Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F27: Electronic Structure Methods II |
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Sponsoring Units: DCOMP Chair: Jia-An Yan, Towson University Room: 501 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F27.00001: Determination of the one-particle Green's function: freedom and constraints Pina Romaniello, Giovanna Lani, Lucia Reining In this work we explore an approach for the calculation of the one-particle Green's function that is an alternative to standard methods based on approximations to the self-energy, namely, the solution of Schwinger's functional integro-differential equations [1]. These equations relate the one-particle Green's function to its functional derivative with respect to an external source. Here we start from an approximate version of these equations, where the Hartree potential is linearized with respect to the source [2]. We show that this set of equations has, in principle, multiple solutions. However, only one can be identified as the physical solution. We provide an expression for the formally exact family of solutions with the help of an auxiliary quantity q. The latter is defined by a number of exact constraints. Our findings suggest that once q is known, the physical solution is uniquely fixed by the limit of vanishing Coulomb interaction. [1] L. P. Kadanoff and G. Baym, Quantum Statistical Mechanics (W.A. Benjamin Inc., New York, 1964) [2] G. Lani, P. Romaniello and L. Reining, New Journal of Physics 14, 013056 (2012); in preparation [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F27.00002: The exact solution of the many-body problem in one-point: insights in approximate Green's function approaches Arjan Berger, Pina Romaniello, Lucia Reining In this work we obtain the exact one-body Green's function in one point by solving the Kadanoff-Baym equation. The result is a family of solutions. We show that only one of these solutions is a physical solution. We compare the exact physical solution to the exact solution of an approximate Kadanoff-Baym equation that was obtained recently [1] as well as to standard approximations such as GW. We show that the iterative solution of the GW equations is not always equal to the exact GW result. \\[4pt] [1] G. Lani, P. Romaniello and L. Reining, New J. Phys. 14, 013056 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F27.00003: Acceleration of screened-exchange density-functional calculations with approximate differential overlap Jonathan Moussa, Peter Schultz We implement the Heyd-Scuseria-Ernzerhof (HSE) screened-exchange density functional in the \textsc{SeqQuest} electronic structure code. HSE calculations are accelerated by approximating differential overlap in the Fock exchange based on an atomic-orbital partitioning scheme. All one-center and two-center exchange integrals are calculated. A subset of three-center exchange integrals are calculated for one-center Fock exchange matrix elements and for exchange mediated by one-center density matrix elements. Four-center exchange integrals are not calculated. We test the validity of this approximation by examining the number and magnitude of these different classes of exchange integrals. Basis set and pseudopotential errors in HSE calculations are benchmarked on atoms. Differential overlap approximation errors are benchmarked on small molecules. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F27.00004: Implementation of DFT$+$DMFT in local-orbital pseudopotential code Hyungju Oh, Choong-ki Lee, Hyoung Joon Choi Density functional theory (DFT) has been remarkably successful at describing ground-state properties of many solids from first principles. This is also the state-of-the-art method for band structure calculations, with the additional assumption that Kohn-Sham eigenvalues can be interpreted as single-particle excitations. However, DFT has limitations for strongly correlated materials. Dynamical mean-field theory (DMFT) is one of various approaches that have been developed for overcoming the shortcomings of DFT. DMFT goes beyond DFT by allowing the interaction potential of the correlated orbitals to be energy (frequency) dependent. This frequency dependent potential, or self-energy, is computed for the correlated orbitals using many-body techniques within an accurate impurity solver. We have implemented DMFT to the SIESTA code based on pseudo-atomic orbital basis set. For an impurity solver, we use exact diagonalization. We calculate electronic states of LaFeAsO using our DFT$+$DMFT code and confirm the band-narrowing, corresponding to an enhancement of the effective masses of quasiparticles. This work was supported by the NRF of Korea (Grant No. 2011-0018306). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2013-C3-008). [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F27.00005: {\it Ab initio} Sternheimer-GW method for quasiparticle calculations Henry Lambert, Feliciano Giustino The GW method has emerged as the standard computational tool for investigating electronic excitations in bulk and nanoscale systems. Recently significant efforts have been devoted to extending the range of applicability of the GW method. With this aim, Ref.