Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q23: Electronic Structure |
Hide Abstracts |
Sponsoring Units: DCOMP Chair: Guang-Lin Zhao, Southern University Room: C125-C126 |
Wednesday, March 17, 2010 11:15AM - 11:27AM |
Q23.00001: GW approximation of the many-body problem and changes in the particle number Fabien Bruneval A stringent test for an exchange-correlation approximation in electronic structure calculations is the equality between the ionization energy of the neutral system and the affinity of the singly positively charged system. All of the commonly used approximations (local, semilocal, hybrid) for the exchange correlation within density functional theory fail badly with this test. They consequently present a localization or delocalization error, resulting in a highest occupied molecular orbital or lowest unoccupied molecular orbital gap over- or underestimation [1]. The GW approximation [2] appears as the best available framework to describe particle number changes [3]. The small remaining error can be further reduced by devising a $Delta$SCF-like method within the GW approximation. The proposed approach is necessary as soon as localized electrons are involved. We then show applications to molecules, ions, and defect states in crystals. \\[4pt] [1] A. J. Cohen, P. Mory-S\'anchez, W. T. Yang, Science \textbf{321}, 792 (2008).\\[0pt] [2] L. Hedin, Phys. Rev. \textbf{139}, A796 (1965).\\[0pt] [3] F. Bruneval, Phys. Rev. Lett. \textbf{103}, 176403 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 11:27AM - 11:39AM |
Q23.00002: Using Hartree-Fock pseudopotentials in GW calculations D.R. Hamann, David Vanderbilt The issue of including shallow ``semi-core'' states as valence has recently resurfaced in the context of self-consistent GW calculations.\footnote{F. Bruneval \textit{et al}., Phys. Rev. Lett. \textbf{97}, 267601 (2006).} Supposing that semi-core-valence exchange is the dominant process necessitating the inclusion of semi-cores, we have investigated whether the use Hartree-Fock pseudopotentials\footnote{W. A. Al-Saidi, E. J. Walter, and A. M. Rappe, Phys. Rev. B \textbf{77}, 075122 (2008).} instead of density-functional psp's might obviate the need for semi-cores. The answers to this question appear to be ``yes'' for the case of CuCl (filled d shell), and ``semi-cores don't matter anyway'' for ScN (empty d shell). Opportunity permitting, additional examples will be discussed. [Preview Abstract] |
Wednesday, March 17, 2010 11:39AM - 11:51AM |
Q23.00003: Quasiparticle properties of ZnO: Effects of semicore states and nonuniform convergence of the Coulomb hole self-energy Peihong Zhang, Bi-ching Shih, Yu Xue Understanding of the quasiparticle properties of ZnO, a supposedly simple $sp$ semiconductors, turns out to be rather challenging. The band gap of ZnO calculated within the local density approximation (LDA) is about 0.7 eV if semicore 3$s$, 3 $p$, and 3$d$ electrons are considered as valence electrons. Subsequent quasiparticle calculations within the {\it conventional} $GW$ approximation gives a band gap of about 2.4 eV, which is more than 1 eV smaller than the recent experimental result. We have recently revisited this problem and found that the severe underestimate of the quasiparticle band gap of ZnO can be resolved if (1) The semicore Zn 3$d$ states are treated properly, and (2) The evaluation of the Coulomb hole self-energy is adequately converged. We are able to reproduce of the experimental quasiparticle band gap of ZnO without the need of performing self-consistent GW calculations. [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:03PM |
Q23.00004: Ab initio calculation of natural band offsets of all group IV, II-VI and III-V semiconductors Su-Huai Wei, Aron Walsh, Yong-Hua Li, Xingao Gong The natural band offset between semiconductors is one of the most fundamental properties in materials physics. It is a necessary quantity to assess charge transport and quantum confinement, and is of particular relevance to the design of optoelectronic devices which feature an interface between two or more materials. However, in the past, the natural band offset calculations were based on the assumption that certain reference levels (core levels, average Coulomb potentials, etc.) have zero absolute deformation potential, and thus align between the bulk and heterostructures. In this study [1], using an all-electron band structure approach, we have systematically calculated the natural band offsets between all group IV, III-V and II-VI semiconductor compounds, taking into account the deformation potential of the core states. This revised approach removes assumptions regarding the deformation potential of the reference levels, and offers a more reliable prediction of the `natural' unstrained offsets. Comparison is made to experimental work, where a noticeable improvement is found compared to previous methodologies. [1] Y.-H. Li et al., Appl. Phys. Lett. \textbf{94}, 212109 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 12:03PM - 12:15PM |
