report the processes and results of statistical analysis on the ionospheric electron density data measured by the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite over a period of 6 years (2005-2010), in order to investigate the correlation between seismic activity and equatorial plasma density variations. To simplify the analysis, three equatorial regions with frequent earthquakes were selected and then one-dimensional time series analysis between the daily seismic activity indices and the equatorial ionization anomaly (EIA) intensity indices, which represent relative equatorial electron density increase, were performed for each region. The statistically significant values of the lagged cross-correlation function, particularly in the region with minimal effects of longitudinal asymmetry, indicate that some of the very large earthquakes with M > 5.0 in the low-latitude region can accompany observable precursory and concurrent EIA enhancements, even though the seismic activity is not the most significant driver of the equatorial ionospheric evolution. The physical mechanisms of the seismo-ionospheric coupling is consistent with our observation, and the possibility of earthquake prediction using the EIA intensity variation is discussed.
Conradie, Jeanet; Patra, Ashis K; Harrop, Todd C; Ghosh, Abhik
2015-02-16
Density functional theory (in the form of the PW91, BP86, OLYP, and B3LYP exchange-correlation functionals) has been used to map out the low-energy states of a series of eight-coordinate square-antiprismatic (D2d) first-row transition metal complexes, involving Mn(II), Fe(II), Co(II), Ni(II), and Cu(II), along with a pair of tetradentate N4 ligands. Of the five complexes, the Mn(II) and Fe(II) complexes have been synthesized and characterized structurally and spectroscopically, whereas the other three are as yet unknown. Each N4 ligand consists of a pair of terminal imidazole units linked by an o-phenylenediimine unit. The imidazole units are the strongest ligands in these complexes and dictate the spatial disposition of the metal three-dimensional orbitals. Thus, the dx(2)-y(2) orbital, whose lobes point directly at the coordinating imidazole nitrogens, has the highest orbital energy among the five d orbitals, whereas the dxy orbital has the lowest orbital energy. In general, the following orbital ordering (in order of increasing orbital energy) was found to be operative: dxy < dxz = dyz ≤ dz(2) < dx(2)-y(2). The square-antiprism geometry does not lead to large energy gaps between the d orbitals, which leads to an S = 2 ground state for the Fe(II) complex. Nevertheless, the dxy orbital has significantly lower energy relative to that of the dxz and dyz orbitals. Accordingly, the ground state of the Fe(II) complex corresponds unambiguously to a dxy(2)dxz(1)dyz(1)dz(2)(1)dx(2)-y(2)(1) electronic configuration. Unsurprisingly, the Mn(II) complex has an S = 5/2 ground state and no low-energy d-d excited states within 1.0 eV of the ground state. The Co(II) complex, on the other hand, has both a low-lying S = 1/2 state and multiple low-energy S = 3/2 states. Very long metal-nitrogen bonds are predicted for the Ni(II) and Cu(II) complexes; these bonds may be too fragile to survive in solution or in the solid state, and the complexes may therefore not be isolable
NASA Astrophysics Data System (ADS)
Min, Byeong June; Jeong, Hae Kyung; Lee, ChangWoo
2015-08-01
We studied via plane wave pseudopotential total-energy calculations within the local spin density approximation (LSDA) the electronic and the structural properties of amino acids (alanine, glycine, and histidine) attached to graphene oxide (GO) by peptide bonding. The HOMO-LUMO gap, the Hirshfeld charges, and the equilibrium geometrical structures exhibit distinctive variations that depend on the species of the attached amino acid. The GO-amino acid system appears to be a good candidate for a biosensor.
Xu, Kang-Ming; Huang, Teng; Liu, Yi-Rong; Jiang, Shuai; Zhang, Yang; Lv, Yu-Zhou; Gai, Yan-Bo; Huang, Wei
2015-07-29
The geometries of gold clusters doped with two phosphorus atoms, (AunP-2, n = 1–8) were investigated using density functional theory (DFT) methods. Various two-dimensional (2D) and three-dimensional (3D) structures of the doped clusters were studied. The results indicate that the structures of dual-phosphorus-doped gold clusters exhibit large differences from those of pure gold clusters with small cluster sizes. In our study, as for Au6P-2, two cis–trans isomers were found. The global minimum of Au8P-2 presents a similar configuration to that of Au-20, a pyramid-shaped unit, and the potential novel optical and catalytic properties of this structure warrant further attention. The higher stability of AunP-2 clusters relative to Au-n+2 (n = 1–8) clusters was verified based on various energy parameters, and the results indicate that the phosphorus atom can improve the stabilities of the gold clusters. We then explored the evolutionary path of (n = 1–8) clusters. We found that AunP-2 clusters exhibit the 2D–3D structural transition at n = 6, which is much clearer and faster than that of pure gold clusters and single-phosphorus-doped clusters. The electronic properties of AunP-2 (n = 1–8) were then investigated. The photoelectron spectra provide additional fundamental information on the structures and molecular orbitals shed light on the evolution of AunP-2 (n = 1–8). Natural bond orbital (NBO) described the charge distribution in stabilizing structures and revealed the strong relativistic effects of the gold atoms.
NASA Astrophysics Data System (ADS)
Güler-Kılıç, Sümeyra; Kılıç, ćetin
2015-06-01
Semilocal and dispersion-corrected density-functional calculations have been performed to study the crystal structure, equation of state, and electronic structure of metal tellurohalides with chemical formula MeTeI where Me=Bi, Au, or Pd. A comparative investigation of the results of these calculations is conducted which reveals the role of van der Waals attraction. It is shown that the prediction of crystal structure of metal tellurohalides is systematically improved thanks to the inclusion of van der Waals dispersion. It is found for BiTeI and AuTeI that the energy versus volume curve is anomalously flat in the vicinity of equilibrium volume and the calculated equation of state has an excessively steep slope in the low-pressure region; these are also fixed in the dispersion-corrected calculations. Analysis based on the computation of the volume and axial compressibilities shows that predicting the anisotropy of BiTeI via the semilocal calculations yields an unrealistic result, whereas the results of dispersion-corrected calculations agree with the experimental compressibility data. Our calculations render that BiTeI (AuTeI) is a narrow band gap semiconductor with Rashba-type spin splitting at the band edges (with an indirect band gap) while PdTeI is a metal with relatively low density of states at the Fermi level. The band gaps of BiTeI and AuTeI obtained via semilocal (dispersion-corrected) calculations are found to be greater (smaller) than the respective experimental values, which is against (in line with) the expected trend. Similarly, the Rashba parameters of BiTeI are bracketed by the respective values obtained via semilocal and dispersion-corrected calculations, e.g., a larger value for the Rashba parameter αR is obtained in association with the reduction of the band gap caused by modification of the crystal structure owing to van der Waals attraction. Excellent agreement with the experimental Rashba parameters is obtained via interpolation of the
Ions in solution: Density corrected density functional theory (DC-DFT)
Kim, Min-Cheol; Sim, Eunji; Burke, Kieron
2014-05-14
Standard density functional approximations often give questionable results for odd-electron radical complexes, with the error typically attributed to self-interaction. In density corrected density functional theory (DC-DFT), certain classes of density functional theory calculations are significantly improved by using densities more accurate than the self-consistent densities. We discuss how to identify such cases, and how DC-DFT applies more generally. To illustrate, we calculate potential energy surfaces of HO·Cl{sup −} and HO·H{sub 2}O complexes using various common approximate functionals, with and without this density correction. Commonly used approximations yield wrongly shaped surfaces and/or incorrect minima when calculated self consistently, while yielding almost identical shapes and minima when density corrected. This improvement is retained even in the presence of implicit solvent.
NASA Astrophysics Data System (ADS)
Guedj, C.; Hung, L.; Zobelli, A.; Blaise, P.; Sottile, F.; Olevano, V.
2014-12-01
The effect of nanocrystal orientation on the energy loss spectra of monoclinic hafnia (m-HfO2) is measured by high resolution transmission electron microscopy (HRTEM) and valence energy loss spectroscopy (VEELS) on high quality samples. For the same momentum-transfer directions, the dielectric properties are also calculated ab initio by time-dependent density-functional theory (TDDFT). Experiments and simulations evidence anisotropy in the dielectric properties of m-HfO2, most notably with the direction-dependent oscillator strength of the main bulk plasmon. The anisotropic nature of m-HfO2 may contribute to the differences among VEELS spectra reported in literature. The good agreement between the complex dielectric permittivity extracted from VEELS with nanometer spatial resolution, TDDFT modeling, and past literature demonstrates that the present HRTEM-VEELS device-oriented methodology is a possible solution to the difficult nanocharacterization challenges given in the International Technology Roadmap for Semiconductors.
Guedj, C.; Hung, L.; Sottile, F.; Zobelli, A.; Blaise, P.; Olevano, V.
2014-12-01
The effect of nanocrystal orientation on the energy loss spectra of monoclinic hafnia (m-HfO{sub 2}) is measured by high resolution transmission electron microscopy (HRTEM) and valence energy loss spectroscopy (VEELS) on high quality samples. For the same momentum-transfer directions, the dielectric properties are also calculated ab initio by time-dependent density-functional theory (TDDFT). Experiments and simulations evidence anisotropy in the dielectric properties of m-HfO{sub 2}, most notably with the direction-dependent oscillator strength of the main bulk plasmon. The anisotropic nature of m-HfO{sub 2} may contribute to the differences among VEELS spectra reported in literature. The good agreement between the complex dielectric permittivity extracted from VEELS with nanometer spatial resolution, TDDFT modeling, and past literature demonstrates that the present HRTEM-VEELS device-oriented methodology is a possible solution to the difficult nanocharacterization challenges given in the International Technology Roadmap for Semiconductors.
Local spin analyses using density functional theory
NASA Astrophysics Data System (ADS)
Abate, Bayileyegn; Peralta, Juan
Local spin analysis is a valuable technique in computational investigations magnetic interactions on mono- and polynuclear transition metal complexes, which play vital roles in catalysis, molecular magnetism, artificial photosynthesis, and several other commercially important materials. The relative size and complex electronic structure of transition metal complexes often prohibits the use of multi-determinant approaches, and hence, practical calculations are often limited to single-determinant methods. Density functional theory (DFT) has become one of the most successful and widely used computational tools for the electronic structure study of complex chemical systems; transition metal complexes in particular. Within the DFT formalism, a more flexible and complete theoretical modeling of transition metal complexes can be achieved by considering noncollinear spins, in which the spin density is 'allowed to' adopt noncollinear structures in stead of being constrained to align parallel/antiparallel to a universal axis of magnetization. In this meeting, I will present local spin analyses results obtained using different DFT functionals. Local projection operators are used to decompose the expectation value
A Robust High Current Density Electron Gun
NASA Astrophysics Data System (ADS)
Mako, F.; Peter, W.; Shiloh, J.; Len, L. K.
1996-11-01
Proof-of-principle experiments are proposed to validate a new concept for a robust, high-current density Pierce electron gun (RPG) for use in klystrons and high brightness electron sources for accelerators. This rugged, long-life electron gun avoids the difficulties associated with plasma cathodes, thermionic emitters, and field emission cathodes. The RPG concept employs the emission of secondary electrons in a transmission mode as opposed to the conventional mode of reflection, i.e., electrons exit from the back face of a thin negative electron affinity (NEA) material, and in the same direction as the incident beam. Current amplification through one stage of a NEA material could be over 50 times. The amplification is accomplished in one or more stages consisting of one primary emitter and one or more secondary emitters. The primary emitter is a low current density robust emitter (e.g., thoriated tungsten). The secondary emitters are thin NEA electrodes which emit secondary electrons in the same direction as the incident beam. Specific application is targeted for a klystron gun to be used by SLAC with a cold cathode at 30-40 amps/cm^2 output from the secondary emission stage, a ~2 μs pulse length, and ~200 pulses/second.
Bachler, Vinzenz
2009-10-01
The electronic spectrum of the neutral nickel complex [Ni(LISQ)2] (LISQ = 3,5-di-tert-butyl-o-diiminobenzosemiquinonate(1-)) and the spectra of its anion and dication have been calculated by means of time-dependent density functional theory. The electronic ground state of the neutral complex exhibits an open shell singlet diradical character. The mandatory multireference problem for this electronic ground state has been treated approximately by using the unrestricted and spin symmetry broken Kohn-Sham Slater determinant as the wave function for the noninteracting reference system in the time-dependent density functional calculations. A reasonable agreement with observed transition energies and band intensities has been achieved. This holds also for the long wavelength transitions that are shown to be of charge transfer type. The charge distributions in the electronic ground state and the corresponding low lying excited states, however, are rather similar. Thus, the known failure of standard time-dependent density functional theory to describe improperly long range charge transfer transitions is absent in this work. The applied computational scheme might be adequate for calculating electronic spectra of transition metal complexes with noninnocent ligands. PMID:19229945
Constrained Density Functional Calculations of alpha and delta Pu
NASA Astrophysics Data System (ADS)
Eriksson, Olle
2002-03-01
The electronic structure of α and δ Pu are described using a modified density functional theory that incorporates localization effects of the 5f shell. It is argued that a Russel-Saunders coupled state involving a 5f^4 multiplet, together with one itinerant 5f electron explains most of the observed ground state properties of δ Pu (equilibrium volume, elastic constants, near degeneracy with the alpha phase). This 5f electrons in the α phase are argued to form itinerant states, that are well described in density functional theory. The two distinctly different electronic ground states give rise to different excitation spectra and a comparison with experimental data is made.
Teaching Chemistry with Electron Density Models
NASA Astrophysics Data System (ADS)
Shusterman, Gwendolyn P.; Shusterman, Alan J.
1997-07-01
Linus Pauling once said that a topic must satisfy two criteria before it can be taught to students. First, students must be able to assimilate the topic within a reasonable amount of time. Second, the topic must be relevant to the educational needs and interests of the students. Unfortunately, the standard general chemistry textbook presentation of "electronic structure theory", set as it is in the language of molecular orbitals, has a difficult time satisfying either criterion. Many of the quantum mechanical aspects of molecular orbitals are too difficult for most beginning students to appreciate, much less master, and the few applications that are presented in the typical textbook are too limited in scope to excite much student interest. This article describes a powerful new method for teaching students about electronic structure and its relevance to chemical phenomena. This method, which we have developed and used for several years in general chemistry (G.P.S.) and organic chemistry (A.J.S.) courses, relies on computer-generated three-dimensional models of electron density distributions, and largely satisfies Pauling's two criteria. Students find electron density models easy to understand and use, and because these models are easily applied to a broad range of topics, they successfully convey to students the importance of electronic structure. In addition, when students finally learn about orbital concepts they are better prepared because they already have a well-developed three-dimensional picture of electronic structure to fall back on. We note in this regard that the types of models we use have found widespread, rigorous application in chemical research (1, 2), so students who understand and use electron density models do not need to "unlearn" anything before progressing to more advanced theories.
Improving Density Functionals with Quantum Harmonic Oscillators
NASA Astrophysics Data System (ADS)
Tkatchenko, Alexandre
2013-03-01
Density functional theory (DFT) is the most widely used and successful approach for electronic structure calculations. However, one of the pressing challenges for DFT is developing efficient functionals that can accurately capture the omnipresent long-range electron correlations, which determine the structure and stability of many molecules and materials. Here we show that, under certain conditions, the problem of computing the long-range correlation energy of interacting electrons can be mapped to a system of coupled quantum harmonic oscillators (QHOs). The proposed model allows us to synergistically combine concepts from DFT, quantum chemistry, and the widely discussed random-phase approximation for the correlation energy. In the dipole limit, the interaction energy for a system of coupled QHOs can be calculated exactly, thereby leading to an efficient and accurate model for the many-body dispersion energy of complex molecules and materials. The studied examples include intermolecular binding energies, the conformational hierarchy of DNA structures, the geometry and stability of molecular crystals, and supramolecular host-guest complexes (A. Tkatchenko, R. A. DiStasio Jr., R. Car, M. Scheffler, Phys. Rev. Lett. 108, 236402 (2012); R. A. DiStasio Jr., A. von Lilienfeld, A. Tkatchenko, PNAS 109, 14791 (2012); A. Tkatchenko, D. Alfe, K. S. Kim, J. Chem. Theory and Comp. (2012), doi: 10.1021/ct300711r; A. Tkatchenko, A. Ambrosetti, R. A. DiStasio Jr., arXiv:1210.8343v1).
