Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G46: Excited State I: Method development: ManyBody Perturbation TheoryFocus Recordings Available

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Sponsoring Units: DCOMP DMP Chair: Feliciano Giustino, University of Texas Room: McCormick Place W470A 
Tuesday, March 15, 2022 11:30AM  12:06PM 
G46.00001: Massively parallel cubicscaling GW calculations with the OpenAtom software Invited Speaker: Sohrab IsmailBeigi OpenAtom is an open source, massively parallel ab initio density functional software package based on plane waves and pseudopotentials (http://charm.cs.uiuc.edu/OpenAtom) that takes advantage of the Charm++ parallel framework. We will describe a cubicscaling massively parallel implementation of the GW approximation (L. Hedin, Phys. Rev. 139, 1965) to the electronic selfenergy that is useful for beyondDFT calculations of electronic band structures. After a brief summary of the OpenAtom and Charm++ frameworks along with the main ideas behind our cubicscaling approach (Kim, Martyna, IsmailBeigi, Phys. Rev. B 101, 2020), we will describe the approach we employ for its parallelization (along with benefits and tradeoffs involved) and some examples of its parallel performance on large supercomputers. 
Tuesday, March 15, 2022 12:06PM  12:18PM 
G46.00002: QSGW calculations for point defects using a cutandpaste approach for the selfenergy Ozan Dernek, Walter R Lambrecht, Dmitry Skachkov, Mark van Schilfgaarde The selfenergy in quasiparticle selfconsistent GW (QSGW) approximation is usually represented in the basis set of density functional theory (DFT) eigenstates. The real space representation of the selfenergy matrix obtained by expanding these eigenstates in an atomcentered basis set, such as linearized muffintin orbitals, has great advantages. In this work, we show that the selfenergy matrix is relatively shortranged in the real space representation. This allows us to create a practical “cutandpaste” method to construct the selfenergy matrix of a large system from those of smaller subunits. This method can be used for the study of pointdefects in a large supercell by constructing the real space matrix elements Σ_{RL,R’+TL’} in terms of the host and defect Σ, with the latter obtained from a much smaller defect containing cell. In the case of the As_{Ga }pointdefect in GaAs, we show that the defect can already be represented in the 8 atom conventional fcc unit cell. We show that the band structure of a 64 atom defect supercell using the cutandpaste method is in good agreement with the exact QSGW calculation for the same cell. The defect band position relative to the valence band maximum and dispersion are virtually identical. 
Tuesday, March 15, 2022 12:18PM  12:30PM 
G46.00003: RealSpace Stochastic GW Calculations Benchmark on GW100 Ishita Shitut, Weiwei Gao, James R Chelikowsky, Amir Natan, Hulikal R Krishnamurthy, Manish Jain Stochastic implementation of GW is a linear scaling method, ideally suited for calculating quasiparticle energies of large systems. This approach uses the stochastic resolution of identity to represent Green's function as a product of a randomly generated orbital at time zero and an evolved random orbital at a later time. It employs real time propagation of stochastic functions to obtain screened coulomb response function. The response function is efficiently stored using stochastic compression. We have implemented the stochastic GW method in realspace density functional theory code PARSEC. We have benchmarked our stochastic GW implementation on GW100 set against the results obtained from the NanoGW code [1]. We find that our results are in good agreement with the results obtained from the NanoGW code. 
Tuesday, March 15, 2022 12:30PM  12:42PM 
G46.00004: Stochastic cRPA calculations of screened interactions in graphene moire states at high pressures Mariya Romanova, Vojtech Vlcek Twisted bilayer graphene (tBLG) hosts correlated electrons in flat bands, stemming from the coupling between mutually rotated monolayers. The coupling is primarily controlled by the twist angle but equally well by the bilayer’s inplane strain or compression. It was found previously that under angle twist screening plays a crucial role in reducing bare Coulomb interactions as one approaches the magic angle. The pressure behavior of screening is however less investigated. 