Q23.00005: Direct calculation of the one particle Green's function: an alternative to the self-energy Giovanna Lani, Pina Romaniello, Lucia Reining Calculations of quasi-particle band structures and photo- emission spectra of materials rely more and more on perturbative many-body techniques like the \textit{GW} approximation. In this approach the one-body Green's function (G) is determined from a Dyson equation containing a \textit{self-energy} kernel $\Sigma$ which is approximated as a product of G and the screened Coulomb interaction W. To go beyond the first order in W, iterative schemes of Hedin's equations \footnote {Hedin, L., Phys. Rev. 139, (1965) A796} have been proposed. While generally successful, GW is known to fail for strongly correlated systems and it is far from obvious that one could improve on this through further iterations of Hedin's equations. In the present work we explore an alternative route: in place of approximating $\Sigma$, we reformulate the problem in terms of a set of coupled first order differential equations for the unknown G. We benchmark this approach on small Hubbard clusters, for which one can calculate the exact one-body G. [Preview Abstract] |
Wednesday, March 17, 2010 12:15PM - 12:27PM |
Q23.00006: GW method in the PAW formalism apply to ZnO and SnO Gabriel Antonius, Martin Stankovski, Anna Miglio, Gian-Marco Rignanese, Michel Cote Zinc oxide (ZnO) and tin oxide (SnO, SnO$_{2})$ are wide band gap semiconductors of interest for the fabrication of transparent conducting oxides (TCO). We present an initio study of these materials combining the projector augmented wave (PAW) formalism with the GW method to obtain quasiparticles band gaps. The PAW allows an accurate description of the d semi-core states which are important to correctly obtain the quasiparticle energies. The effect of these electrons on the calculated gap will be discussed. We also compare the structural and electronic description of the systems resulting from different exchange-correlation functionals (LDA, GGA and the use of a Hubbard U term). Finally, preliminary results of quasiparticle self-consistent calculations will also be presented. [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 12:39PM |
Q23.00007: Accelerating the Computation of Hartree-Fock Exchange using Recursive Bisection Ivan Duchemin, Francois Gygi The high computational cost of Hartree-Fock exchange currently limits the use of hybrid density functionals in First-Principles Molecular Dynamics (FPMD) applications. Efficient algorithms are essential for large-scale electronic structure calculations. We present a parallel implementation based on a recursive bisection algorithm [1] that allows one to reduce the number of significant exchange integrals with controlled accuracy. By adjusting the threshold of truncation of wavefunctions, the calculation is accelerated by a factor of up to 6 for a 32-molecule simulation of water. The accuracy of the truncation can be progressively adjusted during self-consistent iterations for optimal acceleration. We analyze the error in ionic forces as a function of the truncation threshold. The approach is demonstrated in a molecular dynamics simulation of liquid water using the PBE0 hybrid exchange-correlation functional. \\[4pt] [1] F. Gygi, Compact Representations of Kohn-Sham Invariant Subspaces, Phys. Rev. Lett. 102, 166406 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 12:39PM - 12:51PM |
Q23.00008: GW quasi-particle spectra without sums over empty states Paolo Umari, Stefano Baroni We introduce a method that allows for the calculation of quasi-particle spectra in the GW approximation, yet avoiding any explicit reference to empty one-electron states. This is achieved by expressing the irreducible polarizability operator and the self-energy operator through a set of linear response equations, which are solved using a Lanczos-chain algorithm. We first validate our approach by calculating the vertical ionization energies of the benzene molecule and then show its potential by addressing the spectrum of a large molecule such as free-base tetraphenylporphyrin and those of some glassy materials. [Preview Abstract] |
Wednesday, March 17, 2010 12:51PM - 1:03PM |
Q23.00009: First principle modeling of magnetic structure in Fe-C and intermetalics Aurelian Rusanu, Don Nicholson, Markus Eisenbach, G.M. Stocks Magnetic structure in Fe rich materials presents a high dependence on local atomic arrangement affecting mechanical, magneto-caloric, and magnetization properties. Insights into local properties and bulk effects can be obtained from first principle analysis. In this study we deploy the LSMS and VASP methods to model the ground states of various Fe-C systems. Behavior at finite temperature would be discussed in terms of diffusion barriers calculated by nudged elastic band method(VASP) and the energy of the magnetic fluctuations by constrained density functional theory (LSMS). [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:15PM |