Makarewicz, Emilia; Gordon, Agnieszka J; Berski, Slawomir
2015-03-19
Topological analysis of the electron localization function (ELF) has been carried out for the AuNgX (Ng = Ar, Kr, Xe; X = F, Cl, Br, I) molecules using the wave function approximated by the CCSD, MP2, and DFT(B3LYP, M062X) methods including zero-order regular approximation (ZORA). In the Ng-F bond, the bonding disynaptic attractor V(Ng,F) is missing; therefore, there are no signs of the covalent binding. The nature of the Au-Ng bond depends on the computational method used. Analysis of the ELF carried out for the AuArF and AuXeF molecules, with the wave function approximated by the CCSD and MP2 methods, shows the V(Au,Ng) attractor possibly corresponding to a partially covalent binding between the gold and noble gas atom. However, its very small basin population (<1e) and a very large value of the variance of the basin population suggest that the Au-Ng bond has a very delocalized character. Such bond nature may be related to the charge shift concept with a resonance of the Au(-+)NgX, Au(+-)NgX hybrids. The weakest Au-Ng bond, in terms of the smallest amount of electron density for the V(Au,Ng) basin, is found for the AuKrF molecule with the CCSD method (0.13e). The MP2 method, however, does not yield any V(Au, Ng) population; hence, the covalent Au-Kr bond is not confirmed. Because the V(Au,Ng) attractor is also not observed with the DFT method, the proper characterization of the Au-Ng bond requires proper description of correlation effects. Additional studies on the Au2 and [AuXe](+) molecules, performed at the CCSD and B3LYP levels, exhibit no V(Au,Au) and V(Au,Xe) bonding basins either. PMID:25266645
Koitz, Ralph; Seitsonen, Ari P; Iannuzzi, Marcella; Hutter, Jürg
2013-06-21
Hexagonal boron nitride (h-BN) adsorbed on metal surfaces shows great promise for applications in nanoscience. Depending on the nature of the substrate, effects such as an extended corrugation of the monolayer can be observed and utilized, e.g. for the patterning of adsorbed molecules. Here we present an in-depth computational study of the structural and electronic properties of a 6 nm Moiré pattern formed by a rotated layer of h-BN on a Cu(111) surface. In contrast to related systems, the h-BN layer undergoes only minute structural changes upon adsorption. Nevertheless, the projected density of states at various atoms in the cell and the electrostatic potential above the surface are periodically modulated, leading to the experimentally observed electronic corrugation. We rationalize this observation with the variation in adsorption registry resulting in periodic changes of the lateral, rather than vertical, h-BN-Cu distances. PMID:23681111
Duan, Yuhua; Parlinski, K.
2011-01-01
The structural, electronic, lattice dynamical, optical, thermodynamic, and CO{sub 2} capture properties of monoclinic and triclinic phases of Li{sub 4}SiO{sub 4} are investigated by combining density functional theory with phonon lattice dynamics calculations. We found that these two phases have some similarities in their bulk and thermodynamic properties. The calculated bulk modulus and the cohesive energies of these two phases are close to each other. Although both of them are insulators, the monoclinic phase of Li{sub 4}SiO{sub 4} has a direct band gap of 5.24 eV while the triclinic Li{sub 4}SiO{sub 4} phase has an indirect band gap of 4.98 eV. In both phases of Li{sub 4}SiO{sub 4}, the s orbital of O mainly contributes to the lower-energy second valence band (VB{sub 2}) and the p orbitals contribute to the fist valence band (VB{sub 1}) and the conduction bands (CBs). The s orbital of Si mainly contributes to the lower portions of the VB1 and VB{sub 2}, and Si p orbitals mainly contribute to the higher portions of the VB{sub 1} and VB{sub 2}. The s and p orbitals of Li contribute to both VBs and to CBs, and Li p orbitals have a higher contribution than the Li s orbital. There is possibly a phonon soft mode existing in triclinic {gamma}-Li{sub 4}SiO{sub 4}; in the monoclinic Li{sub 4}SiO{sub 4}, there are three phonon soft modes, which correspond to the one type of Li disordered over a few sites. Their LO-TO splitting indicates that both phases of Li{sub 4}SiO{sub 4} are polar anisotropic materials. The calculated infrared absorption spectra for LO and TO modes are different for these two phases of Li{sub 4}SiO{sub 4}. The calculated relationships of the chemical potential versus temperature and CO{sub 2} pressure for reaction of Li{sub 4}SiO{sub 4} with CO{sub 2} shows that Li{sub 4}SiO{sub 4} could be a good candidate for a high-temperature CO{sub 2} sorbent while used for postcombustion capture technology.
NASA Astrophysics Data System (ADS)
Duan, Yuhua; Parlinski, K.
2011-09-01
The structural, electronic, lattice dynamical, optical, thermodynamic, and CO2 capture properties of monoclinic and triclinic phases of Li4SiO4 are investigated by combining density functional theory with phonon lattice dynamics calculations. We found that these two phases have some similarities in their bulk and thermodynamic properties. The calculated bulk modulus and the cohesive energies of these two phases are close to each other. Although both of them are insulators, the monoclinic phase of Li4SiO4 has a direct band gap of 5.24 eV while the triclinic Li4SiO4 phase has an indirect band gap of 4.98 eV. In both phases of Li4SiO4, the s orbital of O mainly contributes to the lower-energy second valence band (VB2) and the p orbitals contribute to the fist valence band (VB1) and the conduction bands (CBs). The s orbital of Si mainly contributes to the lower portions of the VB1 and VB2, and Si p orbitals mainly contribute to the higher portions of the VB1 and VB2. The s and p orbitals of Li contribute to both VBs and to CBs, and Li p orbitals have a higher contribution than the Li s orbital. There is possibly a phonon soft mode existing in triclinic γ-Li4SiO4; in the monoclinic Li4SiO4, there are three phonon soft modes, which correspond to the one type of Li disordered over a few sites. Their LO-TO splitting indicates that both phases of Li4SiO4 are polar anisotropic materials. The calculated infrared absorption spectra for LO and TO modes are different for these two phases of Li4SiO4. The calculated relationships of the chemical potential versus temperature and CO2 pressure for reaction of Li4SiO4 with CO2 shows that Li4SiO4 could be a good candidate for a high-temperature CO2 sorbent while used for postcombustion capture technology.
Joshi, K.B.; Paliwal, U.; Galav, K.L.; Trivedi, D.K.; Bredow, T.
2013-08-15
Stability of B1 and B2 phases of Mg{sub x}Cd{sub 1−x}O is studied by calculating the formation energy within the framework of density functional theory applying the crystalline-orbital program package. Structural and electronic properties of the two polymorphs are reported for x=0.25, 0.50 and 0.75. The equilibrium lattice constants and bulk moduli are computed. Enthalpy calculations show pressure induced B1→B2 phase transitions at 92 GPa, 138 GPa and 212 GPa, respectively, for Mg{sub 0.25}Cd{sub 0.75}O, Mg{sub 0.50}Cd{sub 0.50}O and Mg{sub 0.75}Cd{sub 0.25}O compositions. Formation energy of ternary oxides in the B1 phase is negative with respect to mixing of B2-MgO with B1-CdO. Mixing B1-MgO with B2-CdO also leads to negative formation energy in Cd rich B1 phase ternary oxides (0≤x≤0.5). Band structure calculations predict direct band gaps in the B1 phase and indirect band gaps in the B2 phase ternary oxides. Mulliken population analysis is performed for the two polymorphs to study the charge transfer. - Graphical abstract: Diagram reveals trends in formation energy while mixing B2-MgO with B1-CdO to form B1-Mg{sub x}Cd{sub 1−x}O. Formation energies obtained from mixing isostructural and nonisostructural components are also shown. Display Omitted - Highlights: • Lattice constants and bulk moduli are computed for Mg{sub 0.25}Cd{sub 0.75}O, Mg{sub 0.50}Cd{sub 0.50}O and Mg{sub 0.75}Cd{sub 0.25}O compositions. • Enthalpy calculations signify pressure induced B1→B2 phase transitions at 92 GPa, 138 GPa and 212 GPa, respectively, in Mg{sub 0.25}Cd{sub 0.75}O, Mg{sub 0.50}Cd{sub 0.50}O and Mg{sub 0.75}Cd{sub 0.25}O. • Band structure calculations predict direct band gaps in the B1 phase ternary oxides. • In the B2 phase ternary oxides band structure calculations show valence band maximum along the Γ–X direction and the conduction band minimum at the Γ point of symmetry.
Electron energy-distribution functions in gases
Pitchford, L.C.
1981-01-01
Numerical calculation of the electron energy distribution functions in the regime of drift tube experiments is discussed. The discussion is limited to constant applied fields and values of E/N (ratio of electric field strength to neutral density) low enough that electron growth due to ionization can be neglected. (GHT)
NASA Astrophysics Data System (ADS)
Kim, Namhoon; Martin, Pamela Peña; Rockett, Angus A.; Ertekin, Elif
2016-04-01
We present a systematic assessment of the structural properties, the electronic density of states, the charge densities, and the phase stabilities of AgInSe2 and AuInSe2 using screened-exchange hybrid density functional theory, and compare their properties to those of CuInSe2. For AgInSe2, hybrid density functional theory properly captures several experimentally measured properties, including the increase in the band gap and the change in the direction of the lattice distortion parameter u in comparison to CuInSe2. While the electronic properties of AuInSe2 have not yet been experimentally characterized, we predict it to be a small gap (≈0.15 eV) semiconductor. We also present the phase stability of AgInSe2 and AuInSe2 according to screened-exchange density functional theory, and compare the results to predictions from conventional density functional theory, results tabulated from several online materials data repositories, and experiment (when available). In comparison to conventional density functional theory, the hybrid functional predicts phase stabilities of AgInSe2 in better agreement with experiment: discrepancies in the calculated formation enthalpies are reduced by approximately a factor of 3, from ≈0.20 eV/atom to ≈0.07 eV/atom, similar to the improvement observed for CuInSe2. We further predict that AuInSe2 is not a stable phase, and can only be present under nonequilibrium conditions.
Levy, Mel E-mail: mlevy@tulane.edu; Anderson, James S. M.; Zadeh, Farnaz Heidar; Ayers, Paul W. E-mail: mlevy@tulane.edu
2014-05-14
Properties of exact density functionals provide useful constraints for the development of new approximate functionals. This paper focuses on convex sums of ground-level densities. It is observed that the electronic kinetic energy of a convex sum of degenerate ground-level densities is equal to the convex sum of the kinetic energies of the individual degenerate densities. (The same type of relationship holds also for the electron-electron repulsion energy.) This extends a known property of the Levy-Valone Ensemble Constrained-Search and the Lieb Legendre-Transform refomulations of the Hohenberg-Kohn functional to the individual components of the functional. Moreover, we observe that the kinetic and electron-repulsion results also apply to densities with fractional electron number (even if there are no degeneracies), and we close with an analogous point-wise property involving the external potential. Examples where different degenerate states have different kinetic energy and electron-nuclear attraction energy are given; consequently, individual components of the ground state electronic energy can change abruptly when the molecular geometry changes. These discontinuities are predicted to be ubiquitous at conical intersections, complicating the development of universally applicable density-functional approximations.
The benchmark of gutzwiller density functional theory in hydrogen systems
Yao, Y.; Wang, Cai-Zhuang; Ho, Kai-Ming
2012-02-23
We propose an approximate form of the exchange-correlation energy functional for the Gutzwiller density functional theory. It satisfies certain physical constraints in both weak and strong electron correlation limits. We benchmark the Gutzwiller density functional approximation in the hydrogen systems, where the static correlation error is shown to be negligible. The good transferability is demonstrated by applications to the hydrogen molecule and some crystal structures.
The Benchmark of Gutzwiller Density Functional Theory in Hydrogen Systems
Yao, Yongxin; Wang, Cai-Zhuang; Ho, Kai-Ming
2011-01-13
We propose an approximate form of the exchange-correlation energy functional for the Gutzwiller density functional theory. It satisfies certain physical constraints in both weak and strong electron correlation limits. We benchmark the Gutzwiller density functional approximation in the hydrogen systems, where the static correlation error is shown to be negligible. The good transferability is demonstrated by applications to the hydrogen molecule and some crystal structures. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Sun, Haitao; Ryno, Sean; Zhong, Cheng; Ravva, Mahesh Kumar; Sun, Zhenrong; Körzdörfer, Thomas; Brédas, Jean-Luc
2016-06-14
We propose a new methodology for the first-principles description of the electronic properties relevant for charge transport in organic molecular crystals. This methodology, which is based on the combination of a nonempirical, optimally tuned range-separated hybrid functional with the polarizable continuum model, is applied to a series of eight representative molecular semiconductor crystals. We show that it provides ionization energies, electron affinities, and transport gaps in very good agreement with experimental values, as well as with the results of many-body perturbation theory within the GW approximation at a fraction of the computational costs. Hence, this approach represents an easily applicable and computationally efficient tool to estimate the gas-to-crystal phase shifts of the frontier-orbital quasiparticle energies in organic electronic materials. PMID:27183355
Electron Density Profiles of the Topside Ionosphere
NASA Technical Reports Server (NTRS)
Huang, Xue-Qin; Reinsch, Bodo W.; Bilitza, Dieter; Benson, Robert F.
2002-01-01
The existing uncertainties about the electron density profiles in the topside ionosphere, i.e., in the height region from h,F2 to - 2000 km, require the search for new data sources. The ISIS and Alouette topside sounder satellites from the sixties to the eighties recorded millions of ionograms but most were not analyzed in terms of electron density profiles. In recent years an effort started to digitize the analog recordings to prepare the ionograms for computerized analysis. As of November 2001 about 350000 ionograms have been digitized from the original 7-track analog tapes. These data are available in binary and CDF format from the anonymous ftp site of the National Space Science Data Center. A search site and browse capabilities on CDAWeb assist the scientific usage of these data. All information and access links can be found at http://nssdc.gsfc.nasa.gov/space/isis/isis- status.htm1. This paper describes the ISIS data restoration effort and shows how the digital ionograms are automatically processed into electron density profiles from satellite orbit altitude (1400 km for ISIS-2) down to the F peak. Because of the large volume of data an automated processing algorithm is imperative. The TOPside Ionogram Scaler with True height algorithm TOPIST software developed for this task is successfully scaling - 70% of the ionograms. An <
Effective potential in density matrix functional theory.
Nagy, A; Amovilli, C
2004-10-01
In the previous paper it was shown that in the ground state the diagonal of the spin independent second-order density matrix n can be determined by solving a single auxiliary equation of a two-particle problem. Thus the problem of an arbitrary system with even electrons can be reduced to a two-particle problem. The effective potential of the two-particle equation contains a term v(p) of completely kinetic origin. Virial theorem and hierarchy of equations are derived for v(p) and simple approximations are proposed. A relationship between the effective potential u(p) of the shape function equation and the potential v(p) is established. PMID:15473719
A Safari Through Density Functional Theory
NASA Astrophysics Data System (ADS)
Dreizler, Reiner M.; Lüdde, Cora S.
Density functional theory is widely used to treat quantum many body problems in many areas of physics and related fields. A brief survey of this method covering foundations, functionals and applications is presented here.
Nonlinear eigenvalue problems in Density Functional Theory calculations
Fattebert, J
2009-08-28
Developed in the 1960's by W. Kohn and coauthors, Density Functional Theory (DFT) is a very popular quantum model for First-Principles simulations in chemistry and material sciences. It allows calculations of systems made of hundreds of atoms. Indeed DFT reduces the 3N-dimensional Schroedinger electronic structure problem to the search for a ground state electronic density in 3D. In practice it leads to the search for N electronic wave functions solutions of an energy minimization problem in 3D, or equivalently the solution of an eigenvalue problem with a non-linear operator.
Molecular dynamics simulation of liquid water: Hybrid density functionals
Todorova, T; Seitsonen, A; Hutter, J; Kuo, W; Mundy, C
2005-09-12
The structure, dynamical and electronic properties of liquid water utilizing different hybrid density functionals were tested within the plane wave framework of first principles molecular dynamics simulations. The computational approach, which employs modified functionals with short-ranged Hartree-Fock exchange, was first tested in calculations of the structural and bonding properties of the water dimer and cyclic water trimer. Liquid water simulations were performed at the state point of 350 K at the experimental density. Simulations included three different hybrid functionals, a meta functional, four gradient corrected functionals, the local density and Hartree-Fock approximation. It is found that hybrid functionals are superior in reproducing the experimental structure and dynamical properties as measured by the radial distribution function and self diffusion constant when compared to the pure density functionals. The local density and Hartree-Fock approximations show strongly over- and under-structured liquids, respectively. Hydrogen bond analysis shows that the hybrid functionals give slightly smaller averaged numbers of hydrogen bonds and similar hydrogen bond populations as pure density functionals. The average molecular dipole moments in the liquid from the three hybrid functionals are lower than from the corresponding pure density functionals.
Delgado, Juan C; Selsby, Ronald G
2013-01-01
The ground state configuration of the gas phase cationic dyes pinacyanol chloride and rhodamine B are optimized with HF/6-311 + G(2d,2p) method and basis set. B3PW91/6-311 + G(2df,2p) functional and basis set is used to calculate the Mulliken atom charge distribution, total molecular energy, the dipole moment, the vertical ionization potential, the adiabatic electron affinity and the lowest excited triplet state, the last three as an energy difference between separately calculated open shell and ground states. The triplet and extra electron states are optimized to find the relaxation energy. In the ground state optimization of both dyes the chloride anion migrates to a position near the center of the chromophore. For rhodamine B the benzoidal group turns perpendicular to the chromophore plane. For both dyes, the LUMO is mostly of π character associated with the aromatic part of the molecule containing the chromophore. The highest occupied MOs consist of three almost degenerate eigenvectors involving the chloride anion coordinated with σ electrons in the molecular framework. The fourth highest MO is of π character. For both molecules in the gas phase ionization process the chloride anion loses the significant fraction of electric charge. In electron capture, the excess charge goes mainly on the dye cation. PMID:22891949
Measuring ionospheric electron density using the plasma frequency probe
Jensen, M.D.; Baker, K.D. )
1992-02-01
During the past decade, the plasma frequency probe (PFP) has evolved into an accurate, proven method of measuring electron density in the ionosphere above about 90 km. The instrument uses an electrically short antenna mounted on a sounding rocket that is immersed in the plasma and notes the frequency where the antenna impedance is large and nonreactive. This frequency is closely related to the plasma frequency, which is a direct function of free electron concentration. The probe uses phase-locked loop technology to follow a changing electron density. Several sections of the plasma frequency probe circuitry are unique, especially the voltage-controlled oscillator that uses both an electronically tuned capacitor and inductor to give the wide tuning range needed for electron density measurements. The results from two recent sounding rocket flights (Thunderstorm II and CRIT II) under vastly different plasma conditions demonstrate the capabilities of the PFP and show the importance of in situ electron density measurements of understanding plasma processes. 9 refs.
