Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X23: Many Body I |
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Sponsoring Units: DCOMP Chair: Tara Das, University at Albany-SUNY Room: C125-C126 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X23.00001: An XML-based framework for the validation and verification of electronic structure codes Gary Yuan, Francois Gygi The process of validation and verification of the results of electronic structure calculations involves comparisons of data sets obtained with widely disparate software packages, including a large diversity of data formats. We present a web-based framework aimed at facilitating automatic comparison between electronic structure computation results and first-principles molecular dynamics simulations. The framework makes extensive use of XML in the representation and translation of data sets. An application to the automatic verification of results obtained with the Qbox first-principle simulation code (http://eslab.ucdavis.edu/software/qbox) and the Quantum Espresso code (http://quantum-espresso.org) will be presented [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X23.00002: First Principles Absorption Spectra of Group IB and IIB Atoms and Dimers Kopinjol Baishya, Serdar Ogut We present absorption spectra of group IB and IIB atoms and dimers, obtained with two state-of-the-art computational methods using ab initio pseudopotentials: the many body perturbation technique GWBSE and the time-dependent density functional theory with the local density approximation (TDLDA). We compare the GWBSE and TDLDA spectra with each other and with available experimental data. A recent study has shown that semi-core s and p states are essential to reproduce accurate quasiparticle energies within the GW theory.$^{1}$ We extend this investigation to the case of optical excitations and examine the effect of semi-core states on the absorption spectra by carrying out TDLDA and GWBSE computations with standard and semi-core pseudopotentials. $^{1}$ M.Tiago, J.C.Idrobo, S.Ogut, J.Jellinek, and J.R.Chelikwosky, Phys. Rev. B 79, 155419 (2009). [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X23.00003: GW without empty states J.A. Berger, L. Reining, F. Sottile The GW approximation (GWA) to the self-energy[1] has proved to be very successful in the calculation of quasi-particle energies for a wide range of solids. However, the GWA is computationally expensive which is mainly due to the slow convergence with the number of unoccupied states that have to be taken into account in its standard sum-over-states expression. In this work we will show that the expression for the GW self-energy can be rewritten explicitly in such a way that no unoccupied states enter. This approach leads to a hierarchy of expressions for the self-energy which converges rapidly. The expressions thus obtained are very simple and can be readily implemented leading to an immediate speedup of GW calculations. Truncation of this hierarchy at lowest order already leads to excellent results for the quasiparticle energies. We use a similar scheme to rewrite the sum-over-states expression for the polarizability such that only occupied states are required.\\ \\ $[1]$ L.\ Hedin, Phys.\ Rev.\ 139, A796 (1965). [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X23.00004: Insights in the T-matrix formalism Pina Romaniello, Friedhelm Bechstedt, Lucia Reining In many-body perturbation theory the self-energy $\Sigma=iGW\Gamma$ plays a key role since it contains all the many body effects of the system. The exact self-energy is not known and approximations are needed. As first approximation one can neglect the vertex $\Gamma$, and obtain the GW approximation. In some cases this is not sufficient, and one needs to go beyond this approximation. In this work we elucidate the concept of T-matrix [1] and its relation with Hedin's equations [2]: we look for a unified framework including GW, T-matrix, and GW$\Gamma$. We discuss this in relation to two main shortcomings of the GW approximation: the self-screening error and the incorrect atomic limit [3]. \\ \\ $[1]$ L.\ P.\ Kadanoand G.\ Baym, Quantum Statistical Mechanics, W.\ A.\ Benjamin, Inc.\, New York, (1962).\\ $[2]$ L.\ Hedin, Phys.\ Rev.\ \textbf{139}, A796 (1965).\\ $[3]$ P. Romaniello, S. Guyont, and L. Reining, J.\ Chem.\ Phys.\textbf{131}, 154111 (2009). [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X23.00005: Examination of Variational Procedures for Electronic Structure Studies by Comparison of Results for Energies of Atoms with Experiment and Results from Bruckner-Goldstone Many-Body Perturbation Theory --Lithium Atom H. Paudel, Archana Dubey, R.H. Pink, S.R. Badu, R.H. Scheicher, T.P. Das Through the use of the Bruckner-Goldstone Diagrammatic Many Body Perturbation Theory (BGMBPT) in the recent past, quantitative results in excellent agreement with experiment have been found for atomic properties which can be used to test the accuracy of the variational procedures in use currently for investigations of electronic structures and properties of multicenter systems like molecules and solid state systems by applying them to atomic systems. In the present talk, we shall consider lithium atom in its ground state where an earlier BGMBPT investigation [1] has provided very good agreement with experiment for the total energy including correlation effects. We have focused for the comparison in the present work on the Gaussian basis set based, first --principles Hartree-Fock procedure combined with Many-Body Perturbation Theory, and the B3LYP procedure using DFT based exchange and correlation potentials, for the total energy for lithium. Results of our investigation, and conclusions from our analysis, will be presented. [1]T.P. Das, Phys. Rev. 174, 1 (1968) [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X23.00006: All-electron $GW$ calculations for perovskite transition-metal oxides Christoph Friedrich, Andreas Gierlich, Stefan Bl\"ugel, Arno Schindlmayr The $GW$ approximation for the electronic self-energy, which combines bare exchange with the dynamical screening of the many-electron system