Q23.00010: Sum rule constraints on model dielectric functions Eric L. Shirley, Keith Gilmore The f-sum rule, Kramers-Kronig relations, orthonormality of Kohn- Sham orbitals and properties of the one-particle density matrix all dictate sum rules that can be used to maximize the physicality of model dielectric functions. In this talk, we wish to discuss aspects of the zeroth-frequency moment of the Lindhard polarization function that appear relatively unexploited in earlier work. We shall assess the effects on low- frequency dielectric properties and electron lifetime damping effects. We shall discuss the origin of various sum rules, a proposed method to implement them, and numerical results that can be obtained in a range of materials. [Preview Abstract] |
Wednesday, March 17, 2010 1:15PM - 1:27PM |
Q23.00011: Obtaining core excitation spectra using ABINIT and NBSE John Vinson, Eric Shirley, John Rehr, Joshua Kas We present a hybrid approach for GW/Bethe-Salpeter Equation calculations of core excitation spectra including XAS, NRIXS and EELS. The method dubbed OCEAN, is based on 1) {\it ab initio} ground state wavefunctions from the plane-wave pseudopotential code ABINIT; 2) the NIST core-level Bethe-Salpeter Equation Lanczos solver; and 3) a many-pole GW self-energy model (MPSE) to approximate final state broadening and self-energy shifts. An easy to use interface has been developed to drive the different steps required. Extensions to related spectra are briefy discussed. Examples are presented for the F and Li K-edge XAS in LiF, the O K-edge in ice-1h, and others. [Preview Abstract] |
Wednesday, March 17, 2010 1:27PM - 1:39PM |
Q23.00012: Electron Stopping Power Simulated by Time-dependent Density Functional Theory Jia-An Yan, Kalman Varga, Sokrates T. Pantelides We report results of a study of electron energy loss in a solid using time-dependent density functional theory. The scattering electron is modeled by a wave packet that enters a crystalline thin film with a predetermined velocity. The propagation of the wave packet is simulated in real time and real space. The electron stopping power is extracted from the energy loss of the wave packet during the time propagation through the material. Results for thin Si films will be presented. [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 1:51PM |
Q23.00013: Screened Coulomb interactions between localized electrons in solids Bi-Ching Shih, Peihong Zhang An accurate description of the Coulomb interaction between localized electrons in solids remains a fundamental and challenging problem. In this talk, we present first-principles results for the screened Coulomb and exchange interaction between localized $d$ electrons in transition metals and transition metal oxides. The localized $d$ states are represented by maximally localized Wannier functions whereas the dielectric screening is calculated within the constrained random phase approximation. [Preview Abstract] |
Wednesday, March 17, 2010 1:51PM - 2:03PM |
Q23.00014: Half-Metallicity in Two-Dimensional Nickel Cyanide Molecular Layers Craig Higgins, Gary Beall, Byounghak Lee We investigate the electronic structural properties of undoped- and doped-transition metal nickel cyanide molecular layers. Our theoretical study is motivated by recent experiments synthesizing two-dimensional nickel cyanides from a reaction of nickel tetracyanides and other divalent transition metals [1]. DFT calculations reveal that, while the undoped molecular planar structure is a narrow band gap semiconductor with a band gap of 0.3 eV, the structures with iron doping are half-metals. We discuss the stability of the ferromagnetic phase and the effective magnetic coupling between magnetic ions. We confirm the calculated results by comparing with atomic force microscopy and tunneling electron microscopy measurement. \\[4pt] [1] G. Beall, private communication [Preview Abstract] |
Wednesday, March 17, 2010 2:03PM - 2:15PM |
Q23.00015: Study of $Au/TiO_2$ interface structure using Energy Density Method Min Yu, Dallas R. Trinkle, Richard M. Martin We propose a way to decompose the total energy in a material into the contribution associated with each of the atoms, using the first principles energy density formalism[1]. Although the energy density function is non-unique up to a gauge transformation, we show that unique defect energies can be found by defining volumes for each atoms using the Bader charge analysis[2]. The methods are applied to gold nanoclusters supported on rutile titanium dioxide, which are commonly used as catalysts for chemical reactions. The variation in structure and chemistry of the interface may affect catalytic acticity. In this work, we study four model interfaces; $Au(111)//TiO_2(110)$ and $Au(100)//TiO_2(110)$, with and without bridging oxygen. Calculations are performed using the projector augmented wave method implemented in the Vienna ab initio simulation package[3], and the energy density method computes the interfacial energies to determine the equlibrium interfacial structure. [1]Phys. Rev. B 45, 6074 (1992) [2]Comput. Mater. Sci. 36, 254 (2006) [3]Phys. Rev. B 59, 1758 (1999) [Preview Abstract] |
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