Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session E24: ManyBody Perturbation Theory for Electronic Excitations: Computational AdvancesFocus

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Sponsoring Units: DMP DCOMP Chair: Volker Blum, Duke University Room: 323 
Tuesday, March 15, 2016 8:00AM  8:12AM 
E24.00001: Plasmon Pole Approximations within a GW Sternheimer implementation vincent gosselin, Michel Cote We use an implementation of the GW approximation that exploits a Sternheimer equation and a Lanczos procedure to circumvent the resource intensive sum over all bands and inversion of the dielectric matrix. I will present further improvement of the method that uses Plasmon Pole approximations to evaluate the integral over all frequencies analytically. A comparison study between the von LindenHorsh and EngelFarid approaches for energy levels of various molecules along with benchmarking of the computational ressources needed by the method will be discussed. [Preview Abstract] 
(Author Not Attending)

E24.00002: Inclusion of the electronphonon interaction in the BerkeleyGW computational package Derek VigilFowler, Stephan Lany The BerkeleyGW package is a highly optimized and efficient code for calculating, among others, the dielectric response, bandstructures, lifetimes, and optical absorption of materials from nanostructures and twodimensional sheets to bulk materials. In the past the only interactions included in BerkeleyGW were electronelectron interactions, with other packages being used to include the effect of, say, electronphonon interactions. One common approach is to use Wannier functions to interpolate all needed quantities to a very fine grids in energy and momentum, which leads to very accurate electronphonon couplings and lifetimes. However, in materials with complex, even unknown, chemical environments the generation of Wannier functions can be quite time consuming and constitutes another step in an already difficult calculation. The BerkeleyGW package has a wavefunctionbased interpolation scheme that is used in solving the BetheSalpeter equation and which is much more easily automated than Wannier interpolation. In this talk, we discuss results for the carrier lifetimes due to the electronphonon interaction using this interpolation scheme. In particular, we discuss the computational efficiency and scalability, and the prospects for applying this method to a wide range of materials to get first principles lifetimes, and related quantities, such as mobilities and diffusion lengths. [Preview Abstract] 
Tuesday, March 15, 2016 8:24AM  8:36AM 
E24.00003: Largescale GW software development Minjung Kim, Subhasish Mandal, Eric Mikida, Prateek Jindal, Eric Bohm, Nikhil Jain, Laxmikant Kale, Glenn Martyna, Sohrab IsmailBeigi Electronic excitations are important in understanding and designing many functional materials. In terms of {\it ab initio} methods, the GW and BetheSaltpeter Equation (GWBSE) beyond DFT methods have proved successful in describing excited states in many materials. However, the heavy computational loads and large memory requirements have hindered their routine applicability by the materials physics community. We summarize some of our collaborative efforts to develop a new software framework designed for GW calculations on massively parallel supercomputers. Our GW code is interfaced with the planewave pseudopotential {\it ab initio} molecular dynamics software ``OpenAtom'' which is based on the Charm++ parallel library [1]. The computation of the electronic polarizability is one of the most expensive parts of any GW calculation. We describe our strategy that uses a realspace representation to avoid the large number of fast Fourier transforms (FFTs) common to most GW methods. We also describe an eigendecomposition of the plasmon modes from the resulting dielectric matrix that enhances efficiency. [1] Bohm {\it et al.}, IBM J. RES. \& DEV. vol. 52 no. 1/2, 2008 [Preview Abstract] 
Tuesday, March 15, 2016 8:36AM  8:48AM 
