Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P27: Electronic Structure II |
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Sponsoring Units: DCOMP Chair: Anne Chaka, National Institute of Standards and Technology Room: Baltimore Convention Center 324 |
Wednesday, March 15, 2006 11:15AM - 11:27AM |
P27.00001: Analytic structure of Bloch functions for linear molecular chains Emil Prodan In this talk I will discuss Hamiltonians of the form $H=-{\bf \nabla}^2+v(x,y,z)$, with $v (x,y,z)$ periodic along the $z$ direction, $v(x,y,z+b)=v(x,y,z)$. The wavefunctions of $H$ are the well known Bloch functions $\psi_{n,\lambda}(x,y,z)$, with the fundamental property $\psi_{n,\lambda}(x,y,z+b)=\lambda \psi_{n,\lambda}(x,y,z)$ and $\partial_z\psi_{n, \lambda}(x,y,z+b)=\lambda \partial_z\psi_{n,\lambda}(x,y,z)$. I will give the generic analytic structure (i.e. the Riemann surface) of $\psi_{n,\lambda}(x,y,z)$ and their corresponding energy, $E_n (\lambda)$, as functions of $\lambda$. I will also discuss several applications, like a compact expression of the Green's function or the asymptotic behavior of the density matrix and other correlation functions for insulating molecular chains. [Preview Abstract] |
Wednesday, March 15, 2006 11:27AM - 11:39AM |
P27.00002: First-principles calculation of Hubbard parameter: Constrained local density functional approach with Maximally localized Wannier function Kazuma Nakamura , Ryotaro Arita , Yoshihide Yoshimoto , Shinji Tsuneyuki We present a new ab initio method for calculating effective onsite Coulomb interactions of itinerant and strongly correlated electron systems. The method is based on constrained local density functional theory formulated in terms of maximally localized Wannier functions. This scheme can be implemented with any basis, and thus allows us to perform the constrained calculation with plane-wave-based electronic-structure codes. We apply the developed method to the evaluation of the onsite interaction of 3d transition-metal series. The results are discussed using a heuristic formula for screened Coulomb interactions. This work was supported by NAREGI Nanoscience Project, Ministry of Education, Culture, Sports, Science and Technology, Japan. [Preview Abstract] |
Wednesday, March 15, 2006 11:39AM - 11:51AM |
P27.00003: Electronic structure of Cu$_{2-x}$S and related compounds Pavel Lukashev , Walter R. L. Lambrecht , Takao Kotani , Mark van Schilfgaarde Chalcosite Cu$_2$S and digenite Cu$_{1.8}$S are possibly interesting semiconductors for photovoltaic applications. Their electronic structure is poorly understood because their crystal structure is complex. If consists of a close-packed lattice of S with mobile Cu occupying various types of interstitial sites with a statistical distribution depending on temperature. As a starting point for understanding these materials, we investigated the simpler antifluorite structure. Both local density approximation (LDA) and self-consistent quasiparticle GW calculations with the full-potential linearized muffin-tin orbital method give a semimetallic band structure with the Fermi level pinned at a degenerate Cu-d band state at $\Gamma$. A random distortion of the Cu atoms from the perfect antifluorite positions inside each S cage is found to break the degeneracy of the $d$ state at $\Gamma$ and thus opens up a small gap of about 0.1 eV in LDA. The experimental evidence for a semiconducting gap of about 1 eV is critically examined. To gain further insight into the Cu d and s-band shifts beyond LDA, we considered other Cu compounds such as Cu$_2$O and CuBr. We compare their LDA and GW band structures and determined the effective masses and Kohn-Luttinger Hamiltonian parameters for CuBr. [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P27.00004: The Applicability of Different Quantum Mechanical Methods to Transition Metal Oxides Orkid Coskuner , Emily Jarvis , Thomas Allison , Carlos Gonzalez , Anne Chaka Numerous quantum mechanical methods and basis sets have been applied extensively to organic molecules. However, the performance of these is not well understood for transition metal oxides. We employed different methods along with several basis sets for optimizing the geometries in the gas phase and calculating the IR spectra as well as thermodynamic properties including Gibbs free energy and enthalpy of linear, trigonal and tetrahedral metal oxides. The MCSCF and DFT methods generally give the most accurate results for organic and inorganic molecules. Surprisingly, our studies showed that the results obtained for iron(III) oxides at the GVB and MP2 levels gave more accurate results than the MCSCF and hybrid methods. Similarly, for aluminum and chromium oxides, the calculations with MP2 and PBE yielded thermodynamic properties, which are closer to experimental values. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P27.00005: Zeroth-moment dielectric sum rule applied to electron damping Eric L. Shirley , J.A. Soininen , J.J. Rehr The first and inverse-first frequency moments of the dielectric function, epsilon(q,omega), are given by the f-sum rule and Kramers-Kronig transformation of the static dielectric function. Model expressions for these quantities are plentiful. The square of the zeroth moment must be less than the product of the above two, by Cauchy-Schwartz. (It equals that product in single-plasmon-pole models). In this work, we present simple ways to estimate the zeroth moment as a function of q. This facilitates an improved model for epsilon(q,omega) that requires minimal computation and exhibits realistic behavior without use of a pole model. We apply this to calculating the electron self-energy, particularly lifetime damping effects in insulators near the band gap. [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P27.00006: Relativistic real-space multiple scattering calculations of EELS K. Jorissen , J.J. Rehr , A. Sorini , Z.H. Levine We present an extension of the real space multiple scattering code FEFF8 for {\it ab initio}, relativistic calculations of electron energy loss spectra (EELS), which is applicable both to periodic and non-periodic systems. The approach explains the observed relativistic shifts in the magic angle. \footnote{B. Jouffrey, P. Schattschneider and C. Hebert, Ultramicroscopy {\bf102}, 61 (2004).} In addition, the method can account for experimental parameters such as collection and convergence angles of the microscope and sample orientation. We also discuss relativistic effects on inelastic electron scattering including the density correction to the stopping power. Our results are compared with other approaches and with experiment. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 12:39PM |
P27.00007: Prediction of Born-Oppenheimer Interatomic Forces Using Orbital-Free Density Functional Theory with Approximate Kinetic Energy Functionals S.B. Trickey , V.V. Karasiev , Frank E. Harris Rapid calculation of Born-Oppenheimer forces is essential for driving the so called quantum region of a multi-scale molecular dynamics (MD) simulation. The orbital-free (OF) DFT approach is appealing but has proven difficult to implement because of the challenge of constructing reliable orbital-free approximations to the kinetic energy functional. To be maximally useful for multi-scale simulations, an OF-KE functional must be local (i.e. one-point). In the face of these difficulties, we demonstrate that there is a way forward. By requiring only that the approximate functional deliver high-quality forces, by exploiting the ``conjointness'' hypothesis of Lee, Lee, and Parr, by enforcing a basic positivity constraint, and by parameterizing to a carefully selected, small set of molecules we are able to generate a OF-KE functional that does a good job of describing various H$_q$Si$_m$O$_n$ clusters as well as CO and H$_2$O (providing encouraging evidence of transferability). [Preview Abstract] |
Wednesday, March 15, 2006 12:39PM - 12:51PM |
P27.00008: Fitting of Molecular Densities by Compact, Atom-Centered Expansion V.V. Karasiev , S.B. Trickey , Frank E. Harris Use of an orbital-free (OF) version of DFT requires both a suitable approximate Kohn-Sham kinetic energy functional and a systematic but simple model of the system density. We report useful approximations to the KS density via a very compact expansion in atom-centered functions. Spherically averaged, isolated-atom densities are used as basis functions to expand spherically symmetric atom-centered contributions. A simplified expansion in real spherical harmonics is then added to the spherically symmetric contributions. Although drastically simplified, such representations of the density nevertheless result in impressively small mean square deviations relative to the target KS density. The fitted density can then be combined with an approximate OF-KE functional we have developed \footnote{V.V. Karasiev, S.B. Trickey, and F.E. Harris, J. Comp. Aided Mat. Des. (2005) (accepted).} to generate energy surfaces. These energy surfaces have shapes similar to those arising from true KS densities, and are therefore suitable for calculation of forces to drive molecular dynamics simulations. [Preview Abstract] |
Wednesday, March 15, 2006 12:51PM - 1:03PM |