Density functional theory for carbon dioxide crystal
Chang, Yiwen; Mi, Jianguo Zhong, Chongli
2014-05-28
We present a density functional approach to describe the solid−liquid phase transition, interfacial and crystal structure, and properties of polyatomic CO{sub 2}. Unlike previous phase field crystal model or density functional theory, which are derived from the second order direct correlation function, the present density functional approach is based on the fundamental measure theory for hard-sphere repulsion in solid. More importantly, the contributions of enthalpic interactions due to the dispersive attractions and of entropic interactions arising from the molecular architecture are integrated in the density functional model. Using the theoretical model, the predicted liquid and solid densities of CO{sub 2} at equilibrium triple point are in good agreement with the experimental values. Based on the structure of crystal-liquid interfaces in different planes, the corresponding interfacial tensions are predicted. Their respective accuracies need to be tested.
Electronic structure and electron momentum density in TiSi
NASA Astrophysics Data System (ADS)
Ghaleb, A. M.; Mohammad, F. M.; Sahariya, Jagrati; Sharma, Mukesh; Ahuja, B. L.
2013-03-01
We report the electron momentum density in titanium monosilicide using 241Am Compton spectrometer. Experimental Compton profile has been compared with the theoretical profiles computed using linear combination of atomic orbitals (LCAO). The energy bands, density of states and Fermi surface structures of TiSi are reported using the LCAO and the full potential linearized augmented plane wave methods. Theoretical anisotropies in directional Compton profiles are interpreted in terms of energy bands. To confirm the conducting behavior, we also report the real space analysis of experimental Compton profile of TiSi.
Electron temperature and density measurements of laser induced germanium plasma
NASA Astrophysics Data System (ADS)
Shakeel, Hira; Arshad, Saboohi; Haq, S. U.; Nadeem, Ali
2016-05-01
The germanium plasma produced by the fundamental harmonics (1064 nm) of Nd:YAG laser in single and double pulse configurations have been studied spectroscopically. The plasma is characterized by measuring the electron temperature using the Boltzmann plot method for neutral and ionized species and electron number density as a function of laser irradiance, ambient pressure, and distance from the target surface. It is observed that the plasma parameters have an increasing trend with laser irradiance (9-33 GW/cm2) and with ambient pressure (8-250 mbar). However, a decreasing trend is observed along the plume length up to 4.5 mm. The electron temperature and electron number density are also determined using a double pulse configuration, and their behavior at fixed energy ratio and different interpulse delays is discussed.
Accurate density functional thermochemistry for larger molecules.
Raghavachari, K.; Stefanov, B. B.; Curtiss, L. A.; Lucent Tech.
1997-06-20
Density functional methods are combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. Seven different density functionals are assessed for the evaluation of heats of formation, Delta H 0 (298 K), for a test set of 40 molecules composed of H, C, O and N. The use of bond separation energies results in a dramatic improvement in the accuracy of all the density functionals. The B3-LYP functional has the smallest mean absolute deviation from experiment (1.5 kcal mol/f).
Scarborough, Christopher C; Sproules, Stephen; Weyhermüller, Thomas; DeBeer, Serena; Wieghardt, Karl
2011-12-19
The electron transfer series of complexes [Cr((t)bpy)(3)](n)(PF(6))(n) (n = 3+, 2+, 1+, 0 (1-4)) has been synthesized and the molecular structures of 1, 2, and 3 have been determined by single-crystal X-ray crystallography; the structure of 4 has been investigated using extended X-ray absorption fine structure (EXAFS) analysis. Magnetic susceptibility measurements (4-300 K) established an S = 3/2 ground state for 1, an S = 1 ground state for 2, an S = 1/2 ground state for 3, and an S = 0 ground state for 4. The electrochemistry of this series in CH(3)CN solution exhibits three reversible one-electron transfer waves. UV-vis/NIR spectra and Cr K-edge X-ray absorption spectra (XAS) are reported. The same experimental techniques have been applied for [Cr(III)(tacn)(2)]Br(3)·5H(2)O (5) and [Cr(II)(tacn)(2)]Cl(2) (6), which possess an S = 3/2 and an S = 2 ground state, respectively (tacn = 1,4,7-triazacyclononane, a tridentate, pure σ-donor ligand). The Cr K-edge XAS spectra of the corresponding complexes K(4)[Cr(II)(CN)(6)]·10H(2)O (S = 1) (7) and K(3)[Cr(III)(CN)(6)] (S = 3/2) (8) have also been recorded. All complexes have been studied computationally with density functional theory (DFT) using the B3LYP functional. The molecular and electronic structures of the anionic members of the series [Cr(bpy)(3)](1-,2-,3-) have also been calculated. It is unequivocally shown that all members of the electron transfer series 1-4 and [Cr(bpy)(3)](n) (n = 3+, 2+, 1+, 0, 1-, 2, 3-) possess a central Cr(III) ion ((t(2g))(3), S = 3/2). The three N,N'-coordinated neutral (bpy(0)) ligands in the trication 1 and [Cr(III)(bpy)(3)](3+) are one-electron reduced in a stepwise fashion to localized one, two, and three π-radical anions (bpy(•))(1-) in the dicationic, monocationic, and neutral species, respectively. Complexes 2 and [Cr(bpy)(3)](2+) cannot be described as low-spin Cr(II) species; they are in fact best described as [Cr(III)((t)bpy(•))((t)bpy(0))(2)](2+) and [Cr
Correlated quantum transport of density wave electrons.
Miller, J H; Wijesinghe, A I; Tang, Z; Guloy, A M
2012-01-20
Recently observed Aharonov-Bohm quantum interference of the period h/2e in charge density wave rings strongly suggests that correlated density wave electron transport is a cooperative quantum phenomenon. The picture discussed here posits that quantum solitons nucleate and transport current above a Coulomb blockade threshold field. We propose a field-dependent tunneling matrix element and use the Schrödinger equation, viewed as an emergent classical equation as in Feynman's treatment of Josephson tunneling, to compute the evolving macrostate amplitudes, finding excellent quantitative agreement with voltage oscillations and current-voltage characteristics in NbSe(3). A proposed phase diagram shows the conditions favoring soliton nucleation versus classical depinning. PMID:22400766
Density Functional Theory with Dissipation: Transport through Single Molecules
Kieron Burke
2012-04-30
A huge amount of fundamental research was performed on this grant. Most of it focussed on fundamental issues of electronic structure calculations of transport through single molecules, using density functional theory. Achievements were: (1) First density functional theory with dissipation; (2) Pseudopotential plane wave calculations with master equation; (3) Weak bias limit; (4) Long-chain conductance; and (5) Self-interaction effects in tunneling.
Electron Density Calibration for Radiotherapy Treatment Planning
Herrera-Martinez, F.; Rodriguez-Villafuerte, M.; Martinez-Davalos, A.; Ruiz-Trejo, C.; Celis-Lopez, M. A.; Larraga-Gutierrez, J. M.; Garcia-Garduno, A.
2006-09-08
Computed tomography (CT) images are used as basic input data for most modern radiosurgery treatment planning systems (TPS). CT data not only provide anatomic information to delineate target volumes, but also allow the introduction of corrections for tissue inhomogeneities into dose calculations during the treatment planning procedure. These corrections involve the determination of a relationship between tissue electron density ({rho}e) and their corresponding Hounsfield Units (HU). In this work, an elemental analysis of different commercial tissue equivalent materials using Scanning Electron Microscopy was carried out to characterize their chemical composition. The tissue equivalent materials were chosen to ensure a large range of {rho}e to be included in the CT scanner calibration. A phantom was designed and constructed with these materials to simulate the size of a human head.
Symmetry measures of the electron density.
Casanova, David; Alemany, Pere; Alvarez, Santiago
2010-10-01
In this communication we define electronic symmetry operation and symmetry group measures, eSOM and eSGM, respectively, develop the basic algorithms to obtain them, and give some examples of the possible applications of these new computational tools. These new symmetry measures based on the electron density have been tested in an analysis of (a) the inversion symmetry for heteronuclear diatomic molecules, for the eclipsed and staggered conformations of ethane and tetrafluoroethane, and for a series of octahedral sulfur halides; (b) the reflection symmetry of three different conformers of tetrafluoroethene; and (c) the loss of C(6) symmetry along the B(2u) distortion mode of benzene and an analysis of rotational symmetry for different six-member ring heterocycles. PMID:20652983
Electron Density Calibration for Radiotherapy Treatment Planning
NASA Astrophysics Data System (ADS)
Herrera-Martínez, F.; Rodríguez-Villafuerte, M.; Martínez-Dávalos, A.; Ruiz-Trejo, C.; Celis-López, M. A.; Lárraga-Gutiérrez, J. M.; García-Garduño, A.
2006-09-01
Computed tomography (CT) images are used as basic input data for most modern radiosurgery treatment planning systems (TPS). CT data not only provide anatomic information to delineate target volumes, but also allow the introduction of corrections for tissue inhomogeneities into dose calculations during the treatment planning procedure. These corrections involve the determination of a relationship between tissue electron density (ρe) and their corresponding Hounsfield Units (HU). In this work, an elemental analysis of different commercial tissue equivalent materials using Scanning Electron Microscopy was carried out to characterize their chemical composition. The tissue equivalent materials were chosen to ensure a large range of ρe to be included in the CT scanner calibration. A phantom was designed and constructed with these materials to simulate the size of a human head.
Reanalysis of relativistic electron phase space density
NASA Astrophysics Data System (ADS)
Shprits, Yuri; Chen, Yue; Kondrashov, Dmitri
In this study we perform a reanalysis of the sparse relativistic electron data using a relatively simple one-dimensional radial diffusion model and a Kalman filtering approach. The results of the reanalysis clearly show pronounced peaks in the electron phase space density (PSD), which can not be explained by the variations in the outer boundary, and can only be produced by a local acceleration processes. The location of the innovation vector shows that local acceleration is most efficient at L* = 5.5. To verify that our results are not affected by the limitations of the satellite orbit and coverage, we performed an "identical twin" experiments with synthetic data specified only at the locations for which CRRES observations are available. Our results indicate that the model with data assimilation can accurately reproduce the underlying structure of the PSD even when data is sparse.
Photostriction in Ferroelectrics from Density Functional Theory
NASA Astrophysics Data System (ADS)
Paillard, Charles; Xu, Bin; Dkhil, Brahim; Geneste, Grégory; Bellaiche, L.
2016-06-01
An ab initio procedure allowing the computation of the deformation of ferroelectric-based materials under light is presented. This numerical scheme consists in structurally relaxing the system under the constraint of a fixed ne concentration of electrons photoexcited into a specific conduction band edge state from a chosen valence band state, via the use of a constrained density functional theory method. The resulting change in lattice constant along a selected crystallographic direction is then calculated for a reasonable estimate of ne. This method is applied to bulk multiferroic BiFeO3 and predicts a photostriction effect of the same order of magnitude than the ones recently observed. A strong dependence of photostrictive response on both the reached conduction state and the crystallographic direction (along which this effect is determined) is also revealed. Furthermore, analysis of the results demonstrates that the photostriction mechanism mostly originates from the screening of the spontaneous polarization by the photoexcited electrons in combination with the inverse piezoelectric effect.
Photostriction in Ferroelectrics from Density Functional Theory.
Paillard, Charles; Xu, Bin; Dkhil, Brahim; Geneste, Grégory; Bellaiche, L
2016-06-17
An ab initio procedure allowing the computation of the deformation of ferroelectric-based materials under light is presented. This numerical scheme consists in structurally relaxing the system under the constraint of a fixed n_{e} concentration of electrons photoexcited into a specific conduction band edge state from a chosen valence band state, via the use of a constrained density functional theory method. The resulting change in lattice constant along a selected crystallographic direction is then calculated for a reasonable estimate of n_{e}. This method is applied to bulk multiferroic BiFeO_{3} and predicts a photostriction effect of the same order of magnitude than the ones recently observed. A strong dependence of photostrictive response on both the reached conduction state and the crystallographic direction (along which this effect is determined) is also revealed. Furthermore, analysis of the results demonstrates that the photostriction mechanism mostly originates from the screening of the spontaneous polarization by the photoexcited electrons in combination with the inverse piezoelectric effect. PMID:27367406
Locality of correlation in density functional theory
NASA Astrophysics Data System (ADS)
Burke, Kieron; Cancio, Antonio; Gould, Tim; Pittalis, Stefano
2016-08-01
The Hohenberg-Kohn density functional was long ago shown to reduce to the Thomas-Fermi (TF) approximation in the non-relativistic semiclassical (or large-Z) limit for all matter, i.e., the kinetic energy becomes local. Exchange also becomes local in this limit. Numerical data on the correlation energy of atoms support the conjecture that this is also true for correlation, but much less relevant to atoms. We illustrate how expansions around a large particle number are equivalent to local density approximations and their strong relevance to density functional approximations. Analyzing highly accurate atomic correlation energies, we show that EC → -AC ZlnZ + BCZ as Z → ∞, where Z is the atomic number, AC is known, and we estimate BC to be about 37 mhartree. The local density approximation yields AC exactly, but a very incorrect value for BC, showing that the local approximation is less relevant for the correlation alone. This limit is a benchmark for the non-empirical construction of density functional approximations. We conjecture that, beyond atoms, the leading correction to the local density approximation in the large-Z limit generally takes this form, but with BC a functional of the TF density for the system. The implications for the construction of approximate density functionals are discussed.
Locality of correlation in density functional theory.
Burke, Kieron; Cancio, Antonio; Gould, Tim; Pittalis, Stefano
2016-08-01
The Hohenberg-Kohn density functional was long ago shown to reduce to the Thomas-Fermi (TF) approximation in the non-relativistic semiclassical (or large-Z) limit for all matter, i.e., the kinetic energy becomes local. Exchange also becomes local in this limit. Numerical data on the correlation energy of atoms support the conjecture that this is also true for correlation, but much less relevant to atoms. We illustrate how expansions around a large particle number are equivalent to local density approximations and their strong relevance to density functional approximations. Analyzing highly accurate atomic correlation energies, we show that EC → -AC ZlnZ + BCZ as Z → ∞, where Z is the atomic number, AC is known, and we estimate BC to be about 37 mhartree. The local density approximation yields AC exactly, but a very incorrect value for BC, showing that the local approximation is less relevant for the correlation alone. This limit is a benchmark for the non-empirical construction of density functional approximations. We conjecture that, beyond atoms, the leading correction to the local density approximation in the large-Z limit generally takes this form, but with BC a functional of the TF density for the system. The implications for the construction of approximate density functionals are discussed. PMID:27497544
NASA Astrophysics Data System (ADS)
Zhao, Run-Ning; Yuan, Yan-Hong; Han, Ju-Guang; Duan, Yuhua
2016-03-01
Geometries associated with relative stabilities and energy gaps of W@Bn (n = 20, 24, 28, 32) are systematically investigated by density functional theory. The calculated averaged atomic binding energies reveal that the W@B20 has enhanced stability over other clusters. Interestingly, the irregular W@B24 fullerene with bigger HOMO-LUMO gap is supposed to have stronger chemical activity. Moreover, the interactions between W and B24 cage is strongest one based upon the calculated binding energy between W and B cage, the doped W changes the properties of pure cages. The calculated dipoles of W@Bn reveal that the irregular W@B24 cage is a nonpolar molecule.
Electron density measurements for plasma adaptive optics
NASA Astrophysics Data System (ADS)
Neiswander, Brian W.
Over the past 40 years, there has been growing interest in both laser communications and directed energy weapons that operate from moving aircraft. As a laser beam propagates from an aircraft in flight, it passes through boundary layers, turbulence, and shear layers in the near-region of the aircraft. These fluid instabilities cause strong density gradients which adversely affect the transmission of laser energy to a target. Adaptive optics provides corrective measures for this problem but current technology cannot respond quickly enough to be useful for high speed flight conditions. This research investigated the use of plasma as a medium for adaptive optics for aero-optics applications. When a laser beam passes through plasma, its phase is shifted proportionally to the electron density and gas heating within the plasma. As a result, plasma can be utilized as a dynamically controllable optical medium. Experiments were carried out using a cylindrical dielectric barrier discharge plasma chamber which generated a sub-atmospheric pressure, low-temperature plasma. An electrostatic model of this design was developed and revealed an important design constraint relating to the geometry of the chamber. Optical diagnostic techniques were used to characterize the plasma discharge. Single-wavelength interferometric experiments were performed and demonstrated up to 1.5 microns of optical path difference (OPD) in a 633 nm laser beam. Dual-wavelength interferometry was used to obtain time-resolved profiles of the plasma electron density and gas heating inside the plasma chamber. Furthermore, a new multi-wavelength infrared diagnostic technique was developed and proof-of-concept simulations were conducted to demonstrate the system's capabilities.