E24.00004: Towards highly scalable GW calculations Subhasish Mandal, Minjung Kim, Eric Mikida, Eric Bohm, Prateek Jindal, Nikhil Jain, Laxmikant V. Kale, Glenn J. Martyna, Sohrab IsmailBeigi The GW and BetheSaltpeter Equation (GWBSE) approach is an accurate and useful method beyond DFT to describe excited states of materials. However over the past few decades, most {\it ab initio} GW calculations used have been confined to small units of cells of bulklike materials due to the extreme computational demands of the approach. We will present our collaborative efforts to develop new software that permits large scale GW calculations more efficiently: our GW software is interfaced with the ab initio plane wave pseudopotential OpenAtom software (http://charm.cs.uiuc.edu/OpenAtom/) that uses the Charm++ parallel framework. Here, we focus on describing our work on computing the static (so called ``COHSEX’’) GW selfenergy. We describe the advantages of our realspace approach for quasiparticle calculations and provide information on scaling behavior of the resulting algorithms. [Preview Abstract] 
Tuesday, March 15, 2016 8:48AM  9:00AM 
E24.00005: Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules: A Benchmark of GW Methods Noa Marom, Joseph Knight, Xiaopeng Wang, Lukas Gallandi, Olga Dolgounitcheva, Xinguo Ren, Vincent Ortiz, Patrick Rinke, Thomas Korzdorfer The performance of different GW methods is assessed for a set of 24 organic acceptors. Errors are evaluated with respect to coupled cluster singles, doubles, perturbative triples [CCSD(T)] reference data for the vertical ionization potentials (IPs) and electron affinities (EAs), extrapolated to the complete basis set limit. Additional comparisons are made to experimental data, where available. We consider fully selfconsistent GW (scGW), partial selfconsistency in the Green's function (scGW$_{\mathrm{0}})$, nonselfconsistent G$_{\mathrm{0}}$W$_{\mathrm{0}}$ based on several meanfield starting points, and a ``beyond GW'' second order screened exchange (SOSEX) correction to G$_{\mathrm{0}}$W$_{\mathrm{0}}$. The best performers overall are G$_{\mathrm{0}}$W$_{\mathrm{0}}+$SOSEX and G$_{\mathrm{0}}$W$_{\mathrm{0}}$ based on an IPtuned long range corrected hybrid functional with the former being more accurate for EAs and the latter for IPs. Both provide a balanced treatment of localized vs. delocalized states and valence spectra in good agreement with photoemission spectroscopy (PES) experiments. [Preview Abstract] 
Tuesday, March 15, 2016 9:00AM  9:12AM 
E24.00006: Extrapolation of G$_0$W$_0$ energy levels from small basis sets for elements from H to Cl Tong Zhu, Volker Blum G$_0$W$_0$ calculations based on orbitals from a densityfunctional theory reference are widely used to predict carrier levels in molecular and inorganic materials. Their computational feasibility, however, is limited by the need to evaluate slowconverging sums over unoccupied states, requiring large basis sets paired with unfavorable scaling exponents to evaluate the selfenergy. In the quantum chemistry literature, complete basis set (CBS) extrapolation strategies have been used successfully to overcome this problem for total energies. We here apply the principle of basis set extrapolation to G$_0$W$_0$ energy levels. For a set of 49 small molecules and clusters containing the elements H, Li through F, and Na through Cl, we test established extrapolation strategies based on Dunning's correlationconsistent (cc) basis sets (aug)ccpVNZ (N=25), as well as numeric atomcentered NAOVCCnZ (n=25) basis sets in the FHIaims allelectron code. For the occupied and lowest unoccupied levels, different extrapolation strategies agree within $\pm$50 meV based on large 4Z and 5Z basis sets. We show that extrapolation based on much smaller 2Z and 3Z basis sets with largest errors $\pm$ 100 meV based on a refinement of the NAOVCCnZ basis sets. [Preview Abstract] 
Tuesday, March 15, 2016 9:12AM  9:48AM 
E24.00007: The SternheimerGW method and the spectral signatures of plasmonic polarons Invited Speaker: Feliciano Giustino During the past three decades the GW method has emerged among the most promising electronic structure techniques for predictive calculations of quasiparticle band structures. In order to simplify the GW workflow while at the same time improving the calculation accuracy, we developed the SternheimerGW method [1]. In SternheimerGW both the screened Coulomb interaction and the electron Green's function are evaluated by using exclusively occupied KohnSham states, as in densityfunctional perturbation theory. In this talk I will review the basics of SternheimerGW, and I will discuss two recent