Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M22: FirstPrinciples Modeling of ExcitedState Phenomena in Materials V: Method DevelopmentFocus Live

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Sponsoring Units: DCOMP DCP DMP Chair: Serdar Ogut, University of Illinois at Chicago 
Wednesday, March 17, 2021 11:30AM  12:06PM Live 
M22.00001: Stochastic manybody methods for quasiparticle excitations in realistic nanoscale systems Invited Speaker: Vojtech Vlcek I will present recent developments in predicting electronic excitations using the combination of stochastic computational techniques and manybody theory. The methodology relies on operators' decomposition via random vectors and recasting expectation values as statistical estimators. 
Wednesday, March 17, 2021 12:06PM  12:18PM Live 
M22.00002: Timeresolved exciton wave functions from timedependent densityfunctional theory Jared Williams, Carsten Ullrich, Nicolas TancogneDejean Timedependent densityfunctional theory (TDDFT) is a computationally efficient firstprinciples approach for calculating optical spectra in insulators and semiconductors, including excitonic effects. We show how exciton wave functions can be obtained from TDDFT via the KohnSham transition density matrix, both in the frequencydependent linearresponse regime and in realtime propagation. The method is illustrated using onedimensional model solids. 
Wednesday, March 17, 2021 12:18PM  12:30PM Live 
M22.00003: Lowscaling GW calculations for molecules with benchmark accuracy Jan Wilhelm, Dorothea Golze In traditional GW implementations, the computational cost is growing as O(N^{4}) in the system size N, which prohibits their application to many systems of interest. I present a GW algorithm in a Gaussiantype basis, whose computational cost scales with N^{2} to N^{3}. It will be shown that large minimax grids and resolution of the identity with the truncated Coulomb metric improve the accuracy of the lowscaling GW algorithm to < 0.01 eV for the GW100 test set. Largescale applications of lowscaling GW will be discussed. 
Wednesday, March 17, 2021 12:30PM  12:42PM Live 
M22.00004: Band structure of semiconductors and insulators from Koopmanscompliant functionals Riccardo De Gennaro, Nicola Colonna, Nicola Marzari Koopmanscompliant functionals provide a novel orbitaldensitydependent framework for an accurate evaluation of spectral properties, obtained imposing a generalized piecewiselinearity condition on the total energy of the system with respect to the occupation of each orbital. In 
Wednesday, March 17, 2021 12:42PM  12:54PM Live 
M22.00005: Going beyond the cumulant approximation systematically: Power series expansion of quasiparticle Green’s function. Bipul Pandey, Peter Littlewood The cumulant expansion of quasiparticle Green’s function has been quite popular and successful in going beyond the oneelectron picture and describing electronic correlation that manifest as satellite sidebands^{1}. However, this method is not systematically improvable and only takes into account only some fraction of interband interaction because of the truncation of Dyson series. In this work, we provide a method to systematically improve the quasiparticle Green’s function by using power series corrections and self consistently including the corrections within the self energy. This method does not truncate the Dyson series and the final correction equations are recursive in nature. We also show the derivation of timeordered and retarded cumulant expansion results from this method and elucidate the underlying approximations on self energy in both cases. Finally, we show an application of this method on a simple two band model and compare them to cumulant results. 
Wednesday, March 17, 2021 12:54PM  1:06PM Live 
M22.00006: Improving stochastic Green’s function methods for localized states in lowdimensional heterostructures Mariya Romanova, Vojtech Vlcek First, we present a new embedding approach [1] in the stochastic GW technique that enables efficient treatment of the impurity states with high accuracy and minimal effort. The method is based on a partitioning of the Green’s function and screened Coulomb potential into the deterministic subspace (of localized states) and the stochastic subspace. The enhanced stochasticdeterministic sampling minimizes statistical errors in energies of localized quasiparticles. 
Wednesday, March 17, 2021 1:06PM  1:18PM Live 
M22.00007: Band gaps of crystalline solids from Wannierlocalization based optimal tuning of a screened rangeseparated hybrid functional Dahvyd Wing, Guy Ohad, Jonah Haber, Marina Filip, Stephen E Gant, Jeffrey Neaton, Leeor Kronik Accurate prediction of fundamental band gaps of crystalline solid state systems entirely within density functional theory is a long standing challenge. Here, we present a simple and inexpensive method that achieves this by means of nonempirical optimal tuning of the parameters of a screened rangeseparated hybrid functional. The tuning involves the enforcement of a generalization of the ionization potential theorem to the removal of an electron in an occupied state described by a localized Wannier function in a modestly sized supercell calculation. The method is benchmarked on a set of systems ranging from narrow band gap semiconductors to large band gap insulators, spanning a range of band gaps from 0.2 to 14.2 eV and is found to yield quantitative accuracy across the board, with a mean absolute error of ∼0.1 eV and a maximal error of ∼0.2 eV. 