Rationale for switching to nonlocal functionals in density functional theory
NASA Astrophysics Data System (ADS)
Lazić, P.; Atodiresei, N.; Caciuc, V.; Brako, R.; Gumhalter, B.; Blügel, S.
2012-10-01
Density functional theory (DFT) has been steadily improving over the past few decades, becoming the standard tool for electronic structure calculations. The early local functionals (LDA) were eventually replaced by more accurate semilocal functionals (GGA) which are in use today. A major persisting drawback is the lack of the nonlocal correlation which is at the core of dispersive (van der Waals) forces, so that a large and important class of systems remains outside the scope of DFT. The vdW-DF correlation functional of Langreth and Lundqvist, published in 2004, was the first nonlocal functional which could be easily implemented. Beyond expectations, the nonlocal functional has brought significant improvement to systems that were believed not to be sensitive to nonlocal correlations. In this paper, we use the example of graphene nanodomes growing on the Ir(111) surface, where with an increase of the size of the graphene islands the character of the bonding changes from strong chemisorption towards almost pure physisorption. We demonstrate how the seamless character of the vdW-DF functionals makes it possible to treat all regimes self-consistently, proving to be a systematic and consistent improvement of DFT regardless of the nature of bonding. We also discuss the typical surface science example of CO adsorption on (111) surfaces of metals, which shows that the nonlocal correlation may also be crucial for strongly chemisorbed systems. We briefly discuss open questions, in particular the choice of the most appropriate exchange part of the functional. As the vdW-DF begins to appear implemented self-consistently in a number of popular DFT codes, with numerical costs close to the GGA calculations, we draw the attention of the DFT community to the advantages and benefits of the adoption of this new class of functionals.
A density functional for sparse matter
NASA Astrophysics Data System (ADS)
Langreth, D. C.; Lundqvist, B. I.; Chakarova-Käck, S. D.; Cooper, V. R.; Dion, M.; Hyldgaard, P.; Kelkkanen, A.; Kleis, J.; Kong, Lingzhu; Li, Shen; Moses, P. G.; Murray, E.; Puzder, A.; Rydberg, H.; Schröder, E.; Thonhauser, T.
2009-02-01
Sparse matter is abundant and has both strong local bonds and weak nonbonding forces, in particular nonlocal van der Waals (vdW) forces between atoms separated by empty space. It encompasses a broad spectrum of systems, like soft matter, adsorption systems and biostructures. Density-functional theory (DFT), long since proven successful for dense matter, seems now to have come to a point, where useful extensions to sparse matter are available. In particular, a functional form, vdW-DF (Dion et al 2004 Phys. Rev. Lett. 92 246401; Thonhauser et al 2007 Phys. Rev. B 76 125112), has been proposed for the nonlocal correlations between electrons and applied to various relevant molecules and materials, including to those layered systems like graphite, boron nitride and molybdenum sulfide, to dimers of benzene, polycyclic aromatic hydrocarbons (PAHs), doped benzene, cytosine and DNA base pairs, to nonbonding forces in molecules, to adsorbed molecules, like benzene, naphthalene, phenol and adenine on graphite, alumina and metals, to polymer and carbon nanotube (CNT) crystals, and hydrogen storage in graphite and metal-organic frameworks (MOFs), and to the structure of DNA and of DNA with intercalators. Comparison with results from wavefunction calculations for the smaller systems and with experimental data for the extended ones show the vdW-DF path to be promising. This could have great ramifications.
Choi, Ikjin; Chung, ChinWook; Youn Moon, Se
2013-08-15
In plasma diagnostics with a single Langmuir probe, the electron temperature T{sub e} is usually obtained from the slope of the logarithm of the electron current or from the electron energy probability functions of current (I)-voltage (V) curve. Recently, Chen [F. F. Chen, Phys. Plasmas 8, 3029 (2001)] suggested a derivative analysis method to obtain T{sub e} by the ratio between the probe current and the derivative of the probe current at a plasma potential where the ion current becomes zero. Based on this method, electron temperatures and electron densities were measured and compared with those from the electron energy distribution function (EEDF) measurement in Maxwellian and bi-Maxwellian electron distribution conditions. In a bi-Maxwellian electron distribution, we found the electron temperature T{sub e} obtained from the method is always lower than the effective temperatures T{sub eff} derived from EEDFs. The theoretical analysis for this is presented.
Optimization of an exchange-correlation density functional for water.
Fritz, Michelle; Fernández-Serra, Marivi; Soler, José M
2016-06-14
We describe a method, that we call data projection onto parameter space (DPPS), to optimize an energy functional of the electron density, so that it reproduces a dataset of experimental magnitudes. Our scheme, based on Bayes theorem, constrains the optimized functional not to depart unphysically from existing ab initio functionals. The resulting functional maximizes the probability of being the "correct" parameterization of a given functional form, in the sense of Bayes theory. The application of DPPS to water sheds new light on why density functional theory has performed rather poorly for liquid water, on what improvements are needed, and on the intrinsic limitations of the generalized gradient approximation to electron exchange and correlation. Finally, we present tests of our water-optimized functional, that we call vdW-DF-w, showing that it performs very well for a variety of condensed water systems. PMID:27305990
Optimization of an exchange-correlation density functional for water
NASA Astrophysics Data System (ADS)
Fritz, Michelle; Fernández-Serra, Marivi; Soler, José M.
2016-06-01
We describe a method, that we call data projection onto parameter space (DPPS), to optimize an energy functional of the electron density, so that it reproduces a dataset of experimental magnitudes. Our scheme, based on Bayes theorem, constrains the optimized functional not to depart unphysically from existing ab initio functionals. The resulting functional maximizes the probability of being the "correct" parameterization of a given functional form, in the sense of Bayes theory. The application of DPPS to water sheds new light on why density functional theory has performed rather poorly for liquid water, on what improvements are needed, and on the intrinsic limitations of the generalized gradient approximation to electron exchange and correlation. Finally, we present tests of our water-optimized functional, that we call vdW-DF-w, showing that it performs very well for a variety of condensed water systems.
Diestler, D J
2012-03-22
The Born-Oppenheimer (BO) description of electronically adiabatic molecular processes predicts a vanishing electronic flux density (j(e)),
Fast electronic resistance switching involving hidden charge density wave states
Vaskivskyi, I.; Mihailovic, I. A.; Brazovskii, S.; Gospodaric, J.; Mertelj, T.; Svetin, D.; Sutar, P.; Mihailovic, D.
2016-01-01
The functionality of computer memory elements is currently based on multi-stability, driven either by locally manipulating the density of electrons in transistors or by switching magnetic or ferroelectric order. Another possibility is switching between metallic and insulating phases by the motion of ions, but their speed is limited by slow nucleation and inhomogeneous percolative growth. Here we demonstrate fast resistance switching in a charge density wave system caused by pulsed current injection. As a charge pulse travels through the material, it converts a commensurately ordered polaronic Mott insulating state in 1T–TaS2 to a metastable electronic state with textured domain walls, accompanied with a conversion of polarons to band states, and concurrent rapid switching from an insulator to a metal. The large resistance change, high switching speed (30 ps) and ultralow energy per bit opens the way to new concepts in non-volatile memory devices manipulating all-electronic states. PMID:27181483
Fast electronic resistance switching involving hidden charge density wave states
NASA Astrophysics Data System (ADS)
Vaskivskyi, I.; Mihailovic, I. A.; Brazovskii, S.; Gospodaric, J.; Mertelj, T.; Svetin, D.; Sutar, P.; Mihailovic, D.
2016-05-01
The functionality of computer memory elements is currently based on multi-stability, driven either by locally manipulating the density of electrons in transistors or by switching magnetic or ferroelectric order. Another possibility is switching between metallic and insulating phases by the motion of ions, but their speed is limited by slow nucleation and inhomogeneous percolative growth. Here we demonstrate fast resistance switching in a charge density wave system caused by pulsed current injection. As a charge pulse travels through the material, it converts a commensurately ordered polaronic Mott insulating state in 1T-TaS2 to a metastable electronic state with textured domain walls, accompanied with a conversion of polarons to band states, and concurrent rapid switching from an insulator to a metal. The large resistance change, high switching speed (30 ps) and ultralow energy per bit opens the way to new concepts in non-volatile memory devices manipulating all-electronic states.
Fast electronic resistance switching involving hidden charge density wave states.
Vaskivskyi, I; Mihailovic, I A; Brazovskii, S; Gospodaric, J; Mertelj, T; Svetin, D; Sutar, P; Mihailovic, D
2016-01-01
The functionality of computer memory elements is currently based on multi-stability, driven either by locally manipulating the density of electrons in transistors or by switching magnetic or ferroelectric order. Another possibility is switching between metallic and insulating phases by the motion of ions, but their speed is limited by slow nucleation and inhomogeneous percolative growth. Here we demonstrate fast resistance switching in a charge density wave system caused by pulsed current injection. As a charge pulse travels through the material, it converts a commensurately ordered polaronic Mott insulating state in 1T-TaS2 to a metastable electronic state with textured domain walls, accompanied with a conversion of polarons to band states, and concurrent rapid switching from an insulator to a metal. The large resistance change, high switching speed (30 ps) and ultralow energy per bit opens the way to new concepts in non-volatile memory devices manipulating all-electronic states. PMID:27181483
C library for topological study of the electronic charge density.
Vega, David; Aray, Yosslen; Rodríguez, Jesús
2012-12-01
The topological study of the electronic charge density is useful to obtain information about the kinds of bonds (ionic or covalent) and the atom charges on a molecule or crystal. For this study, it is necessary to calculate, at every space point, the electronic density and its electronic density derivatives values up to second order. In this work, a grid-based method for these calculations is described. The library, implemented for three dimensions, is based on a multidimensional Lagrange interpolation in a regular grid; by differentiating the resulting polynomial, the gradient vector, the Hessian matrix and the Laplacian formulas were obtained for every space point. More complex functions such as the Newton-Raphson method (to find the critical points, where the gradient is null) and the Cash-Karp Runge-Kutta method (used to make the gradient paths) were programmed. As in some crystals, the unit cell has angles different from 90°, the described library includes linear transformations to correct the gradient and Hessian when the grid is distorted (inclined). Functions were also developed to handle grid containing files (grd from DMol® program, CUBE from Gaussian® program and CHGCAR from VASP® program). Each one of these files contains the data for a molecular or crystal electronic property (such as charge density, spin density, electrostatic potential, and others) in a three-dimensional (3D) grid. The library can be adapted to make the topological study in any regular 3D grid by modifying the code of these functions. PMID:22865338
Franco-Pérez, Marco; Ayers, Paul W; Gázquez, José L; Vela, Alberto
2015-12-28
We explore the local and nonlocal response functions of the grand canonical potential density functional at nonzero temperature. In analogy to the zero-temperature treatment, local (e.g., the average electron density and the local softness) and nonlocal (e.g., the softness kernel) intrinsic response functions are defined as partial derivatives of the grand canonical potential with respect to its thermodynamic variables (i.e., the chemical potential of the electron reservoir and the external potential generated by the atomic nuclei). To define the local and nonlocal response functions of the electron density (e.g., the Fukui function, the linear density response function, and the dual descriptor), we differentiate with respect to the average electron number and the external potential. The well-known mathematical relationships between the intrinsic response functions and the electron-density responses are generalized to nonzero temperature, and we prove that in the zero-temperature limit, our results recover well-known identities from the density functional theory of chemical reactivity. Specific working equations and numerical results are provided for the 3-state ensemble model. PMID:26723661
Franco-Pérez, Marco E-mail: ayers@mcmaster.ca E-mail: avela@cinvestav.mx; Ayers, Paul W. E-mail: ayers@mcmaster.ca E-mail: avela@cinvestav.mx; Gázquez, José L. E-mail: ayers@mcmaster.ca E-mail: avela@cinvestav.mx; Vela, Alberto E-mail: ayers@mcmaster.ca E-mail: avela@cinvestav.mx
2015-12-28
We explore the local and nonlocal response functions of the grand canonical potential density functional at nonzero temperature. In analogy to the zero-temperature treatment, local (e.g., the average electron density and the local softness) and nonlocal (e.g., the softness kernel) intrinsic response functions are defined as partial derivatives of the grand canonical potential with respect to its thermodynamic variables (i.e., the chemical potential of the electron reservoir and the external potential generated by the atomic nuclei). To define the local and nonlocal response functions of the electron density (e.g., the Fukui function, the linear density response function, and the dual descriptor), we differentiate with respect to the average electron number and the external potential. The well-known mathematical relationships between the intrinsic response functions and the electron-density responses are generalized to nonzero temperature, and we prove that in the zero-temperature limit, our results recover well-known identities from the density functional theory of chemical reactivity. Specific working equations and numerical results are provided for the 3-state ensemble model.
Nearly degenerate electron distributions and superluminal radiation densities
NASA Astrophysics Data System (ADS)
Tomaschitz, Roman
2010-02-01
Polylogarithmic fugacity expansions of the partition function, the caloric and thermal equations of state, and the specific heat of fermionic power-law distributions are derived in the nearly degenerate low-temperature/high-density quantum regime. The spectral functions of an ultra-relativistic electron plasma are obtained by averaging the tachyonic radiation densities of inertial electrons with Fermi power-laws, whose entropy is shown to be extensive and stable. The averaged radiation densities are put to test by performing tachyonic cascade fits to the γ-ray spectrum of the TeV blazar Markarian 421 in a low and high emission state. Estimates of the thermal electron plasma in this active galactic nucleus are extracted from the spectral fits, such as temperature, number count, and internal energy. The tachyonic cascades reproduce the quiescent as well as a burst spectrum of the blazar obtained with imaging atmospheric Cherenkov detectors. Double-logarithmic plots of the differential tachyon flux exhibit intrinsic spectral curvature, caused by the Boltzmann factor of the electron gas.
NASA Astrophysics Data System (ADS)
Ruzsinszky, Adrienn; Perdew, John P.; Csonka, Gábor I.; Vydrov, Oleg A.; Scuseria, Gustavo E.
2007-03-01
The common density functionals for the exchange-correlation energy make serious self-interaction errors in the molecular dissociation limit when real or spurious noninteger electron numbers N are found on the dissociation products. An "M-electron self-interaction-free" functional for positive integer M is one that produces a realistic linear variation of total energy with N in the range of M -1
NASA Astrophysics Data System (ADS)
Bochevarov, Arteum D.; Friesner, Richard A.
2008-01-01
We investigate one of the fundamental observables, electronic charge density, as produced by a number of popular functionals of the density functional theory (DFT): SVWN5, B3LYP, B3LYP, OLYP, O3LYP, BP86, B3P86, O3P86, and PBE using restricted and unrestricted orbitals. Measuring and comparing the quality of the densities could tell us more about the physical soundness of the functional models. The study is performed on the small molecules He, H2, LiH, H4 in an extensive range of correlation-consistent basis sets. We compare DFT densities to those of full configuration interaction (FCI) under the assumption that the FCI density in the largest employed basis set is sufficiently close to the exact one. For LiH and H4, we also compare the DFT densities to those of CCSD. The SVWN5 functional consistently shows the worst performance. The OPTX exchange functional regularly beats the Becke exchange. Among the best performers are all the hybrid functionals, the novel O3P86 being the most accurate in most cases. The popular functional B3LYP was consistently outmatched by O3LYP, and produced, in fact, some of the poorest densities among the hybrids. CCSD was found to produce much more accurate densities than any DFT functional in the case of LiH in equilibrium geometry, but was sometimes outperformed by DFT in the case of slightly stretched H4, where CCSD theory itself starts to break down. Surprisingly, as one stretches the H2 molecule, BP86 and PBE improve the description of density although such behavior is not observed in other systems. We conclude by reasoning how functionals such as B3LYP, despite being quite average for density, could still be very successful in predicting thermodynamic properties.
Density Functional Model for Nondynamic and Strong Correlation.
