Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N19: Focus Session: Frontiers in Electronic Structure Theory III |
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Sponsoring Units: DCP DCOMP Chair: Peter Gill, Australian National University Room: Colorado Convention Center 104 |
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N19.00001: A New View of the Kondo Effect from an Ab Initio Embedded Configuration Interaction Theory Invited Speaker: Over the past decade, we have been developing an ab initio theory to describe localized correlated many-electron states in condensed matter. This theory embeds a correlated quantum chemistry description into surroundings described by periodic density functional theory (DFT). Recent technical advances in the theory include: (i) implementation of ultrasoft pseudopotentials (USPPs) in a consistent manner across all levels of theory (periodic DFT, CASSCF, and CI), (ii) self-consistent updates of the density of the total system, thereby allowing a fully-self-consistent embedding operator, and (iii) a multi-reference singles and double excitation CI (MRSDCI) treatment of electron correlation in the embedded region. Our current embedded configuration interaction (ECI) theory is now more efficient (via USPPs), less approximate (by use of self-consistent embedding potentials), as well as more accurate (via MRSDCI) than earlier versions that were based either on many-body perturbation theory or valence CI/CASSCF wavefunctions. The current version is now being used to study a variety of systems/phenomena where DFT is known to fail, due to either neglect of many-body effects or self-interaction artifacts. Time permitting, more than one example will be given of how the embedding theory is able to give a \textit{qualitatively (as well as quantitatively)} different view of these systems/phenomena. We will focus on the Kondo effect, a long standing problem in condensed matter physics, which has not had a first principles solution until now. The Kondo effect refers to the observation of an anomalous resistivity minimum at low temperatures for materials containing magnetic transition metal impurities in nonmagnetic host metals. We will show that the ECI theory is able to capture the physics and offer a new view of this phenomenon, while periodic DFT and finite cluster quantum chemistry calculations do not. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N19.00002: First-principles calculations of nanoscale capacitors at finite bias potential Invited Speaker: When the thickness of an oxide film is reduced to few unit cells, its dielectric properties (which are relevant, e.g., for nonvolatile ferroelectric memories and as gate oxides in MOSFET transistors) start to deviate from those predicted by macroscopic models, and cannot be disentangled from the metallic or semiconducting contacts. One particularly important issue related to interfacial effects is the ``dielectric dead layer'', which plagues the performance of thin-film perovskite capacitors by substantially reducing the effective permittivity ($\kappa$) of the active high-$\kappa$ material. The microscopic origins of this reduced permittivity, and in particular whether it stems from defects or from the fundamental properties of a metal/insulator interface, are not well understood. To address this problem from first principles, we will first show how the macroscopic polarization (and the coupling to an external field) can be rigorously defined for a periodic metal-insulator heterostructure, by using techniques and ideas borrowed from Wannier-function theory [1]. We will then demonstrate our new method by calculating the dielectric properties of realistic SrRuO$_3$/SrTiO$_3$/SrRuO$_3$ nanocapacitors [2]. In particular, we demonstrate the existence of an intrinsic dielectric dead layer and analyze its origin by extracting the ionic and electronic contributions to the electrostatic screening. We establish a correspondence between the dead layer and the hardening of the collective SrTiO$_3$ zone-center polar modes, and determine the influence of the electrode by repeating our calculations for Pt/SrTiO$_3$/Pt capacitors. Our results provide practical guidelines for minimizing the deleterious effects of the dielectric dead layer in nanoscale devices. \begin{itemize} \item[{[1]}] \underline{M. Stengel} and N. A. Spaldin, {\em Origin of the dielectric dead layer in nanoscale capacitors}, Nature (London) {\bf 443}, 679 (2006).\\ \item[{[2]}] \underline{M. Stengel} and N. A. Spaldin, {\em Ab-initio theory of metal-insulator interfaces in a finite electric field}, cond-mat/0511042 (2005). \\ \end{itemize} [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N19.00003: \textit{Ab Initio} Quantum Simulations of Liquid Water John Gergely, David Ceperley, Francois Gygi Some recent efforts at simulating liquid water have employed ``\textit{ab initio}'' molecular dynamics (AIMD) methods with forces from a version of density functional theory (DFT)\footnote{E. Schwegler, J.C. Grossman, F. Gygi, G. Galli, J. Chem. Phys \textbf{121}, 5400 (2004).