Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L35: Density Functional Theory: Methods |
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Sponsoring Units: DCOMP Chair: Steven Louie, University of California, Berkeley Room: 405 |
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L35.00001: Improvement of C$_{60}$'s calculated electron-phonon coupling using hybrid functional Jonathan Laflamme Janssen, Michel C\^ot\'e, Steven G. Louie, Marvin L. Cohen Superconductivity in doped C$_{60}$ crystals is generally admitted to be phonon mediated. However, the electron-phonon coupling calculated within density functional theory do not agree with measured values, in contrast to others phonon related properties. This discrepancy hasn't been understood yet. Up to now, only calculations using the local density approximation for the exchange-correlation functional (LDA) were performed. In this study, we demonstrate that using exact-exchange functionals increases the calculated couplings, bringing it closer to experiment, while others properties, such as geometry and phonon frequencies, are little affected. We investigate how such an improvement is possible while little change is seen on more commonly calculated properties. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L35.00002: Complex band structure of a metallic nanowire under plane-wave nonlocal pseudopotential Hamiltonian and non equilibrium quantum transport calculations Maia G. Vergniory, Lin-Wang Wang We present ab initio calculations of the complex band structure of a copper and gold nanowire with a nonlocal plane-wave pseudopotential Hamiltonians. This new method allows us to calculate the evanescent states exactly using plane waves of any metallic electrode. The calculation of the evanescent states is important for quantum transport calculations when the transmisson energy is close to a band structure minimun. Nonlocal pseudopotential effects are introduced using the Kleinman-Bylander implementation. Using the method in Ref[1], where the quantum transport is calculated by means of the exact scattering states using plane waves basis set, we have calculated the nonequilibrium transmission coefficient and conductivities of a di-thiol-benzene (DBT) and other molecules connected by two Cu or Au nano wires. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L35.00003: Towards switchable carbon nanotube interconnects Nicolas Poilvert, Nicola Marzari Carbon nanotubes have attracted much attention both in theory, computation and experiment for the past fifteen years. Despite synthetic challenges those unique quasi-one dimensional systems remain one of the preferred components of future electronic devices beyond silicon technology. We explore here the use of organic addends to functionalize carbon nanotubes and tailor and engineer their conducting properties. Once functionalized metallic nanotubes can become insulators if sp$^2$ to sp$^3$ rehybridization takes place. We explore here with first-principles calculations a number of organic addends that could lead to switchable rehybridization, allowing for real-time on/off control of the conductance. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L35.00004: Hybrid Density Functional Studies of $\delta $-Pu Raymond Atta-Fynn, Asok Ray Hybrid density functionals, which replaces a fraction of a density functional theory exchange with exact Hartree-Fock (HF) exchange, have been used to study the structural, magnetic, and electronic properties of $\delta $-Pu. The fractions of exact Hartree-Fock exchange used were 25{\%}, 40{\%}, and 55{\%}. Compared to the pure PBE functional, the lattice constants expanded with respect to the experimental value when the PBE-HF hybrid functionals were applied. For pure PBE and hybrids functionals with HF exchange amounts of 25{\%} and 40{\%}, the ground state structure was anti-ferromagnetic, while for 55{\%} HF contribution the ground state was non-magnetic. The $5f$ electrons tend to exhibit slight delocalization or itinerancy for the pure PBE functional and well-defined localization for the hybrid functionals, with the degree of 5$f$ electron localization increasing with the amount of HF exchange. Overall, the performance of the hybrid density functionals do not seem superior to pure density functionals for $\delta $ --Pu. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L35.00005: Application of Hybrid Functionals to Semiconductor Surfaces Manish Jain, James Chelikowsky, Steven Louie Hybrid functionals within Generalized Kohn Sham formalism have been shown to give good band gaps for semiconductors and small band gap insulators, when a particular mixing of nonlocal exchange is included. We explore the use of hybrid functionals for semiconductor surfaces. Semiconductor surfaces often have multiple band gaps - the bulk-state gaps and surface-state gaps - which can be quite different. This study examines the applicability of hybrid functionals for such systems. In particular, we focus on diamond and silicon and their (100) and (111) surfaces, where surface states exist and are known to have lower band gaps. We employ the hybrid functionals - PBE0, HSE and B3LYP - to examine the structural and electronic properties of these surfaces. \\[0pt] This work was supported by NSF under DMR-0551195 and DMR07-05941, and the US DOE under DE-FG02-06ER46286, DE-FG02-06ER15760 and DE-AC02-05CH11231. Computer time was provided by NERSC and Teragrid. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L35.00006: An LDA+$U$ study of the photoemission spectra of ground state phase of americium and curium Md Islam, Asok Ray We have investigated the photoemission spectra and other ground state properties such as equilibrium volume and bulk modulus of dhcp americium and the density of states and magnetic properties of dhcp curium using LDA+$U$ method. Our calculations show that spin polarized americium is energetically favorable but spin degenerate configuration produces experimental quantities much better than that calculated using spin polarized configuration. The DOS calculated using LDA+$U$ with both non-magnetic and spin polarized configurations is compared and the non-magnetic DOS is shown to be in good agreement with experimental photoemission spectra when U=4.5 eV. In spin polarized case, $U$ is observed to increase the splitting between occupied and unoccupied bands by enhancing Stoner parameter. The results are shown to be in good agreement with that calculated using dynamical mean field theory for these two heavy actinides. For curium, exchange interaction appears to play the dominant role in its magnetic stability. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L35.00007: Tackling localized $d$-states: a systematic investigation by \textit{GW}@LDA+\textit{U} Hong Jiang, Ricardo I. Gomez-Abal, Patrick Rinke, Matthias Scheffler First-principles modeling of systems with localized $d$-states is currently a great challenge in condensed matter physics. Density-functional theory (DFT) in the standard local-density approximation (LDA) proves to be problematic. This can be partly overcome by including local Hubbard $U$ corrections (LDA+$U$), but itinerant states are still treated on the LDA level. Many-body perturbation theory in the $GW$ approach offers both a quasiparticle perspective (appropriate for itinerant states) and an exact treatment of exchange (appropriate for localized states), and is therefore promising for these systems. Here we present a systematic investigation of the $G_0W_0$ method based on LDA+$U$ ($G_0W_0@$LDA+$U$) for a series of prototype systems: 1) ZnS with semicore $d$-states, 2) ScN and TiO$_2$ with empty $d$-states and 3) late transition metal oxides (MnO, FeO, CoO and NiO) with partially occupied $d$-states. We show that for ZnS, ScN and TiO$_2$, the $G_0W_0$ band gap only weakly depends on $U$, but for the other transition metal oxides the dependence on $U$ is as strong as in LDA+$U$. These different trends can be understood in terms of changes in the hybridization and screening. Our work demonstrates that $G_0W_0@$LDA+$U$ with ``physical'' values of $U$ provides a balanced and accurate description of both localized and itinerant states. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L35.00008: LDA+DMFT Charge Self-consistency Applied to Yb Valence Transition Erik Ylvisaker, Warren Pickett, Andrew McMahan, Jan Kunes Elemental ytterbium metal is known to undergo a gradual transition from a divalent $spd^2f^{14}$ state to a trivalent $spd^3f^{13}$ state in a pressure range of 0 to 34 GPa. We present LDA+DMFT studies of this transition, comparing three impurity solvers (Hirsch-Fye QMC, continuous time QMC and Hubbard I) with each other and with experimental data. All Yb states of interest are kept; no downfolding to a minimum basis is done. This application of DMFT (especially the QMC solvers) to the correlated f-orbitals gives reasonable agreement with the experimental transition. However, the neglect charge self-consistency is questionable for a valence transition where the concentration of valence electrons changes. Therefore we generalize the procedure and compare and contrast LDA+DMFT results with and without charge self-consistency for Yb using the Hubbard I impurity solver. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L35.00009: Computation of phonon spectra from density-functional perturbation theory in the projector augmented-wave approach Marc Torrent, Francois Jollet, Christophe Audouze, Xavier Gonze The density-functional perturbation theory expressions have been derived within the projector augmented-wave formalism (PAW) and compared to those found in the ultrasoft pseudopotential framework~[1]. They have been recently implemented in the \textsc{abinit} package~[2] in the case of perturbations of the atomic-displacement type. We summarize the key points of this implementation: The variational and non-variational forms of the 2nd-order total energy changes are detailed. The resolution of the variational principle by a generalized Sternheimer equation is explained (the 1st-order wave-function change is found with a band-by-band conjugate gradient algorithm). We focus on some difficulties: metallic electronic occupations, response to incommensurate perturbations of periodic systems {\ldots} Results on pure compounds are presented; a comparison with results from pseudopotentials approach is performed in order to highlight the effect of the PAW methodology and its accuracy. [1] Audouze, Jollet, Torrent and Gonze. Phys. Rev. B \textbf{73}, 235101 (2006); \textbf{78}, 035105 (2008) [2] \textit{http://www.abinit.org}. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L35.00010: eliminating the V-representability problem via coarse-graining Paul Lammert The mathematical foundations of density functional theory remain in an incomplete state, with old nagging problems and questions. I propose a coarse-grained approach to eliminating the V-representability problem and present results on differentiability of the Lieb functional. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L35.00011: Density Functional Theory and Semiclassical Methods Peter Elliott, Donghyung Lee, Attila Cangi, Kieron Burke In this work we explore the relationship between semiclassical methods and density functional theory. There is a rich history between the two, for example, the gradient expansion approximation (GEA), on top of which all common generalized gradient approximations (GGAs) are built, may be derived semiclassically. However methods like Thomas-Fermi and the GEA miss important contributions from quantum oscillations, such as shell structure. In Ref. [1] we showed why these are missing and how, for a simple system, one could derive them . This led to approximations to the density and kinetic energy density which were non-local functionals of just the external potential. Interpreting these in the context of DFT, allows us to understand current approximations and improve them. In fact the potential scaling (re-)introduced in Ref. [1] can be used to derive new exact conditions on the universal functional of DFT and its components. This talk will discuss both the development of potential functionals and how they can be used to understand DFT.\\[0pt] [1] P. Elliott, D. Lee, A. Cangi, and K. Burke, Phys. Rev. Lett. {\bf 100}, 256406 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L35.00012: Correlation-Kinetic Contributions in the Mapping to Model Noninteracting Fermion and Boson Systems Xiaoyin Pan, Viraht Sahni In the mapping from a system of electrons in an external field $\vec{\cal{F}}^{ext} = - \nabla v(\vec{r})$ to one of noninteracting fermions or bosons in their ground state with equivalent density $\rho (\vec{r})$, electron correlations due to the Pauli principle, Coulomb repulsion, and Correlation-Kinetic effects must be accounted for. Via Quantal Density Functional Theory$^{\dag}$ (QDFT), it is proved that the contributions due to the Pauli principle and Coulomb repulsion to either mapping are the same. The application to atoms of the QDFT mapping to the model fermion system shows the Correlation-Kinetic energy contribution to be a very small fraction of the electron-interaction energy. In contrast, the same application of the QDFT mapping to the model boson system shows the corresponding Correlation-Kinetic energy to be a substantial fraction of the electron-interaction energy. Thus, whereas Correlation-Kinetic effects are insignificant in the mapping to the fermionic system, they play a significant role in the mapping to the model system of bosons. \\ $^{\dag}$Quantal Density Functional Theory, Springer-Verlag, 2004 [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L35.00013: DFT-MD simulations of shocked Xenon Rudolph J. Magyar, Thomas R. Mattsson Xenon is not only a technologically important element used in laser technologies, jet propulsion and dental anesthesia, but it is also arguably the simplest material in which to study the metal-insulator transition at high pressure. Because of its closed shell electronic configuration, Xenon is often assumed to be chemically inert, interacting almost entirely through the van der Waals interaction, and at liquid density, is typically modeled well using Leonard-Jones potentials. However, such modeling has a limited range of validity as Xenon is known to form compounds at normal conditions and likely exhibits considerably more chemistry at higher densities when hybridization of occupied orbitals becomes significant. In this talk, we present DFT-MD simulations of shocked liquid Xenon with the goal of developing an improved equation of state. The relative importance of the van der Waals interaction compared to other Coulomb interactions is considered, and estimates of the relative accuracy of various density functionals are quantified. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L35.00014: Evaluation of Magnetic Moments using Bader Analysis Tomoharu Shikauchi, Kazuo Tsumuraya Evaluation of the magnetic moments in solids is crucial in the computational physics. The moments have been calculated by an atomic sphere approximation or a Voronoi polyhedron approximation. There has been a method to partition the space with the minimum electron charge density surface, called zero flux planes, around each atom. The space is called Bader region. We apply the method to calculate the local magnetic moments of each atom depending on their circumstance using the first principle electronic structure calculation. We obtain the moments from the Bader charges using the up-spin charge and the down-spin charges. We apply the validity of this scheme to the analyses of the spin moments in Fe-N compounds, fcc Fe, and bcc Fe crystals and compare them with the experimental values. For Fe$_4$N, the difference of the moments between Fe(I) and Fe(II) atoms has been larger than that of the Voronoi method and is better agreement with the experimental values than the Voronoi method. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L35.00015: Layered dilute magnetic semiconductors: A dynamical mean field study Majid Nili, Unjong Yu, Juana Moreno, Mark Jarrell We study ferromagnetism of layered dilute magnetic semiconductors within the Dynamical Mean Field Approximation. Our approach includes the spin-orbit coupling in the host compound and the interaction between the magnetic ions and the itinerant carriers using a modified double-exchange coupling. We simulate heterostructures with different distributions of magnetic ions: uniform doping, delta-doping in one single layer and delta-doping in two layers. We investigate the magnetic properties by changing the hole filling as well as the magnetic doping and the position of the doped layers in the heterostructure. We also include the on-site attraction potential between the magnetic ions and charge carriers to show its effect on the formation of the impurity band. We find that the ferromagnetic transition temperature and other properties strongly depend on the distribution of magnetic ions. [Preview Abstract] |
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