Kong, Jing; Proynov, Emil
2016-01-12
A single-term density functional model for the left-right nondynamic/strong electron correlation is presented based on single-determinant Kohn-Sham density functional theory. It is derived from modeling the adiabatic connection for kinetic correlation energy based on physical arguments, with the correlation potential energy based on the Becke'13 model ( Becke, A.D. J. Chem. Phys . 2013 , 138 , 074109 ). This functional satisfies some known scaling relationships for correlation functionals. The fractional spin error is further reduced substantially with a new density-functional correction. Preliminary tests with self-consistent-field implementation show that the model, with only three empirical parameters, recovers the majority of left-right nondynamic/strong correlation upon bond dissociation and performs reasonably well for atomization energies and singlet-triplet energy splittings. This study also demonstrates the feasibility of developing DFT functionals for nondynamic and strong correlation within the single-determinant KS scheme. PMID:26636190
Khoshkholgh, Mehri Javan; Marsusi, Farah; Abolhassani, Mohammad Reza
2015-02-01
PTBs polymers with thieno[3,4-b]thiophene [TT] and benzodithiophene [BDT] units have particular properties, which demonstrate it as one of the best group of donor materials in organic solar cells. In the present work, density functional theory (DFT) is applied to investigate the optimized structure, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), band gap and dihedral angle of PTB7 at B3LYP/6-31G(d). Two different approaches are applied to carry out these investigations: Oligomer extrapolation technique and periodic boundary condition (PBC) method. The results obtained from PBC-DFT method are in fair agreement with experiments. Based on these reliable outcomes; the investigations continued to perform some derivatives of PTB7. In this study, sulfur is substituted by nitrogen, oxygen, silicon, phosphor or selenium atoms in pristine PTB7. Due to the shift of HOMO and LUMO levels, smaller band gaps are predicted to appear in some derivatives in comparison with PTB7. Maximum theoretical efficiencies, η, of the mentioned derivatives as well as local difference of dipole moments between the ground and excited states (Δμge) are computed. The results indicate that substitution of sulfur by nitrogen or oxygen in BDT unit, and silicon or phosphor in TT unit of pristine PTB7 leads to a higher η as well as Δμge. PMID:25311524
Vo, Trinh; Allmen, Paul von; Huang, Chen-Kuo; Ma, James; Bux, Sabah; Fleurial, Jean-Pierre
2014-10-07
The electronic properties and Seebeck coefficients of Ce{sub 3}Te{sub 4} and La{sub 3}Te{sub 4} are computed using Density Functional Theory with on-site Coulomb interaction correction. We found that the Seebeck coefficients of Ce{sub 3}Te{sub 4} and La{sub 3}Te{sub 4} are almost equal at temperatures larger than the Curie temperature of Ce{sub 3}Te{sub 4}, and in good agreement with the measurements reported by May et al. [Phys. Rev. B 86, 035135 (2012)]. At temperatures below the Curie temperature, the Seebeck coefficient of Ce{sub 3}Te{sub 4} increases due to the ferromagnetic ordering, which leads the f-electron of Ce to contribute to the Seebeck coefficient in the relevant range of electron concentration.
Density functional theory in the solid state
Hasnip, Philip J.; Refson, Keith; Probert, Matt I. J.; Yates, Jonathan R.; Clark, Stewart J.; Pickard, Chris J.
2014-01-01
Density functional theory (DFT) has been used in many fields of the physical sciences, but none so successfully as in the solid state. From its origins in condensed matter physics, it has expanded into materials science, high-pressure physics and mineralogy, solid-state chemistry and more, powering entire computational subdisciplines. Modern DFT simulation codes can calculate a vast range of structural, chemical, optical, spectroscopic, elastic, vibrational and thermodynamic phenomena. The ability to predict structure–property relationships has revolutionized experimental fields, such as vibrational and solid-state NMR spectroscopy, where it is the primary method to analyse and interpret experimental spectra. In semiconductor physics, great progress has been made in the electronic structure of bulk and defect states despite the severe challenges presented by the description of excited states. Studies are no longer restricted to known crystallographic structures. DFT is increasingly used as an exploratory tool for materials discovery and computational experiments, culminating in ex nihilo crystal structure prediction, which addresses the long-standing difficult problem of how to predict crystal structure polymorphs from nothing but a specified chemical composition. We present an overview of the capabilities of solid-state DFT simulations in all of these topics, illustrated with recent examples using the CASTEP computer program. PMID:24516184
Particle conservation in dynamical density functional theory
NASA Astrophysics Data System (ADS)
de las Heras, Daniel; Brader, Joseph M.; Fortini, Andrea; Schmidt, Matthias
2016-06-01
We present the exact adiabatic theory for the dynamics of the inhomogeneous density distribution of a classical fluid. Erroneous particle number fluctuations of dynamical density functional theory are absent, both for canonical and grand canonical initial conditions. We obtain the canonical free energy functional, which yields the adiabatic interparticle forces of overdamped Brownian motion. Using an exact and one of the most advanced approximate hard core free energy functionals, we obtain excellent agreement with simulations. The theory applies to finite systems in and out of equilibrium.
Particle conservation in dynamical density functional theory.
de Las Heras, Daniel; Brader, Joseph M; Fortini, Andrea; Schmidt, Matthias
2016-06-22
We present the exact adiabatic theory for the dynamics of the inhomogeneous density distribution of a classical fluid. Erroneous particle number fluctuations of dynamical density functional theory are absent, both for canonical and grand canonical initial conditions. We obtain the canonical free energy functional, which yields the adiabatic interparticle forces of overdamped Brownian motion. Using an exact and one of the most advanced approximate hard core free energy functionals, we obtain excellent agreement with simulations. The theory applies to finite systems in and out of equilibrium. PMID:27115673
Ramanantoanina, Harry; Kuri, Goutam; Daul, Claude; Bertsch, Johannes
2016-07-28
Ligand field density functional theory (LFDFT) calculations have been used to model the uranium M4,5, N4,5 and O4,5-edge X-ray absorption near edge structure (XANES) in UO2, characterized by the promotion of one electron from the core and the semi-core 3d, 4d and 5d orbitals of U(4+) to the valence 5f. The model describes the procedure to resolve non-empirically the multiplet energy levels originating from the two-open-shell system with d and f electrons and to calculate the oscillator strengths corresponding to the dipole allowed d(10)f(2)→ d(9)f(3) transitions appropriate to represent the d electron excitation process. In the first step, the energy and UO2 unit-cell volume corresponding to the minimum structures are determined using the Hubbard model (DFT+U) approach. The model of the optical properties due to the uranium nd(10)5f(2)→nd(9)5f(3) transitions, with n = 3, 4 and 5, has been tackled by means of electronic structure calculations based on the ligand field concept emulating the Slater-Condon integrals, the spin-orbit coupling constants and the parameters of the ligand field potential needed by the ligand field Hamiltonian from Density Functional Theory. A deep-rooted theoretical procedure using the LFDFT approach has been established for actinide-bearing systems that can be valuable to compute targeted results, such as spectroscopic details at the electronic scale. As a case study, uranium dioxide has been considered because it is a nuclear fuel material, and both atomic and electronic structure calculations are indispensable for a deeper understanding of irradiation driven microstructural changes occurring in this material. PMID:27356168
Electronic structure tuning of diamondoids through functionalization
NASA Astrophysics Data System (ADS)
Rander, Torbjörn; Staiger, Matthias; Richter, Robert; Zimmermann, Tobias; Landt, Lasse; Wolter, David; Dahl, Jeremy E.; Carlson, Robert M. K.; Tkachenko, Boryslav A.; Fokina, Natalie A.; Schreiner, Peter R.; Möller, Thomas; Bostedt, Christoph
2013-01-01
We investigated the changes in electronic structures induced by chemical functionalization of the five smallest diamondoids using valence photoelectron spectroscopy. Through the variation of three parameters, namely functional group (thiol, hydroxy, and amino), host cluster size (adamantane, diamantane, triamantane, [121]tetramantane, and [1(2,3)4]pentamantane), and functionalization site (apical and medial) we are able to determine to what degree these affect the electronic structures of the overall systems. We show that unlike, for example, in the case of halobenzenes, the ionization potential does not show a linear dependence on the electronegativity of the functional group. Instead, a linear correlation exists between the HOMO-1 ionization potential and the functional group electronegativity. This is due to localization of the HOMO on the functional group and the HOMO-1 on the diamondoid cage. Density functional theory supports our interpretations.
Flexoelectricity from density-functional perturbation theory
NASA Astrophysics Data System (ADS)
Stengel, Massimiliano
2013-11-01
We derive the complete flexoelectric tensor, including electronic and lattice-mediated effects, of an arbitrary insulator in terms of the microscopic linear response of the crystal to atomic displacements. The basic ingredient, which can be readily calculated from first principles in the framework of density-functional perturbation theory, is the quantum-mechanical probability current response to a long-wavelength acoustic phonon. Its second-order Taylor expansion in the wave vector q around the Γ (q=0) point in the Brillouin zone naturally yields the flexoelectric tensor. At order one in q we recover Martin's theory of piezoelectricity [Martin, Phys. Rev. B 5, 1607 (1972)], thus providing an alternative derivation thereof. To put our derivations on firm theoretical grounds, we perform a thorough analysis of the nonanalytic behavior of the dynamical matrix and other response functions in a vicinity of Γ. Based on this analysis, we find that there is an ambiguity in the specification of the “zero macroscopic field” condition in the flexoelectric case; such arbitrariness can be related to an analytic band-structure term, in close analogy to the theory of deformation potentials. As a by-product, we derive a rigorous generalization of the Cochran-Cowley formula [Cochran and Cowley, J. Phys. Chem. Solids 23, 447 (1962)] to higher orders in q. This can be of great utility in building reliable atomistic models of electromechanical phenomena, as well as for improving the accuracy of the calculation of phonon dispersion curves. Finally, we discuss the physical interpretation of the various contributions to the flexoelectric response, either in the static or dynamic regime, and we relate our findings to earlier theoretical works on the subject.
NASA Astrophysics Data System (ADS)
Varas, Alejandro; Aguilera-Granja, F.; Rogan, José; Kiwi, Miguel
2015-11-01
The ternary alloy FexCoyNiz nanoparticles are of interest in both theoretical and experimental aspects, particularly due to the possible technological applications and due to the novel structural and magnetic properties in the sub-nanometer region. Here we compute the structural parameters, chemical and magnetic properties of these 13 atom nanoclusters. To the best of our knowledge, this is the first time that such kind of calculation is performed for all the possible compositions of 13-atom ternary nanoclusters. We performed density-functional-theory (DFT) calculations, as implemented in the SIESTA code, for all the possible concentrations (i.e. all x , y and z-values). The seeds for the possible homotops are built using a semi-empirical Gupta potential, and these conformations are thereafter subject to reoptimization by means of the SIESTA code. Based on known results, we focus our attention on just two types of possible nanostructures: icosahedral and compact biplanar. We find that approximately half of the minimum energy conformations are icosahedral, and the other half are biplanar, with most interatomic distances smaller than the bulk values. The binding is strongest for the FeCo rich nanoclusters, and weakest for pristine Ni13. However, a set of highly stable structures, with layer-like ordering, were found close to the Ni-rich region. These nanoalloys are expected to be the most abundant ones in cluster-growth experiments. We conclude that the total magnetic moment varies smoothly over the full composition range, with the large Fe moment being quenched by the addition of Co and/or Ni. By alloying, the magnitude of the moments varies almost continuously, and thus allows for fine tuning magnetism by controlling the composition, which has implications for magnetic recording.
Functional molecules in electronic circuits.
Weibel, Nicolas; Grunder, Sergio; Mayor, Marcel
2007-08-01
Molecular electronics is a fascinating field of research contributing to both fundamental science and future technological achievements. A promising starting point for molecular devices is to mimic existing electronic functions to investigate the potential of molecules to enrich and complement existing electronic strategies. Molecules designed and synthesized to be integrated into electronic circuits and to perform an electronic function are presented in this article. The focus is set in particular on rectification and switching based on molecular devices, since the control over these two parameters enables the assembly of memory units, likely the most interesting and economic application of molecular based electronics. Both historical and contemporary solutions to molecular rectification are discussed, although not exhaustively. Several examples of integrated molecular switches that respond to light are presented. Molecular switches responding to an electrochemical signal are also discussed. Finally, supramolecular and molecular systems with intuitive application potential as memory units due to their hysteretic switching are highlighted. Although a particularly attractive feature of molecular electronics is its close cooperation with neighbouring disciplines, this article is written from the point of view of a chemist. Although the focus here is largely on molecular considerations, innovative contributions from physics, electro engineering, nanotechnology and other scientific disciplines are equally important. However, the ability of the chemist to correlate function with structure, to design and to provide tailor-made functional molecules is central to molecular electronics. PMID:17637951
NASA Astrophysics Data System (ADS)
Allali, D.; Bouhemadou, A.; Safi, E. Muhammad Abud Al; Bin-Omran, S.; Chegaar, M.; Khenata, R.; Reshak, A. H.
2014-06-01
We report ab initio density functional theory calculations of the structural, electronic and optical properties of the spinel oxides SiMg2O4, SiZng2O4, and SiCd2O4 using the full-potential linearized augmented plane-wave method. The structural parameters calculated using both the local density and generalized gradient approximations to the exchange-correlation potential are consistent with the literature data. To calculate the electronic properties, the exchange-correlation potential is treated with various functionals, and we find that the newly developed Tran-Blaha-modified Becke-Johnson functional significantly improves the band gap. We predict a direct band gap in all of the considered SiB2O4 compounds, and the band gaps continuously decrease as the atomic size of the B element increases. The decrease in the fundamental direct band gap (Γ-Γ) from SiMg2O4 to SiZn2O4 to SiCd2O4 can be attributed to p-d mixing in the upper valence bands of SiZn2O4 and SiCd2O4. The lowest conduction band is well dispersive, similar to that found for transparent conducting oxides such as ZnO. This band is mainly defined by the s and p electrons of the Si and B (B=Mg, Zn, Cd) atoms. The topmost valence band is considerably less dispersive and is defined by O-2p and B-d electrons. The charge-carrier effective masses are evaluated at the topmost valence band and at the bottommost conduction band that were calculated. The frequency-dependent complex dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity and electron energy loss function were estimated. We find that the value of the zero-frequency limit of the dielectric function ε(0) increases as the band gap decreases. The origins of the peaks and structures in the optical spectra are determined in terms of the calculated energy band structures.
Matar, S.F.; Chevalier, B.; Etourneau, J.; Eyert, V.
1997-02-05
The electronic structures of U{sub 2}Fe{sub 2}Sn and UFe{sub 2}Ge{sub 2} are self-consistently calculated within the local density functional theory using the augmented spherical wave (ASW) method. Calculations are scalar relativistic. The experimentally observed Pauli paramagnetic behavior of the two systems is accounted for and the influence of hybridization between the different l-states on the chemical bonding is discussed from the site-projected densities of states (DOS) as well as from the modulation of the DOS by the sign and magnitude of the overlap integral, i.e., with the so-called COOP. From this, we propose a mechanism for the evolution of bonding within the series to which the two compounds belong. 12 refs., 3 figs.
Nishikawa, Takeshi
2014-07-15
Most conventional atomic models in a plasma do not treat the effect of the plasma on the free-electron state density. Using a nearest neighbor approximation, the state densities in hydrogenic plasmas for both bound and free electrons were evaluated and the effect of the plasma on the atomic model (especially for the state density of the free electron) was studied. The model evaluates the electron-state densities using the potential distribution formed by the superposition of the Coulomb potentials of two ions. The potential from one ion perturbs the electronic state density on the other. Using this new model, one can evaluate the free-state density without making any ad-hoc assumptions. The resulting contours of the average ionization degree, given as a function of the plasma temperature and density, are shifted slightly to lower temperatures because of the effect of the increasing free-state density.
Electron density distributions in the high-latitude magnetosphere
NASA Technical Reports Server (NTRS)
Persoon, Ann M.
1988-01-01
Electron density profiles were constructed to study the plasma density depletions in the nightside auroral zone and the density variations with increasing altitude in the polar cap, using electric field spectrum measurements from the plasma wave instrument on DE-1. Sharply defined regions of depleted plasma densities were commonly observed on nightside auroral field lines, in which electron densities were strongly depleted in relation to the adjacent plasmaspheric and polar densities, forming a low-density cavity at about 70 deg invariant latitude. A correlation was found between low auroral plasma densities, upflowing ion distributions, and an energetic precipitating electron population, indicating that electron density depletions in the nightside auroral zone are directly associated with auroral acceleration processes.
Density-functional expansion methods: Grand challenges
Giese, Timothy J.; York, Darrin M.
2016-01-01
We discuss the source of errors in semiempirical density functional expansion (VE) methods. In particular, we show that VE methods are capable of well-reproducing their standard Kohn-Sham density functional method counterparts, but suffer from large errors upon using one or more of these approximations: the limited size of the atomic orbital basis, the Slater monopole auxiliary basis description of the response density, and the one- and two-body treatment of the core-Hamiltonian matrix elements. In the process of discussing these approximations and highlighting their symptoms, we introduce a new model that supplements the second-order density-functional tight-binding model with a self-consistent charge-dependent chemical potential equalization correction; we review our recently reported method for generalizing the auxiliary basis description of the atomic orbital response density; and we decompose the first-order potential into a summation of additive atomic components and many-body corrections, and from this examination, we provide new insights and preliminary results that motivate and inspire new approximate treatments of the core-Hamiltonian. PMID:27293378
Comments on the locality in density-functional theory
Lindgren, Ingvar; Salomonson, Sten
2003-05-01
The 'locality hypothesis' in density-functional theory (DFT), implying that the functional derivative is equivalent to a multiplicative local function, forms the basis of models of Kohn-Sham type. This has been generally accepted by the community since the advent of the model, and has later been formally proved for a large class of functionals. The hypothesis has recently been questioned by Nesbet [Phys. Rev. A 58, R12 (1998) and Phys. Rev. A 65, 010502 (2001)], who claims that it fails for the kinetic-energy functional for a system with more than two noninteracting electrons with a nondegenerate ground state. This conclusion has been questioned by Gal [Phys. Rev. A 62, 044501 (2000)] and by Holas and March [Phys. Rev. A 64, 016501 (2001)]. We claim that the arguments of Nesbet are incorrect, since the orbital functional used for the kinetic energy is not a unique functional of the total density in the domain of unnormalized orbitals. We have demonstrated that with a proper definition of the kinetic energy, which is a unique density functional also in the unnormalized region, the derivative can be represented by a single local multiplicative function for all v-representable densities. Therefore, we consider the controversy connected with the issue raised by Nesbet as resolved. We believe that the proof of the differentiability given here can be extended to larger groups of DFT functionals, and works along these lines are in progress.