Wednesday, March 17, 2021 1:18PM  1:30PM Live 
M22.00008: Averaged cluster approach to including chemical shortrange order in KKRCPA Vishnu Raghuraman, Yang Wang, Michael Widom The singlesite KorringaKohnRostoker coherent potential approximation (KKRCPA) ignores shortrange ordering present in disordered metallic systems. We establish a technique to fix this shortcoming by embedding an averaged cluster that displays chemical shortrange order (SRO). The degree of SRO can be tuned by externally defined order parameters. This averaged cluster can be embedded in the singlesite CPA medium or a selfconsistently obtained effective medium that contains SRO information. The validity of this method is demonstrated by applying it to two alloy systems—the CuZn bodycentered cubic (BCC) solid solution, and AlCrTiV, a fourelement BCC high entropy alloy. A comparison between the nonselfconsistent and selfconsistent modes is also provided for the two abovementioned systems. 
Wednesday, March 17, 2021 1:30PM  1:42PM Live 
M22.00009: Kerker mixing scheme for selfconsistent muffintin based allelectron electronic structure calculations Miriam Winkelmann, Edoardo Di Napoli, Daniel Wortmann, Rudolf Zeller, Stefan Bluegel We propose a computationally efficient Kerker mixing scheme for robust, rapidly and systemsize independent converging selfconsistentfield calculations using allelectron firstprinciples electronic structure methods based on the muffintin partitioning of space. We construct the Kerker preconditioner by determining the screened Coulomb potential in real space, solving a modified Helmholtz equation by adopting Weinert’s pseudocharge method for calculating the Poisson equation for periodic charge densities. We have related the preconditioning parameter to the density of states of the delocalized electrons at the Fermi energy and developed a model to choose the preconditioning parameter either prior to the calculation or on the fly. The method has been extend to the slabmodel for 2D films with semiinfinte surfaces. Results are presented using the FLAPW method FLEUR (www.flapw.de) and the KKRGreenfunction code KKRnano (jukkr.fzjuelich.de). 
Wednesday, March 17, 2021 1:42PM  1:54PM Live 
M22.00010: Multiscale polarizable embedding of quasiparticle and electronhole excitations from manybody Green’s functions (GWBSE) with VOTCAXTP Gianluca Tirimbo, Björn Baumeier We present the opensource VOTCAXTP software [1] for the calculation of the excitedstate electronic structure of molecules using manybody Green’s functions theory in the GW approximation with the Bethe–Salpeter Equation (BSE). Its implementation is based on Gaussian orbitals, including, i .a., resolutionofidentity techniques, different methods for the frequency integration of the selfenergy, acceleration with a hybrid OpenMP/Cuda scheme, or inclusion of a classical polarizable environment in a coupled quantum and molecularmechanics (QM/MM) scheme. Two showcase examples are discussed: First, a multiscale simulation framework for quantitative predictions of the highenergy part of ultraviolet photoelectron spectroscopy (UPS) spectra of amorphous molecular solids [2]. Second, we investigate the intermolecular chargetransfer excited states in morphologies of lowdonor content rubreneC60 mixtures using different variants of GWBSE/MM setup. 
Wednesday, March 17, 2021 1:54PM  2:06PM Live 
M22.00011: Effects of screening from electronhole interactions on quasiparticle excitations within the GW approach Meng Wu, Zhenglu Li, Steven G Louie For firstprinciples excitedstate calculations, there exist in the literature various schemes to do “selfconsistent” GW calculations at different levels going beyond the oneshot G_{0}W_{0} approach. For singleparticle excitations (e.g., the quasiparticle bandgap in semiconductors), a straightforward selfconsistent update of both the singleparticle Green’s function G and the screened Coulomb interaction W within the randomphase approximation (RPA) for the selfenergy generally gives less satisfactory results than those from the G_{0}W_{0} approach as compared to experiments. This is because the dielectric screening is underestimated within RPA using quasiparticle energies and the error will be accumulated, since electronhole interactions (excitonic effects) are neglected in the RPA polarizability. In this work, we investigate the importance of electronhole interactions in modifying W in the GW selfenergy. The theoretical formalism is presented, along with firstprinciples results for various semiconductors. 
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