applications to semiconductors and superconductors. In the case of semiconductors we calculated complete energy and momentumresolved spectral functions by combining SternheimerGW with the cumulant expansion approach. This study revealed the existence of band structure replicas which arise from electronplasmon interactions [2]. In the case of superconductors we calculated the Coulomb pseudopotential from first principles, and combined this approach with the Eliashberg theory of the superconducting critical temperature [3]. [1] H. Lambert, F. Giustino, Phys. Rev. B 88, 075117 (2013). [2] F. Caruso, H. Lambert, F. Giustino, Phys. Rev. Lett. 114, 146404 (2015). [3] E. R. Margine, F. Giustino, Phys. Rev. B 87, 024505 (2013). [Preview Abstract] 
Tuesday, March 15, 2016 9:48AM  10:00AM 
E24.00008: Combination of Hedin's $GW$ and dynamical meanfield theory tested on H$_2$ molecule Juho Lee, Kristjan Haule We compare various flavors of ``$GW$+DMFT" approach with LDA+DMFT for the simplest strongly correlated system, the H$_2$ molecule. The following $GW$+DMFT methodologies are compared: (i) the fully selfconsistent GW+DMFT, (ii) the quasiparticle selfconsistent QSGW+DMFT with dynamic doublecounting, (iii) QSGW+DMFT with static doublecounting schemes. We found that fully selfconsistent $GW$+DMFT with exact doublecounting yields very precise spectra around equilibrium HH distance, as well as reasonable total energy (comparable to LDA+DMFT). However, this scheme breaks down in the correlated regime due to causality violation. The QSGW+DMFT approaches, which are not derivable from a functional, yield similar spectra as full GW+DMFT near equilibrium distance, and in static doublecounting schemes, can also be extended into correlated regime. However, the total energy of these approaches is much worse than the total energy of LDA+DMFT. In summary, this toy model of correlated physics suggests that QSGW+DMFT with constant doublecounting should give accurate predictions of spectra, but not total energy, while LDA+DMFT gives very precise total energy, but somewhat less precise spectra. [Preview Abstract] 
Tuesday, March 15, 2016 10:00AM  10:12AM 
E24.00009: Electronphonon coupling using manybody GW theory Bartomeu Monserrat, David Vanderbilt Electronphonon coupling drives a plethora of phenomena, such as superconductivity in metals, or the temperature dependence of optical properties in semiconductors. There is increasing evidence that semilocal density functional theory (DFT) is not adequate for the description of electronphonon coupling, and instead effects such as electronic correlation need to be included. Unfortunately, methods beyond semilocal DFT are computationally demanding, limiting the study of these phenomena. In this talk we will introduce the idea of ``thermal lines'', which can be used to explore the vibrational phase space of solids and molecules at small computational cost. In particular, we will describe how thermal lines can be exploited to calculate the temperature dependence of band structures beyond semilocal DFT, by using manybody GW theory, or by including the effects of spinorbit coupling. We will present firstprinciples results showing the effects of electron correlation on the strength of electronphonon coupling, and the effects of electronphonon coupling on topological states of matter. [Preview Abstract] 
Tuesday, March 15, 2016 10:12AM  10:24AM 
E24.00010: Plane wave based selfconsistent solution of the GW Dyson equation LinWang Wang, Huawei cao We have developed a selfconsistent procedure to calculate the full Dyson equation based on plane wave basis set. The whole formalism is based on the Greens function matrix of the plane wave Gvector. There is no truncation of the conduction band when the dielectric function is calculated. The Dyson equation is the variational minimum solution of the total energy in terms of the Greens function. The calculation uses the "spacetime" method, with special algorithm for imaginary time integration and Fourier transformation. We have tested isolated molecules and periodic systems. The effects of selfconsistency compared to the G0W0 results will be presented. We will also discuss some special techniques used in the kpoint summation for the periodic system. Massive parallelization is used to carry out such calculations. [Preview Abstract] 