Van der Waals density functional applied to adsorption systems
NASA Astrophysics Data System (ADS)
Hamada, Ikutaro
2013-03-01
The van der Waals density functional (vdW-DF) is a promising density functional to describe the van der Waals forces within density functional theory. However, despite the recent efforts, there is still room for further improvement, especially for describing molecular adsorption on metal surfaces. I will show that by choosing appropriate exchange and nonlocal correlation functionals, it is possible to calculate geometries and electronic structures for adsorption systems accurately within the framework of vdW-DF. Applicability of the present approach will be illustrated with its applications to graphene/metal, fullerene/metal, and water/graphene interfaces. This work is partly supported by a Grant-in-Aid for Scientific Research on Innovative Area (No. 23104501). AIMR was established by the World Premier International Research Center Initiative (WPI), MEXT, Japan.
Density functional theory studies of etoricoxib
NASA Astrophysics Data System (ADS)
Sachdeva, Ritika; Kaur, Prabhjot; Singh, V. P.; Saini, G. S. S.
2016-05-01
Etoricoxib is a COX-2 selective inhibitor drug with molecular formula C18H15ClN2O2S. It is primarily used for the treatment of arthritis(rheumatoid, psoriatic, osteoarthritis), ankylosing spondylitis, gout and chronic low back pain. Theoretical studies of the molecule including geometry optimization and vibrational frequency calculations were carried out with the help of density functional theory calculations using 6-311++ g (d, p) basis set and B3LYP functional.
Boda, Anil; Ali, Sk Musharaf; Rao, Hanmanth; Ghosh, Sandip K
2012-08-01
The structures, energetic and thermodynamic parameters of model crown ethers with different donor, cavity and electron donating/ withdrawing functional group have been determined with ab initio MP2 and density functional theory in gas and solvent phase. The calculated values of binding energy/ enthalpy for lithium ion complexation are marginally higher for hard donor based aza and oxa crown compared to soft donor based thia and phospha crown. The calculated values of binding enthalpy for lithium metal ion with 12C4 at MP2 level of theory is in good agreement with the available experimental result. The binding energy is altered due to the inductive effect imparted by the electron donating/ withdrawing group in crown ether, which is well correlated with the values of electron transfer. The role of entropy for extraction of hydrated lithium metal ion by different donor and functional group based ligand has been demonstrated. The HOMO-LUMO gap is decreased and dipole moment of the ligand is increased from gas phase to organic phase because of the dielectric constant of the solvent. The gas phase binding energy is reduced in solvent phase as the solvent molecules weaken the metal-ligand binding. The theoretical values of extraction energy for LiCl salt from aqueous solution in different organic solvent is validated by the experimental trend. The study presented here should contribute to the design of model host ligand and screening of solvent for metal ion recognition and thus can contribute in planning the experiments. PMID:22318713
NASA Astrophysics Data System (ADS)
Das, Debashish; Ghosh, Subhradip
2015-10-01
Using the DFT+U method and generalized gradient approximation (GGA) we perform the first systematic study of the chromite series ACr2O4 (A = Mn, Fe, Co, Ni) by computing their structural and magnetic properties. The results are analyzed by their electronic structures. We find that in spite of varying structural distortions, the electronic structures are very similar across the series. Such similarities are responsible for qualitative uniformities in their magnetic phases at low temperatures, as observed in the experiments. We find that the strong electron-electron correlation, along with competing magnetic exchange splitting and the crystal field splitting, are responsible for their electronic properties such as the electronic band gaps. Our results regarding the magnetic exchange parameters are in good agreement with the available results and show the relative importance of the pairwise exchange interactions in each of the compounds. The ground state magnetic spin structures and the ferrimagnetic transition temperatures obtained from these exchange parameters, in combination with a phenomenological theory, qualitatively agree with the experiments and other theoretical results.
Murphy, M. W.; Yiu, Y. M. Sham, T. K.; Ward, M. J.; Liu, L.; Hu, Y.; Zapien, J. A.; Liu, Yingkai
2014-11-21
The electronic structure and optical properties of a series of iso-electronic and iso-structural CdS{sub x}Se{sub 1−x} solid solution nanostructures have been investigated using X-ray absorption near edge structure, extended X-ray absorption fine structure, and X-ray excited optical luminescence at various absorption edges of Cd, S, and Se. It is found that the system exhibits compositions, with variable local structure in-between that of CdS and CdSe accompanied by tunable optical band gap between that of CdS and CdSe. Theoretical calculation using density functional theory has been carried out to elucidate the observations. It is also found that luminescence induced by X-ray excitation shows new optical channels not observed previously with laser excitation. The implications of these observations are discussed.
Nuclear moments in covariant density functional theory
NASA Astrophysics Data System (ADS)
Meng, J.; Zhao, P. W.; Zhang, S. Q.; Hu, J. N.; Li, J.
2014-05-01
Recent progresses on microscopic and self-consistent description of the nuclear moments in covariant density functional theory based on a point-coupling interaction are briefly reviewed. In particular, the electric quadrupole moments of Cd isotopes and the magnetic moments of Pb isotopes are discussed.
NASA Astrophysics Data System (ADS)
Joseph, L.; Arunsasi, B. S.; Sajan, D.; Shettigar, V.
2014-11-01
A new chalcone derivative, 1-(4-Bromophenyl)-3-(3,4-dimethoxy-phenyl)prop-2-en-1-one (DMBC) was synthesized and single crystals were grown by slow evaporation technique. The FT-Raman and FT-IR spectra of the sample were recorded in the region 3500-50 cm-1 and 4000-400 cm-1 respectively. The spectra were interpreted with the aid of normal coordinate analysis, following structure optimizations and force field calculations based on density functional theory (DFT) at the B3LYP/6-31G(d,p) level of theory. Normal coordinate calculations were performed using the DFT force field corrected by a recommended set of scaling factors yielding fairly good agreement between the observed and calculated wavenumbers. DMBC is thermally stable up to 265.0 °C and optically transparent in the visible region. The total electron density and molecular electrostatic potential surfaces of the molecules were constructed by Natural Bond Orbital analysis using B3LYP/6-311++G(d,p) method to display electrostatic potential (electron + nuclei) distribution, molecular shape, size, and dipole moments of the molecule. The electronic properties, HOMO and LUMO energies were measured.
Cutoff probe using Fourier analysis for electron density measurement
NASA Astrophysics Data System (ADS)
Na, Byung-Keun; You, Kwang-Ho; Kim, Dae-Woong; Chang, Hong-Young; You, Shin-Jae; Kim, Jung-Hyung
2012-01-01
This paper proposes a new method for cutoff probe using a nanosecond impulse generator and an oscilloscope, instead of a network analyzer. The nanosecond impulse generator supplies a radiating signal of broadband frequency spectrum simultaneously without frequency sweeping, while frequency sweeping method is used by a network analyzer in a previous method. The transmission spectrum (S21) was obtained through a Fourier analysis of the transmitted impulse signal detected by the oscilloscope and was used to measure the electron density. The results showed that the transmission frequency spectrum and the electron density obtained with a new method are very close to those obtained with a previous method using a network analyzer. And also, only 15 ns long signal was necessary for spectrum reconstruction. These results were also compared to the Langmuir probe's measurements with satisfactory results. This method is expected to provide not only fast measurement of absolute electron density, but also function in other diagnostic situations where a network analyzer would be used (a hairpin probe and an impedance probe) by replacing the network analyzer with a nanosecond impulse generator and an oscilloscope.
Raghunath, P; Reddy, M Ananth; Gouri, C; Bhanuprakash, K; Rao, V Jayathirtha
2006-01-26
Molecular level parameters are investigated computationally to understand the factors that are responsible for the higher efficiency in derivatives of 9,10-bis(1-naphthyl)anthracene (alpha-ADN), 9,10-bis(2-naphthyl)anthracene (beta-ADN), their tetramethyl derivatives (alpha,beta-TMADN) and the t-Bu derivative (beta-TBADN) as blue light emitting electroluminescent (EL) layers in organic light emitting diodes (OLEDs). DFT studies at the B3LYP/6-31G(d,p) level have been carried out on the substituted anthracenes. The absorption spectra are simulated using time dependent DFT methods (TD-DFT) whereas the emission spectra are approximated by optimizing the excited state by HF/CI-Singles and then carrying out the vertical CI calculations by the TD-DFT method. The reorganization energy for estimating the hole and electron transport is calculated. The transfer integrals between parallely stacked molecules in the bulk state are estimated by calculating the electronic splitting. The substituted anthracenes are compared with unsubstituted anthracene and yet untested 9,10-dianthrylanthracene (TANTH). A larger and slower buildup of the electrons and holes in the EL layer, due to the higher reorganization energy and smaller electronic coupling between the adjacent molecules could lead to an increase in hole-electron recombination in the layer and thus increase the efficiency. PMID:16420020
Probability density function learning by unsupervised neurons.
Fiori, S
2001-10-01
In a recent work, we introduced the concept of pseudo-polynomial adaptive activation function neuron (FAN) and presented an unsupervised information-theoretic learning theory for such structure. The learning model is based on entropy optimization and provides a way of learning probability distributions from incomplete data. The aim of the present paper is to illustrate some theoretical features of the FAN neuron, to extend its learning theory to asymmetrical density function approximation, and to provide an analytical and numerical comparison with other known density function estimation methods, with special emphasis to the universal approximation ability. The paper also provides a survey of PDF learning from incomplete data, as well as results of several experiments performed on real-world problems and signals. PMID:11709808
Miller, William H; Cotton, Stephen J
2016-08-28
It is pointed out that the classical phase space distribution in action-angle (a-a) variables obtained from a Wigner function depends on how the calculation is carried out: if one computes the standard Wigner function in Cartesian variables (p, x), and then replaces p and x by their expressions in terms of a-a variables, one obtains a different result than if the Wigner function is computed directly in terms of the a-a variables. Furthermore, the latter procedure gives a result more consistent with classical and semiclassical theory-e.g., by incorporating the Bohr-Sommerfeld quantization condition (quantum states defined by integer values of the action variable) as well as the Heisenberg correspondence principle for matrix elements of an operator between such states-and has also been shown to be more accurate when applied to electronically non-adiabatic applications as implemented within the recently developed symmetrical quasi-classical (SQC) Meyer-Miller (MM) approach. Moreover, use of the Wigner function (obtained directly) in a-a variables shows how our standard SQC/MM approach can be used to obtain off-diagonal elements of the electronic density matrix by processing in a different way the same set of trajectories already used (in the SQC/MM methodology) to obtain the diagonal elements. PMID:27586896
Electron density power spectrum in the local interstellar medium
NASA Technical Reports Server (NTRS)
Armstrong, J. W.; Rickett, B. J.; Spangler, S. R.
1995-01-01
Interstellar scintillation (ISS), fluctuations in the amplitude and phase of radio waves caused by scattering in the interstellar medium, is important as a diagnostic of interstellar plasma turbulence. ISS is also of interest because it is noise for other radio astronomical observations. The unifying concern is the power spectrum of the interstellar electron density. Here we use ISS observations through the nearby (less than or approximately =1 kpc) (ISM) to estimate the spectrum. From measurements of angular broadening of pulsars and extragalactic sources, decorrelation bandwidth of pulsars, refractive steering of features in pulsar dynamic spectra, dispersion measured fluctuations of pulsars, and refractive scintillation index measurements, we construct a composite structure function that is approximately power law over 2 x 10(exp 6) m less than scale less than 10(exp 13) m. The data are consistent with the structure function having a logarithmic slope versus baseline less than 2; thus there is a meaningful connection between scales in the radiowave fluctuation field and the scales in the electron density field causing the scattering. The data give an upper limit to the inner scale, l(sub o) less than or approximately 10(exp 8) m and are consistent with much smaller values. We construct a composite electron density spectrum that is approximately power law over at least the approximately = 5 decade wavenumber range 10(exp -13)/m less than wavenumber less than 10(exp -8)/m and that may extend to higher wavenumbers. The average spectral index of electron density over this wavenumber range is approximately = 3.7, very close to the value expected for a Kolmogorov process. The outer scale size, L(sub o), must be greater than or approximately = 10(exp 13) m (determined from dispersion measure fluctuations). When the ISS data are combined with measurements of differential Faraday rotation angle, and gradients in the average electron density, constraints can be put on the
Density gradient expansion of correlation functions
NASA Astrophysics Data System (ADS)
van Leeuwen, Robert
2013-04-01
We present a general scheme based on nonlinear response theory to calculate the expansion of correlation functions such as the pair-correlation function or the exchange-correlation hole of an inhomogeneous many-particle system in terms of density derivatives of arbitrary order. We further derive a consistency condition that is necessary for the existence of the gradient expansion. This condition is used to carry out an infinite summation of terms involving response functions up to infinite order from which it follows that the coefficient functions of the gradient expansion can be expressed in terms of the local density profile rather than the background density around which the expansion is carried out. We apply the method to the calculation of the gradient expansion of the one-particle density matrix to second order in the density gradients and recover in an alternative manner the result of Gross and Dreizler [Gross and Dreizler, Z. Phys. AZPAADB0340-219310.1007/BF01413038 302, 103 (1981)], which was derived using the Kirzhnits method. The nonlinear response method is more general and avoids the turning point problem of the Kirzhnits expansion. We further give a description of the exchange hole in momentum space and confirm the wave vector analysis of Langreth and Perdew [Langreth and Perdew, Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.21.5469 21, 5469 (1980)] for this case. This is used to derive that the second-order gradient expansion of the system averaged exchange hole satisfies the hole sum rule and to calculate the gradient coefficient of the exchange energy without the need to regularize divergent integrals.
Ionospheric E-region electron density and neutral atmosphere variations
NASA Technical Reports Server (NTRS)
Stick, T. L.
1976-01-01
Electron density deviations from a basic variation with the solar zenith angle were investigated. A model study was conducted in which the effects of changes in neutral and relative densities of atomic and molecular oxygen on calculated electron densities were compared with incoherent scatter measurements in the height range 100-117 km at Arecibo, Puerto Rico. The feasibility of determining tides in the neutral atmosphere from electron density profiles was studied. It was determined that variations in phase between the density and temperature variation and the comparable magnitudes of their components make it appear improbable that the useful information on tidal modes can be obtained in this way.
Density Functional Theory for Steady-State Nonequilibrium Molecular Junctions
NASA Astrophysics Data System (ADS)
Liu, Shuanglong; Nurbawono, Argo; Zhang, Chun
2015-10-01
We present a density functional theory (DFT) for steady-state nonequilibrium quantum systems such as molecular junctions under a finite bias. Based on the steady-state nonequilibrium statistics that maps nonequilibrium to an effective equilibrium, we show that ground-state DFT (GS-DFT) is not applicable in this case and two densities, the total electron density and the density of current-carrying electrons, are needed to uniquely determine the properties of the corresponding nonequilibrium system. A self-consistent mean-field approach based on two densities is then derived. The theory is implemented into SIESTA computational package and applied to study nonequilibrium electronic/transport properties of a realistic carbon-nanotube (CNT)/Benzene junction. Results obtained from our steady-state DFT (SS-DFT) are compared with those of conventional GS-DFT based transport calculations. We show that SS-DFT yields energetically more stable nonequilibrium steady state, predicts significantly lower electric current, and is able to produce correct electronic structures in local equilibrium under a limiting case.
Hedegård, Erik Donovan Knecht, Stefan; Reiher, Markus; Kielberg, Jesper Skau; Jensen, Hans Jørgen Aagaard
2015-06-14
We present a new hybrid multiconfigurational method based on the concept of range-separation that combines the density matrix renormalization group approach with density functional theory. This new method is designed for the simultaneous description of dynamical and static electron-correlation effects in multiconfigurational electronic structure problems.
Probing Electron Dynamics with the Laplacian of the Momentum Density
Sukumar, N.; MacDougall, Preston J.; Levit, M. Creon
2012-09-24
This chapter in the above-titled monograph presents topological analysis of the Laplacian of the electron momentum density in organic molecules. It relates topological features in this distribution to chemical and physical properties, particularly aromaticity and electron transport.
Kinetic-energy density functional: Atoms and shell structure
Garcia-Gonzalez, P.; Alvarellos, J.E.; Chacon, E. |
1996-09-01
We present a nonlocal kinetic-energy functional which includes an anisotropic average of the density through a symmetrization procedure. This functional allows a better description of the nonlocal effects of the electron system. The main consequence of the symmetrization is the appearance of a clear shell structure in the atomic density profiles, obtained after the minimization of the total energy. Although previous results with some of the nonlocal kinetic functionals have given incipient structures for heavy atoms, only our functional shows a clear shell structure for most of the atoms. The atomic total energies have a good agreement with the exact calculations. Discussion of the chemical potential and the first ionization potential in atoms is included. The functional is also extended to spin-polarized systems. {copyright} {ital 1996 The American Physical Society.}
Density-Functional Theory of Thermal Transport
NASA Astrophysics Data System (ADS)
Eich, F. G.; Principi, A.; di Ventra, M.; Vignale, G.