} and, in some cases, imaginary-time path integrals (PI) to study quantum effects of the protons. Although AIMD methods have met with many successes, errors introduced by the approximations and choices of simulation parameters are not fully understood. We report on path integral Monte Carlo (PIMC) studies of liquid water using DFT energies that provide quantitative benchmarks for PI-AIMD work. Specifically, we present convergence studies of the path integrals and address whether the Trotter number can be reduced by improving the form of the (approximate) action. Also, we assess $1)$ whether typical AIMD simulations are sufficiently converged in simulation time, i.e., if there is reason to suspect that nonergodic behavior in PI-AIMD methods leads to poor convergence, and $2)$ the relative efficiency of the methods. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N19.00004: Adaptive multilevel Finite Element Method for Solving the Electronic Schr\"{o}dinger Equation Eric Bylaska, Mike Holst, John Weare It is widely appreciated that to use computational methods for the design of materials encompassing a wide assortment of elements from the Periodic Table, highly efficient methods based as closely as possible on accurate quantum mechanics are needed. We have developed an O(N) ab initio molecular dynamics method based on an adaptive multilevel finite element first principles solver with an efficacious implementation of hybrid functionals . The matrix representations of the discrete Hamiltonian operator in the finite element basis are always sparse due to the local support nature of finite element basis functions. As a result, application of the Hamiltonian operator to a discrete function has complexity which is linear in the number of discretization points. This development also makes use of completely unstructured simplex meshes that have the advantage of giving resolution of the near singular features around atomic nuclei using minimal computational resources. Various aspects of the implementation and computational efficiencies will be discussed. This method has been applied to several systems including excitons in quartz, transition metal dimers, and aqueous complexes. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N19.00005: GW calculations of large model structures Paolo Umari, Stefano Baroni We introduce a novel approach for performing first-principles GW calculations of large model structures. A description of the valence and conduction manifolds in terms of non-orthogonal generalized Wannier functions permits to minimize the dimension of the basis set required for describing the space of single electron transitions. This dimension scales linearly with the size of the system. Then a space-time approach is used to calculate the self-energy operator in the space of Kohn-Sham eigenstates. Ultrasoft pseudopotentials are straightforwardly implemented within this scheme.We validate our approach by calculating the vertical ionization energies of small molecules and find excellent agreement with the experiment. Then we shows its potentiality by addressing a model structure of vitreous silica. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N19.00006: Toward an accurate and practical description of Xe/Cu(111) physisorption Garold Murdachaew, Stefano de Gironcoli, Patrick Huang, Emily Carter, Giacinto Scoles The physisorption of rare gases on metal surfaces has often been described by density functional theory. However, standard DFT has shown very limited success due to its well-known shortcomings when applied to weak interactions. A possible approach which at least includes the relevant missing physics is to use a blend of ``corrected" DFT coupled with a damped-dispersion interaction. Alternatively, one may model the surface by a cluster since it is possible to apply highly accurate quantum chemical methods to small clusters. Unfortunately, cluster model approximations do not give a good description of the physisorption process on the surface. In particular, the site preference of Xe/Cu(111) physisorption as given by cluster models is qualitatively incorrect. For this reason, an approach which better simulates the surface is required. Some recent results obtained using the embedded cluster approach of E. A. Carter, P. Huang, and coworkers [P. Huang and E. A. Carter, J. Chem. Phys. {\bf 125}, 084102 (2006)] will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N19.00007: Boundary Conditions for States with Maximally Broken Time-Reversal Symmetry Roger Haydock, C.M.M. Nex For non-crystalline materials, electronic states can only be calculated for finite clusters, and the results are sensitive to the boundary conditions. States which go to zero on the boundary have infinite life-times, appropriate for isolated clusters, but not for macroscopic materials whose states have finite life-times. Instead, we chose a boundary condition for which the states have minimal life-times, in other words, one for which the states have maximally broken time-reversal symmetry. This approach is tested for a variety of systems and compared with its close relative, the maximum entropy approximation. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N19.00008: Importance of second neighborhood ensembles on PdAu bimetallic surfaces Dingwang Yuan, Ruqian Wu, Xingao Gong Atomic configurations of two or three Pd substituents on the Au(111) and Au(001) surface are investigated using the first-principles pseudopotential plane wave approach. Pd atoms are found to form second neighborhoods on PdAu surfaces. The Pd-d band becomes narrow and well below the Fermi level, very different from those in a Pd film or bulk Pd. Yet the surface Pd atoms are still active and serve as independent attractive centers towards adsorbates. Through studies of example reactions such as CO oxidation, ethylene dehydrogenation and vinyl acetate synthesis, we demonstrate the importance of special ensembles in catalyzing reactions by confining reactants in a small region. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N19.00009: Dielectric function by FLAPW method. Tatsuya Shishidou, Tamio Oguchi Response functions, which describe how electrons respond to external fields, are the central quantity in solid state physics. Many physical properties, such as optical spectra, phonon spectra, dielectric constant, magnetic and structural instabilities, and so on, are accessible if one can calculate the corresponding response function. Moreover, the response functions play important role in the application of many-body perturbation theory. In this paper, we present a way to calculate dynamical inverse dielectric function $\varepsilon^{-1}(r,r',\omega)$ within the framework of the all-electron full-potential linearized augmented plane wave (FLAPW) method. We work with the random phase approximation (RPA) instead of the plasmon pole approximation. Local field effects are taken into account. Details of our method and implementation will be given, focusing on its efficiency and the treatment of the Coulomb singularity at $\Gamma$ point. Calculations for semiconductors, ferromagnetic $3d$ transition metals, and insulating antiferromagnetic transition-metal oxides will be presented and compared with available experiments and theories. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N19.00010: State-of-the Art Procedure for the Calculation of Quartic Force Fields: Application to HO$_{2}^{+}$ Timothy Lee, Xinchuan Huang In the 1990's, ab initio methods began to yield quartic force fields for use in the calculation of ro-vibrational spectra with an accuracy that previously had been unimaginable. The main reason for this advance was the development of efficient computer programs for calculating singles and doubles coupled-cluster energies that included an estimate for connected triple excitations, denoted CCSD(T). Thus the advent of CCSD(T) quartic force fields computed with large one-particle basis sets changed the paradigm for the ab initio calculation of ro-vibrational spectra. Small correction terms have now been successfully incorporated into these procedures, including core-correlation, scalar relativistic, and others. Previously, we investigated procedures where all of these correction terms are appended in one way or another to a base calculation. In the current work, we develop a new procedure where most of these correction terms are included from the beginning, while still minimizing the overall computational cost. Our new procedure is detailed and its application to the lowest triplet and singlet states of HO$_{2}^{+}$ presented. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N19.00011: A Correct, Density Functional Description of Semiconductors D. Bagayoko, G. Zhao, L. Franklin, H. Jon The profusely reported inability of some density functional calculations to describe correctly the band gaps of semiconductors has been ascribed to the derivative discontinuity of the exchange correlation energy, the self-interaction associated with approximate potentials, and other factors, i.e., pd repulsion in the case of wurtzite InN. From 1998 to present, we have studied several semiconductors with local density approximation (LDA) and generalized gradient approximation (GGA) potentials. Upon applying the Bagayoko, Zhao, and Williams (BZW) method to the implementation of the linear combination of atomic orbital (LCAO) formalism, we have obtained band gaps and electron effective masses in excellent agreement with experiment for BaTiO$_{3}$, GaN, GaAs Si, Ge, 3C-SiC, 4H-SiC, ZnSe, ZnO, carbon nanotubes, InN, and AlAs among others. This ab-initio method avoids a basis set and variational effect inherently associated with LCAO calculations -- irrespective of the selected potential. We present a summary of the BZW method and of the aforementioned results, including \textit{the correct description of low-lying conduction bands as verified by agreements with measured optical transition energies and dielectric functions.} These results clearly point to an urgent need to revisit (a) the above presumed causes of reported failures of DFT and (b) computational methods suffering from the identified and well-defined basis set and variational effect. [Preview Abstract] |
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