~[1] introduced the Sternheimer-GW method, reformulating the standard GW approach so that no unoccupied electronic states are required in the calculations. Here we present the implementation of the Sternheimer-GW method using planewaves and norm-conserving pseudopotentials [2]. In our method we calculate the complete position- and energy-dependent GW self-energy operator, and as a by-product we obtain the entire $G_{0}W_{0}$ quasiparticle spectral function. We have validated our method by calculating the quasiparticle band structures of standard semiconductors and insulators (Si, SiC, diamond, LiCl) and by comparing the results with previous GW calculations. This method is currently being used for investigating the electronic structure of novel materials of reduced dimensionality. \\[4pt] [1] F.\ Giustino, M.\ L.\ Cohen, and S.\ G.\ Louie, Phys.\ Rev.\ B {\bf 81}, 115105 (2010).\\[0pt] [2] H.\ Lambert and F.\ Giustino, Phys.\ Rev.\ B.\ {\bf 88}, 075117 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F27.00006: Efficient calculation of random-phase approximation correlation energies using Lanczos chains and an optimal basis set Dario Rocca A new \emph{ab initio} approach is introduced to compute the correlation energy within the adiabatic connection fluctuation dissipation theorem in the random phase approximation. First, an optimally small basis set to represent the response functions is obtained by diagonalizing an approximate dielectric matrix containing the kinetic energy contribution only [1]. Then, the Lanczos algorithm is used to compute the full dynamical dielectric matrix and the correlation energy [1,2]. The convergence issues with respect to the number of empty states or the dimension of the basis set are avoided and the dynamical effects are easily kept into account. To demonstrate the accuracy and efficiency of this approach the binding curves for three different configurations of the benzene dimer are computed: T-shaped, sandwich, and slipped parallel.\\[4pt] [1] D. Rocca, J. Chem. Phys. (2014), to appear in the special issue Advances in DFT Methodology. \\[0pt] [2] T.A. Pham, H.-V. Nguyen, D. Rocca, G. Galli, Phys. Rev. B 87, 155148 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F27.00007: Double-counting corrections to the LDA+DMFT method in the exact density limit Andrei Valentin Plamada, Peter Staar, Anton Kozhevnikov, Bart Ydens, Thomas C. Schulthess The LDA+U method is commonly used for ab-initio studies of strongly correlated electron materials, and it has been successful in predicting spectral properties of prototypical systems such as NiO when used in conjunction with Dynamical Mean Field Theory (DMFT). Presently the method still includes an empirical term to correct doubly counted correlations. Assuming the double-counting correction is a constant $\mu_{DC}$ multiplied by the identity operator in the correlated subspace and that the electron density is well approximated with the Local Density Approximation (LDA) to Density Functional Theory, we devise a method to determine $\mu_{DC}$ directly from LDA and DMFT calculations. The method has been validated for prototypical transition metal oxides and shows promising results that agree with commonly used values for the double counting correction in the respective systems. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F27.00008: An experimentally realized 1-D correlated system for which DFT, DFT$+U$, and DFT$+$DMFT fall short Nader Zaki, Hyowon Park, Richard Osgood, Andrew Millis, Chris Marianetti Density functional theory (DFT) has been immensely successful in its ability to predict physical properties of condensed matter systems, and it is generally qualitatively correct when predicting structural properties. Here, however, we show that DFT qualitatively fails to predict the dimerized structural phase for a monatomic Co wire system that is self-assembled on a vicinal, i.e. stepped, Cu(111) substrate [1]. To elucidate the nature of this failure, we compute the energetics of a Co chain on a Cu surface, step, notch, and embedded in bulk, which demonstrates that increasing coordination and hybridization extinguishes the dimerization. We attribute the failure of DFT for Co on the Cu step to excessive hybridization, which both weakens the ferromagnetic correlations that drive the dimerization and increases the bonding that opposes dimerization. Additionally, we show that accounting for local interactions via DFT$+U$ or DFT$+$DMFT also fails at predicting the correct structural phase for the step-substrate supported wire, though the Co wire does dimerize in DFT$+$DMFT for the isolated vacuum case. [1] N. Zaki, et al, Phys. Rev. B \textbf{87}, 161406 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F27.00009: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F27.00010: Development of noncollinear-spin DFT$+$U method with spin-orbit interaction Eunjung Ko, Hyungjun Lee, Hyungju Oh, Se Young Park, Hyoung Joon Choi We developed a DFT$+$U$+$SOI method by incorporating spin-orbit interaction (SOI) into a noncollinear-spin generalization of the density functional theory (DFT) plus Coulomb interaction among $d$ electrons, parameterized by U and J. The Coulomb interaction, which is based on the rotationally invariant form, is generalized for noncollinear-spin configuration, and the fully localized limit is adopted for the double-counting term. The spin-orbit interaction is treated in the $l$-dependent fully separable nonlocal form using additional Kleinman-Bylander projectors generated by relativistic calculations of atoms. We implemented our DFT$+$U$+$SOI method into the SIESTA code and performed test calculations for the 4$d$ or 5$d$ transition metal oxides, the all-in-all-out noncollinear magnetic insulator Cd$_{\mathrm{2}}$Os$_{\mathrm{2}}$O$_{\mathrm{7}}$, the canted antiferromagnetic order insulator Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$, and