2014-03-01
We have recently introduced a non-equilibrium density-functional theory of local temperature and associated energy density that is suitable for the study of thermoelectric phenomena from first principles. This theory rests on a local temperature field coupled to the energy-density operator. Here we apply the theory to a simple two-terminal setup, in which the terminals are held at different temperatures. We show that our treatment becomes equivalent to the standard Landauer-Büttiker formulation of thermal transport in the non-interacting limit. We gratefully acknowledge support from DOE under Grant No. DE-FG02-05ER46203 (FGE, AP, GV) and DE-FG02-05ER46204 (MD).
Javaid, Saqib; Javed Akhtar, M.
2014-07-14
We have investigated the behavior of orthoferrite LaFeO{sub 3} at ambient conditions and under pressure using DFT (generalized gradient approximation (GGA)) + U approach. Ground state electronic (band gap) and magnetic properties are considerably improved due to the Hubbard correction. Moreover, the experimentally observed pressure-driven phase transition, namely, the simultaneous occurrence of spin crossover, isostructural volume collapse, and drastic reduction in electrical resistance (electronic phase transition) is nicely described by GGA + U calculations. In particular, despite a sharp drop in resistance, a small band gap still remains in the low spin state indicating an insulator to semiconductor phase transition, in good agreement with the experiments but in contrast to GGA, which predicts metallic behavior in low spin state. We discuss the origin of variation in electronic structure of LaFeO{sub 3} in low spin state as obtained from GGA to GGA + U methods. These results emphasize the importance of correlation effects in describing the pressure-driven phase transition in LaFeO{sub 3} and other rare-earth orthoferrites.
Effect of electron correlation in Sr(Ca)Ru{sub 1-x}Cr{sub x}O{sub 3}: Density functional calculation
Hadipour, H.; Akhavan, M.
2010-07-15
We have investigated the electronic structure of Sr(Ca)Ru{sub 1-x}Cr{sub x}O{sub 3} using the full potential linearized augmented plane wave method by different approximation such as LSDA and LSDA+U. The LSDA calculation suggest that Cr{sup 4+}-Ru{sup 4+} hybridization is responsible for the high Curie temperature T{sub C} in SrRu{sub 1-x}Cr{sub x}O{sub 3}, but it cannot completely describe its physical behavior. Our LSDA+U DOS results for SrRu{sub 1-x}Cr{sub x}O{sub 3} clearly establishes renormalization of the intra-atomic exchange strength at the Ru sites, arising from the Cr-Ru hybridization. The antiferromagnetic coupling of Cr{sup 3+} with Ru{sup 4+,5+} lattice increases the screening, which is consistent with the low magnetic moment of the Ru ions. The more distorted Ca-based compounds as compared to the Sr-based systems shows that the hybridization mechanism is not relevant for these compounds. The bigger exchange splitting of Ru 4d and Cr 3d at the Fermi level with Ru{sup 4+,5+} and Cr{sup 3+,4+} orbital occupancies of CaRu{sub 0.75}Cr{sub 0.25}O{sub 3} in the LSDA+U calculation, compared with that of the LSDA calculation, shows that repulsion between electrons tend to keep the localized spins from overlapping. The low screening of the Ru t{sub 2g} electrons increases T{sub C} in the Ca-based systems, which is consistent with the both high Ru exchange splitting and magnetic moment. The insulating behavior of the high Cr-doped systems can be explained by considering the Ru{sup 4+}+Cr{sup 4+{yields}}Ru{sup 5+}+Cr{sup 3+} charge transfer. - Graphical Abstract: We have investigated the electronic structure of Sr(Ca)Ru{sub 1-x}Cr{sub x}O{sub 3} using different ab-initio calculation such as LSDA and LSDA+U approximation. The antiferromagnetic hybridization of Cr{sup 3+} with Ru{sup 4+,5+} lattice increases the screening, which is consistent with the low magnetic moment of the Ru ions. The LSDA+U calculation for the more distorted Cr impurity doped Ca
A linked electron pair functional.
Knowles, Peter J; Cooper, Bridgette
2010-12-14
A modification of the variational configuration interaction functional in the first-order interacting space for molecular electronic structure is presented. The modified functional is a fully linked expression that by construction is extensive and invariant to transformations of the underlying orbital basis and is exact for an ensemble of separated two-electron subsystems. In addition, an approximation to variational coupled cluster is generated through truncation of the exponential cluster operator. When combined, these methods demonstrate accuracy that exceeds that of the standard coupled-cluster method, in particular in situations where the reference Slater determinant is not a good approximation. PMID:21171682
Density functional calculations on model tyrosyl radicals.
Himo, F; Gräslund, A; Eriksson, L A
1997-01-01
A gradient-corrected density functional theory approach (PWP86) has been applied, together with large basis sets (IGLO-III), to investigate the structure and hyperfine properties of model tyrosyl free radicals. In nature, these radicals are observed in, e.g., the charge transfer pathways in photosystem II (PSII) and in ribonucleotide reductases (RNRs). By comparing spin density distributions and proton hyperfine couplings with experimental data, it is confirmed that the tyrosyl radicals present in the proteins are neutral. It is shown that hydrogen bonding to the phenoxyl oxygen atom, when present, causes a reduction in spin density on O and a corresponding increase on C4. Calculated proton hyperfine coupling constants for the beta-protons show that the alpha-carbon is rotated 75-80 degrees out of the plane of the ring in PSII and Salmonella typhimurium RNR, but only 20-30 degrees in, e.g., Escherichia coli, mouse, herpes simplex, and bacteriophage T4-induced RNRs. Furthermore, based on the present calculations, we have revised the empirical parameters used in the experimental determination of the oxygen spin density in the tyrosyl radical in E. coli RNR and of the ring carbon spin densities, from measured hyperfine coupling constants. Images FIGURE 1 FIGURE 5 PMID:9083661
Density Functional Theory Models for Radiation Damage
NASA Astrophysics Data System (ADS)
Dudarev, S. L.
2013-07-01
Density functional theory models developed over the past decade provide unique information about the structure of nanoscale defects produced by irradiation and about the nature of short-range interaction between radiation defects, clustering of defects, and their migration pathways. These ab initio models, involving no experimental input parameters, appear to be as quantitatively accurate and informative as the most advanced experimental techniques developed for the observation of radiation damage phenomena. Density functional theory models have effectively created a new paradigm for the scientific investigation and assessment of radiation damage effects, offering new insight into the origin of temperature- and dose-dependent response of materials to irradiation, a problem of pivotal significance for applications.
Teaching Density Functional Theory Through Experiential Learning
NASA Astrophysics Data System (ADS)
Narasimhan, Shobhana
2015-09-01
Today, quantum mechanical density functional theory is often the method of choice for performing accurate calculations on atomic, molecular and condensed matter systems. Here, I share some of my experiences in teaching the necessary basics of solid state physics, as well as the theory and practice of density functional theory, in a number of workshops held in developing countries over the past two decades. I discuss the advantages of supplementing the usual mathematically formal teaching methods, characteristic of graduate courses, with the use of visual imagery and analogies. I also describe a successful experiment we carried out, which resulted in a joint publication co-authored by 67 lecturers and students participating in a summer school.
NASA Astrophysics Data System (ADS)
Kristyán, Sándor
1995-12-01
The effect of basis set in Kohn-Sham theory on estimating correlation energy was investigated using a density functional subroutine and a Hartree-Fock SCF one-electron density program (working with arbitrary basis set). Lee-Yang-Parr density functional estimation, MP2 perturbation calculation and Davidson's accurate configuration interactions calculations for correlation energies of atoms were used for this test. In the density functional estimation a simple 3-21G basis set has produced the same results as the huge triple-zeta basis set with polarization functions within the chemical accuracy (1 kcal/mol).
Applications of large-scale density functional theory in biology
NASA Astrophysics Data System (ADS)
Cole, Daniel J.; Hine, Nicholas D. M.
2016-10-01
Density functional theory (DFT) has become a routine tool for the computation of electronic structure in the physics, materials and chemistry fields. Yet the application of traditional DFT to problems in the biological sciences is hindered, to a large extent, by the unfavourable scaling of the computational effort with system size. Here, we review some of the major software and functionality advances that enable insightful electronic structure calculations to be performed on systems comprising many thousands of atoms. We describe some of the early applications of large-scale DFT to the computation of the electronic properties and structure of biomolecules, as well as to paradigmatic problems in enzymology, metalloproteins, photosynthesis and computer-aided drug design. With this review, we hope to demonstrate that first principles modelling of biological structure-function relationships are approaching a reality.
Applications of large-scale density functional theory in biology.
Cole, Daniel J; Hine, Nicholas D M
2016-10-01
Density functional theory (DFT) has become a routine tool for the computation of electronic structure in the physics, materials and chemistry fields. Yet the application of traditional DFT to problems in the biological sciences is hindered, to a large extent, by the unfavourable scaling of the computational effort with system size. Here, we review some of the major software and functionality advances that enable insightful electronic structure calculations to be performed on systems comprising many thousands of atoms. We describe some of the early applications of large-scale DFT to the computation of the electronic properties and structure of biomolecules, as well as to paradigmatic problems in enzymology, metalloproteins, photosynthesis and computer-aided drug design. With this review, we hope to demonstrate that first principles modelling of biological structure-function relationships are approaching a reality. PMID:27494095
Equatorial electron energy and number densities in the Jovian magnetosphere
NASA Technical Reports Server (NTRS)
Luthey, J. L.
1972-01-01
A synchrotron model with a Maxwellian energy distribution of the form e to the (-E/E sub 0) power is used in a comparison with spatially resolved radio interferometric measurements of the Jovian emission. The observations of the decimeter radiation as a function of equatorial distance at 10.4 and 21 cm wavelength were reduced to source emission/cc of source electrons in each of 16 concentric rings. The peak energies for isotropically distributed electrons exceeded the maximum energy for flat orbiting electrons, and the peaks were generally located from 2.25 to 3 Jupiter radii. Beyond 3 radii, the order of magnitude on number density became a sensitive function of pitch angle distribution. The total equatorial intensities at 75 cm wavelength were computed for (E sub 0)(r) and n(r) at different values of B sub 0. The radiative half life for electrons of initial energy E sub 0 in a dipole field was calculated and found to be nearly constant at one year or less for altitudes at and below the position in peak energy.
NASA Astrophysics Data System (ADS)
Benam, M. R.; Abdoshahi, N.; Majidiyan Sarmazdeh, M.
2014-08-01
In this paper the effect of pressure on the structural and electronic properties of cubic-LaAlO3 including the equilibrium lattice constant, bulk modulus, derivative of bulk modulus and band structure have been calculated by density functional theory (DFT) using GGA, LDA, and PBEsol exchange correlation potentials. It is found that the change of the lattice constant with pressure has an exponential behavior: with increasing pressure, the lattice constant decreases first sharply at low pressures, and then more slowly at high pressures. Furthermore, the lattice constant calculated by the PBEsol method and the bulk modulus calculated by LDA and PBEsol methods are closer to the available experimental values than those obtained using other exchange correlation potentials. Regarding the electronic properties, it is shown that an increase in pressure increases the band gap, the change being 0.26 eV at 34.00 GPa. The total density of state (t-DOS) calculations demonstrate that increasing pressure has a significant effect on the core and conduction band, but little effect on the valence band. The band structure calculations indicate that, in this material, the band gap changes from indirect to direct at a pressure of about 25 GPa. Also, increasing pressure produces a clear curvature in the band structure near the bottom of the conduction band, a behavior consistent with the strong pressure dependence of the transport properties.
NASA Astrophysics Data System (ADS)
Tian, Yali; Zhou, Wei; Wu, Ping
2016-05-01
The effects of Ni and Pd addition on the mechanical, thermodynamic, and electronic properties of AuSn4-based intermetallic compounds (IMCs) have been investigated by first-principles calculations to reveal the essence of Au embrittlement. Three kinds of doped (namely Ni-doped, Pd-doped, and Ni/Pd-codoped) IMCs are considered in this work. The polycrystalline elastic properties are deduced from single-crystal elastic constants. It is found that the doped systems together with nondoped AuSn4 are all ductile phases. For Ni-doped AuSn4, the modulus, hardness, brittleness, Debye temperature, and minimum thermal conductivity increase with the Ni fraction, but this is not the case for the Pd-doped material, since Au0.75Pd0.25Sn4 is the more brittle phase. For Au0.5Pd0.25Ni0.25Sn4, the mechanical, thermodynamic, and electronic properties are similar to those of Au0.5Pd0.5Sn4.
NASA Astrophysics Data System (ADS)
Tian, Yali; Zhou, Wei; Wu, Ping
2016-08-01
The effects of Ni and Pd addition on the mechanical, thermodynamic, and electronic properties of AuSn4-based intermetallic compounds (IMCs) have been investigated by first-principles calculations to reveal the essence of Au embrittlement. Three kinds of doped (namely Ni-doped, Pd-doped, and Ni/Pd-codoped) IMCs are considered in this work. The polycrystalline elastic properties are deduced from single-crystal elastic constants. It is found that the doped systems together with nondoped AuSn4 are all ductile phases. For Ni-doped AuSn4, the modulus, hardness, brittleness, Debye temperature, and minimum thermal conductivity increase with the Ni fraction, but this is not the case for the Pd-doped material, since Au0.75Pd0.25Sn4 is the more brittle phase. For Au0.5Pd0.25Ni0.25Sn4, the mechanical, thermodynamic, and electronic properties are similar to those of Au0.5Pd0.5Sn4.
Ligand identification using electron-density mapcorrelations
Terwilliger, Thomas C.; Adams, Paul D.; Moriarty, Nigel W.; Cohn,Judith D.
2006-12-01
A procedure for the identification of ligands bound incrystal structuresof macromolecules is described. Two characteristics ofthe density corresponding to a ligand are used in the identificationprocedure. One is the correlation of the ligand density with each of aset of test ligands after optimization of the fit of that ligand to thedensity. The other is the correlation of a fingerprint of the densitywith the fingerprint of model density for each possible ligand. Thefingerprints consist of an ordered list of correlations of each the testligands with the density. The two characteristics are scored using aZ-score approach in which the correlations are normalized to the mean andstandard deviation of correlations found for a variety of mismatchedligand-density pairs, so that the Z scores are related to the probabilityof observing a particular value of the correlation by chance. Theprocedure was tested with a set of 200 of the most commonly found ligandsin the Protein Data Bank, collectively representing 57 percent of allligands in the Protein Data Bank. Using a combination of these twocharacteristics of ligand density, ranked lists of ligand identificationswere made for representative (F-o-F-c) exp(i phi(c)) difference densityfrom entries in the Protein Data Bank. In 48 percent of the 200 cases,the correct ligand was at the top of the ranked list of ligands. Thisapproach may be useful in identification of unknown ligands in newmacromolecular structures as well as in the identification of whichligands in a mixture have bound to a macromolecule.
Electronic structure of oxygen functionalized graphene nanoribbons
NASA Astrophysics Data System (ADS)
Simbeck, Adam; Gu, Deyang; Kharche, Neerav; Nayak, Saroj
2013-03-01
We investigate the electronic and magnetic properties of armchair graphene nanoribbons whose edges are passivated by oxygen. Using a first-principles density functional approach and the many-body GW method we find that oxygen-passivation results in a rich geometrical environment which in turn determines the electronic and magnetic properties of the ribbon. For planar systems we report magnetic ground states whose electronic structure depends upon the magnetic coupling between edges. For non-planar ribbons we report a nonmagnetic ground state with a band gap that decreases as a function of increasing ribbon width. Our results will be discussed in light of previous experimental and computational studies. Interconnect Focus Center (MARCO program), State of New York, NSF IGERT program, Grant no. 0333314, and computing resources of the Computationial Center for Nanotechnology Innovation (CCNI), RPI
Properties of the cosmological density distribution function
NASA Astrophysics Data System (ADS)
Bernardeau, Francis; Kofman, Lev
1995-04-01
The properties of the probability distribution function (PDF) of the cosmological continuous density field are studied. We focus our analysis on the quasi-linear regime where various calculations, based on dynamically motivated methods, have been presented: either by using the Zel'dovich approximation (ZA) or by using the perturbation theory to evaluate the behavior of the moments of the distribution function. We show how these two approaches are related to each other and that they can be used in a complementary way. For that respect, the one-dimensional dynamics, where the ZA is exact solution, has first been used as a testing ground. In particular, we show that, when the density PDF obtained with the ZA is regularized, its various moments exhibit the behavior expected by the perturbation theory applied to the ZA. We show that ZA approach can be used for arbitrary initial conditions (not only Gaussian) and that the nonlinear evolution of the moments can be obtained. The perturbation theory can be used for the exact dynamics. We take into account the final filtering of the density field both for ZA and perturbation theory. Applying these techniques, we got the generating function of the moments for the one-dimensional dynamics, the three-dimensional ZA, with and without smoothing effects. We also suggest methods to build PDFs. One is based on the Laplace inverse transform of the moment generating function. The other, the Edgeworth expansion, is obtained when the previous generating function is truncated at a given order and allows evaluation of the PDF out of limited number of moments. It provides insight on the relationship between the moments and the shape of the density PDF. In particular, it provides an alternative method to evaluate the skewness and kurtosis by measuring the PDF around its maximum. Eventually, results obtained from a numerical simulation with cold dark matter initial conditions have been used to validate the accuracy of the considered
Perspective: Fundamental aspects of time-dependent density functional theory
NASA Astrophysics Data System (ADS)
Maitra, Neepa T.