Tuesday, March 15, 2022 12:42PM  12:54PM 
G46.00005: Efficient Treatment of Dynamical Renormalizations and Multiscale Approaches Through Stochastic Manybody Methods Vojtech Vlcek, Mariya Romanova, Guorong Weng, Carlos Mejuto Zaera Efficient numerical implementations enable applying the manybody perturbation theory to realistic systems. Stochastic approaches based on sampling singleparticle states, propagators, and interactions are particularly successful in combination with oneshot perturbative corrections. These methods allow simulating nanoscale problems with thousands and tens of thousands of electrons. I will present our work on expanding this framework to (i) enable selfconsistent treatments, and (ii) describe more complex quantum phenomena arising from strong interactions of excited states. The new implementation allows to quickly access both diagonal and offdiagonal elements of the selfenergy, update the singleparticle states, and identify energy regions characterized by strong couplings among quasiparticles. Further, I will present new randomized sampling methods to efficiently compute the dynamical renormalization via stochastic constrained RPA (and beyond RPA) methods within arbitrarily selected strongly interacting subspace. The approach is general and requires only minimal computational resources (in the order of 100s of CPU hours for systems with 10,000 electrons). I will outline the route towards multiscale simulations in which the weakly (and moderately) correlated electronic states are treated by stochastic perturbation theory combined with the embedding of strongly correlated states. 
Tuesday, March 15, 2022 12:54PM  1:06PM 
G46.00006: Making the intractable tractable: GWBSE calculation of the nature of interlayer and intralayer moiré excitons in largearea moiré superlattices. Mit H Naik, Zhenglu Li, YangHao Chan, Felipe H da Jornada, Steven G Louie Recent experimental measurements have demonstrated signatures of novel exciton states in the moiré superlattices of transition metal dichalcogenide (TMD) bilayer heterostructures. However, the exact nature of the moiré excitons is difficult, and has yet, to be determined experimentally. An accurate description of these excitons requires a completely account of the electron and hole degrees of freedom. Firstprinciples theoretical study of moiré excitons is strongly hindered by the O(N^{4}) scaling of the ab initio GWBethe Salpeter equation method with the number of atoms (N). To overcome this challenge, we develop a Pristine Unitcell Matrix Projection (PUMP) method, which speeds up the computation by at least six orders of magnitude. In this method, we leverage the smooth modulation of the electronic wavefunctions in the moiré superlattice to expand them in an efficient (i.e., small) basis of pristine unitcell wavefunctions. The electronhole interaction kernel matrix of the Bethe Salpeter equation is similarly expanded in terms of the pristine unitcell kernel matrix elements. Using this method, we have computed the optical spectra of TMD heterobilayers and discovered novel interlayer and intralayer moiré excitons of dramatically different characters. 
Tuesday, March 15, 2022 1:06PM  1:18PM 
G46.00007: Extending the ab initio BetheSalpeter equation approach to include phonon screening Antonios Alvertis, Jonah B Haber, Marina R Filip, Jeffrey B Neaton Exciton properties are critical to the optoelectronic response of materials, making their theoretical and computational description important to future technological devices. Manybody perturbation theory (MBPT), and specifically the ab initio GWBetheSalpeter (BSE) equation approach [1], is a highly accurate method for predicting exciton properties that has been successfully applied in diverse systems. However, recent work has shown that electronic effects alone are not enough to provide an accurate description of exciton physics, and the effect of phonons can lead to significant renormalization of exciton peak positions [2] and electronhole interactions [3]. An extension of the GWBSE method to account for these phonon effects is therefore necessary. Here we present a firstprinciples approach based on MBPT, building on the framework introduced in prior work [3] to rigorously implement phonon screening effects into the ab initio BSE method. We apply our method to a variety of semiconductors, quantifying the role of phonon screening on exciton binding energies. We also discuss the effect of temperature on phonon screening of excitons, connecting our formulation to model electronhole interactions that include excitonphonon coupling, such as the Haken potential. 
Tuesday, March 15, 2022 1:18PM  1:30PM 
G46.00008: OptimallyTuned Starting Points for OneShot GW Calculations of Solids Stephen E Gant, Jonah B Haber, Francisca Sagredo, Guy Ohad, Dahvyd Wing, Marina R Filip, Leeor Kronik, Jeffrey B Neaton Accurate quasiparticle (QP) band gaps can be obtained via ab initio manybody perturbation theory within the GW approximation, where G is the oneelectron Green's function and W is the screened Coulomb interaction. In practice, oneshot G0W0 calculations which do not selfconsistently update G or W are often carried out to reduce computational expense and complexity. However, G0W0 calculations exhibit a strong startingpoint dependence on the eigensystem used to construct G and W, limiting the predictive power of this approach. Here, we present G0W0 calculations performed using a Wannierlocalized, optimally tuned screened rangeseparated hybrid (WOTSRSH) starting point. This functional is optimally tuned per system to obey the ionization potential theorem and has been shown to produce band gaps for semiconductors and insulators in excellent agreement with experiment [1]. We show that G0W0@WOTSRSH leads to QP band gaps and band structures comparable in accuracy to those produced by selfconsistent GW schemes for a range of insulators and semiconductors at a reduced cost. We also discuss the sensitivity of G0W0@WOTSRSH to perturbations in the underlying parameters in the SRSH functional about their optimal values, as well as the implications for optical gaps and spectra. 