Tuesday, March 15, 2016 10:24AM  10:36AM 
E24.00011: PostGW energies from an extended BetheSalpeter scheme Emanuele Maggio, Georg Kresse Hedin's breakthrough in manybody physics is a computationally manageable scheme to implicitly account for manybody effects thanks to the introduction of a selfenergy, whose expression is known but in practice approximated by truncation at some order in the interparticle interaction. \\ Hedin's scheme allows the computation of quasiparticle addition and removal energies. The introduction of an added particle (or hole) to the system will trigger the formation of higher order neutral excitations (particle/hole pairs formation). The widespread GW approximation only partially accounts for these effects by replacing the bare interparticle interaction with a dressed one. Other effects are contained in the vertex function and are typically disregarded.\\ In the present work, we move beyond the GW level by including vertex effects in the selfenergy. This is implemented by expressing the selfenergy in terms of the reducible twoparticle scattering amplitude. The latter is related to the kernel of the BetheSalpeter equation and to the corresponding polarisation propagator. The proposed implementation allows us to evaluate the quality of quasiparticle spectra for a range of realistic solids and molecular systems. [Preview Abstract] 
Tuesday, March 15, 2016 10:36AM  10:48AM 
E24.00012: GW Calculations of Materials on the Intel XeonPhi Architecture Jack Deslippe, Felipe H. da Jornada, Derek VigilFowler, Ariel Biller, James R. Chelikowsky, Steven G. Louie Intel XeonPhi processors are expected to power a large number of HighPerformance Computing (HPC) systems around the United States and the world in the near future. We evaluate the ability of GW and prerequisite Density Functional Theory (DFT) calculations for materials on utilizing the XeonPhi architecture. We describe the optimization process and performance improvements achieved. We find that the GW method, like other higher level ManyBody methods beyond standard local/semilocal approximations to KohnSham DFT, is particularly well suited for manycore architectures due to the ability to exploit a large amount of parallelism over planewaves, bandpairs and frequencies. [Preview Abstract] 
Tuesday, March 15, 2016 10:48AM  11:00AM 
E24.00013: Recent Advances in Modeling Transition Metal Oxides for Photoelectrochemistry Maytal Caspary Toroker Computational research offers a wide range of opportunities for materials science and engineering, especially in the energy arena where there is a need for understanding how material composition and structure control energy conversion, and for designing materials that could improve conversion efficiency. Potential inexpensive materials for energy conversion devices are metal oxides. However, their conversion efficiency is limited by at least one of several factors: a too large band gap for efficiently absorbing solar energy, similar conduction and valence band edge characters that may lead to unfavorably high electronhole recombination rates, a valence band edge that is not positioned well for oxidizing water, low stability, low electronic conductivity, and low surface reactivity. I will show how we model metal oxides with abinitio methods, primarily DFT$+$U. Our previous results show that doping with lithium, sodium, or hydrogen could improve iron (II) oxide's electronic properties, and alloying with zinc or nickel could improve iron (II) oxide's optical properties. Furthermore, doping nickel (II) oxide with lithium could improve several key properties including solar energy absorption. In this talk I will highlight new results on our understanding of the mechanism of iron (III) oxide's surface reactivity. Our theoretical insights bring us a step closer towards understanding how to design better materials for photoelectrochemistry. References: 1. O. Neufeld and M. Caspary Toroker, ``Ptdoped Fe2O3 for enhanced water splitting efficiency: a DFT$+$U study'', J. Phys. Chem. C 119, 5836 (2015). 2. M. Caspary Toroker, ``Theoretical Insights into the Mechanism of Water Oxidation on Nonstoichiometric and Ti  doped Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$(0001)'', J. Phys. Chem. C, 118, 23162 (2014). [Preview Abstract] 
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