P27.00009: Magnetic Field Effects upon Exchange-Correlation in the Hooke's Atom Wuming Zhu , Samuel Trickey Extending Density Functional Theory (DFT) to coulombic systems in a non-vanishing magnetic field in a computationally feasible way is highly desirable. Even though the current DFT (CDFT) formalism is long-established, there still are no generally applicable, reliable $E_{xc}, {\mathbf A}_{xc}$ functionals analogous with the LDA. Progress can be made by comparison study on a solvable correlated system. Hooke's atom is well-known in ordinary DFT because its Schr\"{o}dinger equation can be solved exactly for some coupling strengths and numerically with high accuracy for the rest. Hence exact Kohn-Sham quantities are readily available. Using our extensions (exact and numerical) to non-zero B-field, we examined the effects on exchange- correlation holes and energies and considered possible ways to include the essential ones in $E_{xc}, {\mathbf A}_{xc} $. In our tests, the CDFT vorticity variable, $\nu$, turns out to be a computationally difficult quantity which may not be appropriate in practice to describe external B field effects on $E_{xc}, {\mathbf A}_{xc}$. [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P27.00010: The problems in the density functional theory with the total spin and space symmetry and the invariant properties of the electron density. I.G. Kaplan The problems in the density functional theory (DFT) arising when it is applied to the spin and space multiplets are discussed. It is rigorously proved that the electron density of an arbitrary $N$-electron system does not depend upon the value of the total spin $S$ of the state and preserves the same analytical form for all states with the definite $S$. It is also proved that the diagonal element of the full density matrix is invariant respecting all operation of the group symmetry of the state, i.e, it is a group invariant. From these results follows that the problems in DFT with the total spin and degenerated states cannot be solved within the framework of density matrix formalism. [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P27.00011: Dehydrogenation in catalyst activated MgH$_{2}$ Sa Li , Puru Jena , Rajeev Ahuja Dehydrogenation in catalyst activated magnesium hydride (MgH$_{2})$ has been investigated using \textit{ab initio} Molecular Dynamics (MD) simulation and Nudged Elastic Band (NEB) method. Our calculation explains why small amount of Nb$_{2}$O$_{5}$ catalyst can substantially improve the thermodynamics and kinetics of MgH$_{2}$. We show that Nb$_{2}$O$_{5}$ promotes the creation of Mg vacancies and that the hydrogen desorption from the vicinity of Mg vacancies occurs in molecular form and is exothermic. The activation energy barrier for H$_{2}$ desorption in vacancy containing magnesium hydride (1.02 eV) is much lower than that in the pure magnesium hydride (3.30 eV). Therefore, the effective catalyst for dehydrogenation in MgH$_{2}$ will be one that can easily facilitate MgO formation. [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P27.00012: A method for biased surface electronic structure: a planewave non-repeated slab approach Minoru Otani , Osamu Sugino We have developed a new formalism for calculating electronic structures in a symmetric/asymmetric slab model [1]. The method can treat not only surfaces exposed to vacuum but also biased surfaces. To solve the Kohn-Sham equation, we adopt a conventional Kohn-Sham solver in a repeated slab model. On the other hand, for the Poisson equation, we solve it in a whole space along surface normal direction. Owing to this treatment we can easily obtain work functions of the surface and we can calculate polarized surfaces without dipole correction. By introducing an effective screening medium and imposing appropriate boundary conditions to the Poisson equation, we can calculate a surface that is placed in front of electrode. In this model it is possible to apply a bias voltage to the surface by changing the Fermi energy of the surface. Thus we can calculate electronic and geometric structures of the biased surface. This model corresponds to the experimental setup for the scanning tunneling microscopy or back-gate field effect transistor. The important advantage of the method is that we can easily implement it in a conventional first-principles calculation method. References: [1] M. Otani and O. Sugino, submitted to PRB. [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P27.00013: Many-Body Electronic Structure of Curium metal Antonina Toropova , Kristjan Haule , Gabriel Kotliar We report computer-based simulations for the many-body electronic structure of Curium metal. Cm belongs to the actinide series and has a half-filled shell with seven $5f$ electrons. As a function of pressure, curium exhibits five different crystallographic phases. At low temperatures all phases demonstrate either antiferromagnetic or ferrimagnetic ordering. In this study we perform LDA+DMFT calculations for the antiferromagnetic state of high-pressure fcc modification of Curium metal. [Preview Abstract] |
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