2016-06-01
In the thirty-two years since the birth of the foundational theorems, time-dependent density functional theory has had a tremendous impact on calculations of electronic spectra and dynamics in chemistry, biology, solid-state physics, and materials science. Alongside the wide-ranging applications, there has been much progress in understanding fundamental aspects of the functionals and the theory itself. This Perspective looks back to some of these developments, reports on some recent progress and current challenges for functionals, and speculates on future directions to improve the accuracy of approximations used in this relatively young theory.
Electron density depletions in the nightside auroral zone
NASA Technical Reports Server (NTRS)
Persoon, A. M.; Gurnett, D. A.; Peterson, W. K.; Waite, J. H., Jr.; Burch, J. L.; Green, J. L.
1988-01-01
Dynamics Explorer 1 measurements are used to investigate regions of low electron density in the nightside auroral zone. Sharply defined regions of low electron density are found in auroral zone crossings from the predusk hours until the early morning hours at all radial distances up to at least 4.6 earth radii. Densities in the auroral cavity are shown to fall to values below 0.3/cu cm. Within the auroral cavity, electron-density-profile variations of a factor of 2 or more on spatial scales of tens of kilometers are found, and the electron plasma frequency to electron cyclotron frequency ratios are 0.02-0.4. The results suggest associations between the density depletions in the nightside auroral zone and auroral acceleration processes.
Density Functional Study of Perovskite Superconductor MgCNi3
NASA Astrophysics Data System (ADS)
Kumar, Jagdish; Sharma, Devina; Kumar, Ranjan; Awana, V. P. S.; Ahluwalia, P. K.
2011-12-01
We here report the first principle density functional study of MgCNi3 which crystallize in cubic perovskite structure having critical transition temperature of 8 K. The interesting aspect of this compound is that in normal state it is non magnetic in nature despite conduction electrons in it are derived from partially filled Ni d states, which typically lead to ferromagnetism in metallic Ni and many Ni-based binary alloys. To investigate the detailed microscopic origin of the non magnetic nature we have done density functional based calculations on this compound. The lattice constant is calculated using minimum energy criteria from total energy versus lattice constant plot. By taking the calculated values of lattice constant we have done the precise calculations on the compound using Full Potential Linear Augmented Plane Wave (FP-LAPW) method implemented in ELK code. The electronic density of states is found spin degenerate that corresponds to a non-magnetic ground state. The density of states (DOS) at Fermi level, N(EF) is dominated by Ni-d states. The sharp peak observed just below Fermi level corresponds to van Hove singularity (vHs). The projected density of states (PDOS) suggests a strong hybridization of Ni-3d and C-2p states which is responsible for the observed non magnetic nature of MgCNi3.
NASA Astrophysics Data System (ADS)
Wang, C. X.; Lv, Z. Q.; Fu, W. T.; Li, Y.; Sun, S. H.; Wang, B.
2011-08-01
First-principles calculations have been performed to investigate the magnetic properties and phase stability of alloyed cementite with Co or Ni. The calculations show Co and Ni reduce the stability of cementite and (Fe,M) 3C (X = Co/Ni) form more difficultly than cementite. We predict that Ni and Co additions in high Co and Ni alloyed steels will promote the dissolution of the cementite. The magnetic moments (Ms) of Fe 2CoC, Co 2FeC, Fe 2NiC and Ni 2FeC are 4.86, 4.23, 3.95 and 2.41 μ B/f.u, respectively. The Ms of Co in Fe 2CoC (1.03 μ B) and Co 2FeC (1.09 μ B) are different due to replacing different Fe atoms. The Ms of Ni in Fe 2NiC and Ni 2FeC are 0.14 μ B and 0.15 μ B, respectively. The Ms of alloyed cementite are mainly contributed by 3d electrons of metal atoms.
Ritzmann, Andrew M; Dieterich, Johannes M; Carter, Emily A
2016-04-28
Reducing operating temperatures is a key step in making solid oxide fuel cell (SOFC) technology viable. A promising strategy for accomplishing this goal is employing mixed ion-electron conducting (MIEC) cathodes. La1-xSrxCo1-yFeyO3-δ (LSCF) is the most widely employed MIEC cathode material; however, rational optimization of the composition of LSCF requires fundamental insight linking its electronic structure to its defect chemistry. To provide the necessary insight, density functional theory plus U (DFT+U) calculations are used to investigate the electronic structure of LSCF (xSr = 0.50, yCo = 0.25). The DFT+U calculations show that LSCF has a significantly different electronic structure than La1-xSrxFeO3 because of the addition of cobalt, but that minimal electronic structure differences exist between La0.5Sr0.5Co0.25Fe0.75O3 and La0.5Sr0.5Co0.5Fe0.5O3. The oxygen vacancy formation energy (ΔEf,vac) is calculated for residing in different local environments within La0.5Sr0.5Co0.25Fe0.75O3. These results show that configurations have the highest ΔEf,vac, while have the lowest ΔEf,vac and may act as traps for . We conclude that compositions with more Fe than Co are preferred because the additional sites would lead to higher overall ΔEf,vac (and lower concentrations), while the trapping strength of the sites is relatively weak (∼0.3 eV). PMID:27079696
A Wigner Monte Carlo approach to density functional theory
NASA Astrophysics Data System (ADS)
Sellier, J. M.; Dimov, I.
2014-08-01
In order to simulate quantum N-body systems, stationary and time-dependent density functional theories rely on the capacity of calculating the single-electron wave-functions of a system from which one obtains the total electron density (Kohn-Sham systems). In this paper, we introduce the use of the Wigner Monte Carlo method in ab-initio calculations. This approach allows time-dependent simulations of chemical systems in the presence of reflective and absorbing boundary conditions. It also enables an intuitive comprehension of chemical systems in terms of the Wigner formalism based on the concept of phase-space. Finally, being based on a Monte Carlo method, it scales very well on parallel machines paving the way towards the time-dependent simulation of very complex molecules. A validation is performed by studying the electron distribution of three different systems, a Lithium atom, a Boron atom and a hydrogenic molecule. For the sake of simplicity, we start from initial conditions not too far from equilibrium and show that the systems reach a stationary regime, as expected (despite no restriction is imposed in the choice of the initial conditions). We also show a good agreement with the standard density functional theory for the hydrogenic molecule. These results demonstrate that the combination of the Wigner Monte Carlo method and Kohn-Sham systems provides a reliable computational tool which could, eventually, be applied to more sophisticated problems.
Lithium adsorption on graphite from density functional theory calculations.
Valencia, Felipe; Romero, Aldo H; Ancilotto, Francesco; Silvestrelli, Pier Luigi
2006-08-01
The structural, energetic, and electronic properties of the Li/graphite system are studied through density functional theory (DFT) calculations using both the local spin density approximation (LSDA), and the gradient-corrected Perdew-Burke-Ernzerhof (PBE) approximation to the exchange-correlation energy. The calculations were performed using plane waves basis, and the electron-core interactions are described using pseudopotentials. We consider a disperse phase of the adsorbate comprising one Li atom for each 16 graphite surface cells, in a slab geometry. The close contact between the Li nucleus and the graphene plane results in a relatively large binding energy (larger than 1.1 eV). A detailed analysis of the electronic charge distribution, density difference distribution, and band structures indicates that one valence electron is entirely transferred from the atom to the surface, which gives rise to a strong interaction between the resulting lithium ion and the cloud of pi electrons in the substrate. We show that it is possible to explain the differences in the binding of Li, Na, and K adatoms on graphite considering the properties of the corresponding cation/aromatic complexes. PMID:16869593
Electron and ion densities in interstellar clouds
NASA Technical Reports Server (NTRS)
Glassgold, A. E.; Langer, W. D.
1974-01-01
A quantitative theory of ionization in diffuse clouds is developed which includes H(+) charge exchange with O. Dissociative charge exchange of He(+) with H2 plays an important role in the densities of H(+) and He(+). The abundance of HD is also discussed.
Progress at the interface of wave-function and density-functional theories
Gidopoulos, Nikitas I.
2011-04-15
The Kohn-Sham (KS) potential of density-functional theory (DFT) emerges as the minimizing effective potential in a variational scheme that does not involve fixing the unknown single-electron density. Using Rayleigh Schroedinger (RS) perturbation theory (PT), we construct ab initio approximations for the energy difference, the minimization of which determines the KS potential directly - thereby bypassing DFT's traditional algorithm to search for the density that minimizes the total energy. From second-order RS PT, we obtain variationally stable energy differences to be minimized, solving the severe problem of variational collapse of orbital-dependent exchange-correlation functionals based on second-order RS PT.
Density functional theory calculations of Rh-β-diketonato complexes.
Conradie, J
2015-01-28
Density functional theory (DFT) results on the geometry, energies and charges of selected Rh-β-diketonato reactants, products and transition states are discussed. Various DFT techniques are used to increase our understanding of the orientation of ligands coordinated to Rh, to identify the lowest energy geometry of possible geometrical isomers and to get a molecular orbital understanding of ground and transition states. Trends and relationships obtained between DFT calculated energies and charges, experimentally measured values and electronic parameters describing the electron donating power of groups and ligands, enable the design of ligands and complexes of specific reactivity. PMID:25429658
Petrenko, Taras; Kossmann, Simone; Neese, Frank
2011-02-01
In this paper, we present the implementation of efficient approximations to time-dependent density functional theory (TDDFT) within the Tamm-Dancoff approximation (TDA) for hybrid density functionals. For the calculation of the TDDFT/TDA excitation energies and analytical gradients, we combine the resolution of identity (RI-J) algorithm for the computation of the Coulomb terms and the recently introduced "chain of spheres exchange" (COSX) algorithm for the calculation of the exchange terms. It is shown that for extended basis sets, the RIJCOSX approximation leads to speedups of up to 2 orders of magnitude compared to traditional methods, as demonstrated for hydrocarbon chains. The accuracy of the adiabatic transition energies, excited state structures, and vibrational frequencies is assessed on a set of 27 excited states for 25 molecules with the configuration interaction singles and hybrid TDDFT/TDA methods using various basis sets. Compared to the canonical values, the typical error in transition energies is of the order of 0.01 eV. Similar to the ground-state results, excited state equilibrium geometries differ by less than 0.3 pm in the bond distances and 0.5° in the bond angles from the canonical values. The typical error in the calculated excited state normal coordinate displacements is of the order of 0.01, and relative error in the calculated excited state vibrational frequencies is less than 1%. The errors introduced by the RIJCOSX approximation are, thus, insignificant compared to the errors related to the approximate nature of the TDDFT methods and basis set truncation. For TDDFT/TDA energy and gradient calculations on Ag-TB2-helicate (156 atoms, 2732 basis functions), it is demonstrated that the COSX algorithm parallelizes almost perfectly (speedup ~26-29 for 30 processors). The exchange-correlation terms also parallelize well (speedup ~27-29 for 30 processors). The solution of the Z-vector equations shows a speedup of ~24 on 30 processors. The
NASA Astrophysics Data System (ADS)
Petrenko, Taras; Kossmann, Simone; Neese, Frank
2011-02-01
In this paper, we present the implementation of efficient approximations to time-dependent density functional theory (TDDFT) within the Tamm-Dancoff approximation (TDA) for hybrid density functionals. For the calculation of the TDDFT/TDA excitation energies and analytical gradients, we combine the resolution of identity (RI-J) algorithm for the computation of the Coulomb terms and the recently introduced "chain of spheres exchange" (COSX) algorithm for the calculation of the exchange terms. It is shown that for extended basis sets, the RIJCOSX approximation leads to speedups of up to 2 orders of magnitude compared to traditional methods, as demonstrated for hydrocarbon chains. The accuracy of the adiabatic transition energies, excited state structures, and vibrational frequencies is assessed on a set of 27 excited states for 25 molecules with the configuration interaction singles and hybrid TDDFT/TDA methods using various basis sets. Compared to the canonical values, the typical error in transition energies is of the order of 0.01 eV. Similar to the ground-state results, excited state equilibrium geometries differ by less than 0.3 pm in the bond distances and 0.5° in the bond angles from the canonical values. The typical error in the calculated excited state normal coordinate displacements is of the order of 0.01, and relative error in the calculated excited state vibrational frequencies is less than 1%. The errors introduced by the RIJCOSX approximation are, thus, insignificant compared to the errors related to the approximate nature of the TDDFT methods and basis set truncation. For TDDFT/TDA energy and gradient calculations on Ag-TB2-helicate (156 atoms, 2732 basis functions), it is demonstrated that the COSX algorithm parallelizes almost perfectly (speedup ˜26-29 for 30 processors). The exchange-correlation terms also parallelize well (speedup ˜27-29 for 30 processors). The solution of the Z-vector equations shows a speedup of ˜24 on 30 processors. The
Modulation Based on Probability Density Functions
NASA Technical Reports Server (NTRS)
Williams, Glenn L.
2009-01-01
A proposed method of modulating a sinusoidal carrier signal to convey digital information involves the use of histograms representing probability density functions (PDFs) that characterize samples of the signal waveform. The method is based partly on the observation that when a waveform is sampled (whether by analog or digital means) over a time interval at least as long as one half cycle of the waveform, the samples can be sorted by frequency of occurrence, thereby constructing a histogram representing a PDF of the waveform during that time interval.
Density Functional Theory of Biologically Relevant Metal Centers
NASA Astrophysics Data System (ADS)
Siegbahn, Per E. M.; Blomberg, Margareta R. A.
1999-10-01
Recent applications of density functional theory to biologically relevant metal centers are reviewed. The emphasis is on reaction mechanisms, structures, and modeling. The accuracy of different functionals is discussed for standard benchmark tests of first- and second-row molecules and for transition metal systems. Modeling aspects of the protein metal complexes are discussed regarding both the size of the model being treated quantum mechanically and the treatment of the protein surrounding it. To illustrate the effects, structures computed without the effects of the protein are compared with experimental structures from enzymes, and results from simple dielectric models of the protein for electron transfer processes are described. The choice of spin state is discussed for multimetal complexes. Examples of mechanisms studied recently by density functional theory are described, such as O2 and methane activation in methane monooxygenase and O2 formation in photosystem II.
Measurement of electron density using reactance cutoff probe
NASA Astrophysics Data System (ADS)
You, K. H.; You, S. J.; Kim, D. W.; Na, B. K.; Seo, B. H.; Kim, J. H.; Seong, D. J.; Chang, H. Y.
2016-05-01
This paper proposes a new measurement method of electron density using the reactance spectrum of the plasma in the cutoff probe system instead of the transmission spectrum. The highly accurate reactance spectrum of the plasma-cutoff probe system, as expected from previous circuit simulations [Kim et al., Appl. Phys. Lett. 99, 131502 (2011)], was measured using the full two-port error correction and automatic port extension methods of the network analyzer. The electron density can be obtained from the analysis of the measured reactance spectrum, based on circuit modeling. According to the circuit simulation results, the reactance cutoff probe can measure the electron density more precisely than the previous cutoff probe at low densities or at higher pressure. The obtained results for the electron density are presented and discussed for a wide range of experimental conditions, and this method is compared with previous methods (a cutoff probe using the transmission spectrum and a single Langmuir probe).
Generalization of the Kohn-Sham system that can represent arbitrary one-electron density matrices
NASA Astrophysics Data System (ADS)
van Dam, Hubertus J. J.
2016-05-01
Density functional theory is currently the most widely applied method in electronic structure theory. The Kohn-Sham method, based on a fictitious system of noninteracting particles, is the workhorse of the theory. The particular form of the Kohn-Sham wave function admits only idempotent one-electron density matrices whereas wave functions of correlated electrons in post-Hartree-Fock methods invariably have fractional occupation numbers. Here we show that by generalizing the orbital concept and introducing a suitable dot product as well as a probability density, a noninteracting system can be chosen that can represent the one-electron density matrix of any system, even one with fractional occupation numbers. This fictitious system ensures that the exact electron density is accessible within density functional theory. It can also serve as the basis for reduced density matrix functional theory. Moreover, to aid the analysis of the results the orbitals may be assigned energies from a mean-field Hamiltonian. This produces energy levels that are akin to Hartree-Fock orbital energies such that conventional analyses based on Koopmans' theorem are available. Finally, this system is convenient in formalisms that depend on creation and annihilation operators as they are trivially applied to single-determinant wave functions.
Generalization of the Kohn-Sham system that can represent arbitrary one-electron density matrices
Hubertus J. J. van Dam
2016-05-23
Density functional theory is currently the most widely applied method in electronic structure theory. The Kohn-Sham method, based on a fictitious system of noninteracting particles, is the workhorse of the theory. The particular form of the Kohn-Sham wave function admits only idempotent one-electron density matrices whereas wave functions of correlated electrons in post-Hartree-Fock methods invariably have fractional occupation numbers. Here we show that by generalizing the orbital concept and introducing a suitable dot product as well as a probability density, a noninteracting system can be chosen that can represent the one-electron density matrix of any system, even one with fractionalmore » occupation numbers. This fictitious system ensures that the exact electron density is accessible within density functional theory. It can also serve as the basis for reduced density matrix functional theory. Moreover, to aid the analysis of the results the orbitals may be assigned energies from a mean-field Hamiltonian. This produces energy levels that are akin to Hartree-Fock orbital energies such that conventional analyses based on Koopmans' theorem are available. Lastly, this system is convenient in formalisms that depend on creation and annihilation operators as they are trivially applied to single-determinant wave