the paramagnetic insulator Ca$_{\mathrm{2}}$RuO$_{\mathrm{4}}$. This work was supported by NRF of Korea (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2012-C3-046). [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F27.00011: Multi-orbital time-dependent spin-density functional theory for strongly correlation systems: Application to Ce and YTiO$_{3}$ Volodymyr Turkowski, Syed Islamuddin Shah, Talat S. Rahman We present a methodology for examining the spectral properties and nonequilibrium response of strongly-correlated electron systems within multi-orbital time-dependent spin-density functional theory. The key element of the theory -- exchange-correlation (XC) kernel - is derived from dynamical mean-field theory (DMFT) expressions for two-particle susceptibilities and the electron self-energy for the effective Hubbard model. We demonstrate that the appropriate description of strongly-correlated materials requires a non-adiabatic (time non-local) XC kernel, though the spatial locality in general is not necessary. We apply the formalism to study the spectral properties of cerium and YTiO$_{3}$, and establish that the method is capable of describing both metallic and insulating systems. In addition, we present results of the nonequilibrium response of YTiO$_{3}$ under an applied short laser pulse. In particular, we analyze the role of inter-orbital interactions in the relaxation dynamics of the system. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F27.00012: Developments in Coupled Cluster Theory for the Homogenous Electron Gas James J. Shepherd, Tom M. Henderson, Andreas Gr\"uneis, Gustavo E. Scuseria In a series of recent communications the correlation energy for the ground-state homogeneous electron gas has been precisely determined by full configuration interaction quantum Monte Carlo. The power of this new approach is that energies going beyond fixed-node approxmation and at finite basis set sizes are now available. This has opened up the possibility of benchmarking and further developing quantum chemical methods which involve finite basis sets for periodic systems, in particular coupled cluster theory. We will discuss: A) extensivity and divergences in approximate correlation energies, B) diagrammatic channels in the gas, and C) screening and range-separation in modern coupled cluster theory. This talk will draw on material from: 1) Phys. Rev. B 85, 081103 (2012); 2) Phys. Rev. B 86, 035111 (2012); 3) Phys. Rev. Lett., 110, 226401 (2013); 4) arXiv: 1310.6425; 5) arXiv:1310.6806. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F27.00013: Spin-Orbit Effects in the Quasiparticle Bandstructure of Noble Metals Jamal Mustafa, Steven Louie Applications of the $GW$ approximation to the electron self-energy have proven quite successful for calculating the quasiparticle properties of materials. We find that for the noble metals, in line with previous work in such calculations, the semicore states need to be taken into account. We show that, with these semicore states, a large cutoff must be used to describe the screening and, in turn, a large number of empty states must be included. Taking all of this into account, and carefully checking convergence, shows $G_{0}W_{0}$ can describe experimental results from angle-resolved photoemission spectroscopy quite well when the effects of spin-orbit coupling is also included. We compare our results to recent self-consistent $GW$ calculations on gold. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F27.00014: Effect of spin fluctuations on quasiparticles in simple metals Johannes Lischner, Timur Bazhirov, Allan MacDonald, Marvin Cohen, Steven Louie We present a first-principles theory for quasiparticle excitations in condensed matter systems that includes their interaction with spin fluctuations. We apply this theory to sodium and lithium. Despite several previous studies, the importance of spin fluctuations in these materials and, in particular, their effect on the occupied band width remains unclear. We show that the coupling to spin fluctuations does not significantly change the occupied band width, but gives an important contribution to the quasiparticle lifetime. To obtain quantitative agreement with experiment for the occupied band width, we find that it is necessary to include vertex corrections beyond the random-phase approximation in the screening by charge fluctuations. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F27.00015: The computational foundations of time dependent density functional theory James Whitfield The mathematical foundations of TDDFT are established through the formal existence of a fictitious non-interacting system (known as the Kohn-Sham system), which can reproduce the one-electron reduced probability density of the actual system. We build upon these works and show that on the interior of the domain of existence, the Kohn-Sham system can be efficiently obtained given the time-dependent density. Since a quantum computer can efficiently produce such time-dependent densities, we present a polynomial time quantum algorithm to generate the time-dependent Kohn-Sham potential with controllable error bounds. Further, we find that systems do not immediately become non-representable but rather become ill-representable as one approaches this boundary. A representability parameter is defined in our work which quantifies the distance to the boundary of representability and the computational difficulty of finding the Kohn-Sham system. [Preview Abstract] |
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