within the random-phase approximation, yields quasiparticle band structures in very good agreement with experiment. While most implementations today employ the pseudopotential approximation, our recently developed realization (http://www.flapw.de/spex) is based on the all-electron full-potential linearized-augmented-planewave (FLAPW) method, where core and valence electrons are treated on the same footing. Within this method a large variety of materials can be studied, including d- and f-electron systems, oxides and magnetic systems. In this work we present results for selected perovskite transition-metal oxides, such as SrTiO$_3$, BaTiO$_3$, PbTiO$_3$ and others, which are widely used in technical applications. Their band gaps are difficult to measure experimentally and have been under debate for a long time. Most ab-initio studies so far were based on density-functional theory and showed a strong underestimation of the band gap. Our all-electron $GW$ calculations overcome this problem and yield band gaps very close to experiment. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X23.00007: Accounting for dynamical effects in \emph{ab initio} NMR calculations Mark Robinson, Peter Haynes The \emph{ab initio} calculation of chemical shifts using density functional theory (DFT) is now routine. For many rigid crystals and molecules isotropic chemical shifts have been computed agreeing with experiment to better than 1ppm. However systems that exhibit greater dynamical motion, such as in biological systems, the computed shifts are found to be in error. We study an example of one such system, an L-Alanine molecular crystal, using the NMR-CASTEP code. A straightforward calculation results in a discrepancy between computed and experimental chemical shifts of nearly 4ppm. Previous work by Dumez and Pickard has shown that dynamics are a contributing factor to this error [1]. To incorporate dynamics into the calculation of the chemical shifts we average over an ensemble of configurations representational of the motion of the system. These configurations are generated using molecular dynamics (MD). This can pose a problem for \emph{ab initio} MD since the time scale of such dynamics can be of the order of a picosecond. We overcome this by fitting a force field to DFT forces for the system under study. Classical MD is then used to generate uncorrelated configurations from which the chemical shifts are averaged. Using this procedure we are able to improve the computed chemical shifts for L-Alanine significantly. [1] J-N.~Dumez and C.J.~Pickard, J.~Chem.~Phys., 130 (2009) 104701. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X23.00008: Examining the role of pseudopotentials in exact-exchange-based Kohn-Sham gaps Adi Makmal, Rickard Armiento, Eberhard Engel, Leeor Kronik, Stephan Kuemmel We present exact-exchange optimized effective potential calculations of the Kohn-Sham gap, using highly accurate grid-based all-electron and pseudopotential approaches for prototypical diatomic molecules. Results obtained with pseudopotentials that have been constructed in a manner consistent with the exact-exchange functional agree with the all electron results for the cases studied. This confirms the reliability of the pseudopotential approximation for orbital-dependent functionals such as exact exchange. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X23.00009: Test of Variational Ab Initio Methods for Studying Magnetic Hyperfine Interactions in the Phosphorus Atom. R.H. Pink, S.R. Badu, H. Paudel, Archana Dubey, Lee Chow, R.H. Scheicher, T.P. Das The half-filled 3p valence shell of the phosphorus atom provides an interesting test of the accuracy of calculated instantaneous correlation effects. When no correlation effects are included, one obtains a sizable negative magnetic electron-nuclear hyperfine constant [1] while experiment [1] predicts a sizable positive hyperfine interaction. This is because static spin polarization effects are dominated by the 2s shell which is exchange polarized by the 3p electrons. When dynamic correlation effects are included [1], providing instantaneous admixture of s-states with the p-states, one obtains a positive contribution which overcomes the static spin-polarization effects with good agreement with experiment. We have compared the magnetic hyperfine constants calculated by currently used variational methods involving HFMBPT and DFT procedures with the earlier results [1] to examine the extent to which these two procedures successfully include instantaneous correlation effects. [1] N.C. Dutta, C. Matsubara, R.T. Pu and T.P. Das, Phys. Rev. Lett. 20, 1139(1968). [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X23.00010: Improved GW$\Gamma $ scheme for the first-principles calculation of the electron self-energy Yasutami Takada Improvements are made on the self-consistent calculation scheme for the electron self-energy with the vertex function $\Gamma $ satisfying the Ward identity, originally proposed in 2001 [1]. Although it is basically equivalent to the original one, this improved scheme not only shortens the computational time by about one hundredth but also opens new horizons in its applications: (i) If it is applied to semiconductors and insulators, the obtained quasiparticle dispersion is virtually the same as that in the one-shot GW approximation (or G$_{0}$W$_{0}$A), indicating that the G$_{0}$W$_{0}$A actually takes proper account of both vertex and high-order self-energy corrections in a mutually cancelling manner [2]. (ii) If it is applied to the Tomonaga-Luttinger model, it is reduced to the Dzyaloshinskii-Larkin theory, implying that it is a unified theory to treat both Fermi- and Luttinger-liquids on the same footing. (iii) In contrast with the original one, it can provide the convergent self-consistent solution for the low-density electron liquid where an intrinsic difficulty arises due the dielectric catastrophe associated with the negative electronic compressibility. [1]YT, PRL\textbf{87}, 226402 (2001). [2] S. Ishii, H. Maebashi, and YT, unpublished. [Preview Abstract] |
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