Tuesday, March 15, 2022 1:30PM  1:42PM 
G46.00009: Can vertexcorrected perturbative approaches be improved through symmetry breaking? Carlos Mejuto Zaera, Vojtech Vlcek Manybody perturbation theory (MBPT) approaches are ubiquitous in the study of materials. The most extended such approach is arguably the GW approximation, which includes classical, electrostatic screening on top of a meanfield reference, but neglects multiquasiparticle interactions. In principle, such terms can be recovered by introducing nontrivial components of the interaction vertex Γ, set to zero in GW. However, it is only recently that systematic (in the Hedin sense) implementations of vertexcorrected GWΓ have started to be applied in ab initio calculations. Hence, important questions regarding the computational and physical nature of these approaches remain open. In this talk, we address one such issue: the impact of the meanfield reference on the MBPT result, particularly regarding symmetry broken (SB) solutions, in the context of selfconsistent GWΓ (scGWΓ). The introduction of symmetry breaking formally absent in the true manybody ground state can improve meanfield approximations. However, scGW cannot typically recover the symmetry, resulting in ultimately worse results than the SB meanfield. We test whether it is possible to formulate vertex corrections that recover symmetries at selfconsistency, thus allowing the use of SB meanfield reference states. 
Tuesday, March 15, 2022 1:42PM  1:54PM 
G46.00010: Photoemission spectroscopy from the threebody Green's function. gabriele riva, Arjan Berger, Pina Romaniello We present an original approach for the calculation of direct and inverse photoemission spectra from first principles. 
Tuesday, March 15, 2022 1:54PM  2:06PM 
G46.00011: Validation of Power Series Expansion for Single Particle Green's Function in Model Hamiltonians and Simple Material Systems. Bipul Pandey, Peter B Littlewood The GW method for single particle Green's function overlooks selfconsistency to achieve computational cost effectiveness. This manifests as incorrect estimation of the effect of collective modes (plasmons) on the quasiparticle and incorrect satelline band energies. The cumulant approximation which was developed to mitigate this is ad hoc, unsystematic and fails at intermediate to strong electroncollective mode coupling. We have developed a general scheme to reestablish selfconsistency by postprocessing GW calculations by using a power series. We show that we can not only derive the cumulant results but also get exact green’s function for simple multiband model hamiltonians at intermediate couplings. Finally, we use our scheme to produce corrected bandstructures in simple crystal systems and compare them to GW and cumulants. 
Tuesday, March 15, 2022 2:06PM  2:18PM 
G46.00012: Electronic properties of crystalline solids from Wannierlocalization–based optimal tuning of a screened rangeseparated hybrid functional Guy Ohad, Dahvyd Wing, Marina R Fillip, Ayala V Cohen, Jonah B Haber, Stephen E Gant, Francisca Sagredo, Jeffery B Neaton, Leeor Kronik Accurate prediction of fundamental band gaps of crystalline solidstate systems, entirely within density functional theory, has been a longstanding challenge. Previously, we developed a simple and inexpensive method that achieves this by means of nonempirical optimal tuning of the parameters of a screened rangeseparated hybrid functional [1]. The tuning involves the enforcement of an ansatz that generalizes the ionization potential theorem to the removal of an electron from an occupied state described by a localized Wannier function in a modestly sized supercell calculation. Here we present applications of the method to band gaps of more complex semiconductors and insulators, including halide perovskites and metal oxides, demonstrating quantitative accuracy. 
Tuesday, March 15, 2022 2:18PM  2:30PM 
G46.00013: Relativistic SelfConsistentGW: Exact TwoComponent Formalism with OneElectron Approximation ChiaNan Yeh, Avijit Shee, Dominika Zgid, Emanuel C Gull We present a formulation of relativistic selfconsistent GW based on the exact twocomponent formalism with oneelectron approximation (X2C1e) and nonrelativistic Coulomb interactions. Our theory allows us to study scalar relativistic effects, spinorbit coupling, and the interplay of relativistic effects with electron correlation. Our allelectron implementation is fully ab initio and requires no pseudopotential constructed from relativistic atomic calculations. We examine the effect of the X2C1e approximation by comparison to DFT calculations and reach excellent agreement with the established fourcomponent theory. The simplicity of X2C1e makes it feasible to construct higher order theories, such as embedding theories, on top of GW. 
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