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 R22: First-Principles Modeling of Excited-State Phenomena in Materials VI: Method DevelopmentFocus Live
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Sponsoring Units: DCOMP DCP DMP Chair: Li Yang, Washington University, St. Louis |
Thursday, March 18, 2021 8:00AM - 8:12AM Live |
R22.00001: Ground and excited states of open-shell molecules and atoms from a spin-flip Bethe-Salpeter approach David A Strubbe, Bradford Barker Open-shell systems, including molecules and defects, are interesting platforms for spin physics and quantum information, but their multi-determinantal states are difficult to handle with conventional first-principles calculations. A solution is the spin-flip approach: the ground and excited states are considered as spin-flipping excitations of a single-determinant high-spin reference state. By analogy to time-dependent density-functional theory (TDDFT), we introduced spin-flip to the GW/Bethe-Salpeter approach (GW/BSE). The BSE equations have a similar structure to TDDFT, but with an ab initio long-ranged and non-local interaction kernel that enables more accurate calculations of excited states. We implemented this spin-flip BSE approach in the BerkeleyGW code, and demonstrated success on two benchmark systems: torsion of the ethylene molecule (C2H4) and singlet-triplet splittings in atoms, with good agreement with experiments and higher-level calculations. While spin contamination (not having eigenstates of S2) is a major problem for spin-flip TDDFT, our analysis of <S2> in spin-flip BSE shows only minor deviations from expected values. This method has promise for further applications, particularly in condensed phases. |
Thursday, March 18, 2021 8:12AM - 8:24AM Live |
R22.00002: Substrate Screening Effect on Quasiparticle Energies and Optical Properties of Lattice-mismatched Two-dimensional Interfaces Chunhao Guo, Junqing Xu, Dario Rocca, Yuan Ping Two-dimensional (2D) materials and their interfaces have recently emerged as promising platforms for exotic physical phenomena. Previous methods of including substrate screening for quasiparticle energies can be only applicable to interfaces of two systems' lattice constants with certain integer proportion, which often requires a few percentage of strain. We developed an efficient and accurate reciprocal-space interpolation technique for dielectric matrices that made quasiparticle energy calculations possible for arbitrarily mismatched interfaces free of strain [1]. We applied this method to obtain quasiparticle corrections at GW approximation for arbitrary mismatched 2D interfaces, by interfacing hBN with SnS2 and phosphorene at their natural lattice constants. We then employed this method to study the effect of substrates on optical properties of 2D materials, by solving the Bethe-Salpeter equation. We obtained perfect agreement with experimental non-rigid 1s and 2s excitonic shift with increasing layer thickness of WS2. At the end, we predicted optical spectra and radiative lifetime of hBN with different substrates and explained their microscopic mechanism. [1] C. Guo et al, arXiv:2007.07982 |
Thursday, March 18, 2021 8:24AM - 8:36AM Live |
R22.00003: Dielectric embedding GW: An accurate and efficient approach for molecule-substrate interfaces Zhenfei Liu Many nanoscale energy-conversion processes take place at heterogeneous molecule-substrate interfaces. An accurate characterization of the quasiparticle energy level alignment at the interface is central to understanding the interfacial charge dynamics. Although the first-principles GW approach is considered state-of-the-art in computing quasiparticle properties of molecular and extended systems, its routine application to large interface systems has been limited by the relatively high computational cost. In this talk, we present a newly developed dielectric embedding GW approach designed specifically for heterogeneous molecule-substrate interfaces. The idea is to confine expensive GW self-energy calculations within small simulation cells that only consist of either the adsorbate or the substrate, while accurately accounting for the dielectric effect of the other via the inclusion of its Kohn-Sham polarizability. Using a few prototypical molecule-metal and molecule-semiconductor interfaces, we discuss the strengths and limitations of this approach by comparing our approach with direct GW and other related methods. |
Thursday, March 18, 2021 8:36AM - 8:48AM Live |
R22.00004: Are multi-quasiparticle interactions important in molecular ionization? Carlos Mejuto Zaera, Guorong Weng, Mariya Romanova, Stephen J. Cotton, Birgitta K Whaley, Norm Tubman, Vojtech Vlcek Understanding multi-quasiparticle (MQP) phenomena is a key step towards the design of quantum materials with tailored opto-electronic properties. Photo-emission spectra (PES) offer an experimental probe for these effects, but accurate theoretical descriptions of the quasiparticle interactions underlying the MQP regime are still lacking. We tackle this challenge by studying the inner valence PES of closed-shell molecules using the fully-correlated adaptive sampling configuration interaction (ASCI) method. Our results show a rich satellite structure, hallmark of MQP physics, even in these deceivingly simple systems. We complement the ASCI spectra with perturbative calculations, namely GW and vertex corrected GWΓ, and conclude that the satellite features originate from correlated quasiparticles. The vertex corrections in GWΓ seem recover the excitonic interactions necessary to qualitatively capture these satellites, improving the GW results. |
Thursday, March 18, 2021 8:48AM - 9:00AM Live |
R22.00005: Developing Python framework for atomated GW-BSE calculations Tathagata Biswas, Sydney Olson, Arunima Singh We develop an open-source Python code for performing automated first-principles calculations within GW-BSE (Bethe-Salpeter) framework. GW-BSE framework is the state-of-the-art methodology to explore quasiparticle (QP) and excitonic properties using many body perturbation theory. GW-BSE framework is effective for overcoming bandgap underestimation issues of Density Functional Theory (DFT) and to obtain absorption spectra directly comparable with experimental observations. Our code is built upon open-source Python packages developed by the Materials Project, such as pymatgen, FireWorks, and atomate to achieve complete automation of the entire multi-step GW-BSE computational framework that requires several convergence parameters. Wannier90, a program for calculating maximally-localized Wannier functions (MLWF) has been used to perform the interpolation required to obtain quasiparticle bandstructure. We have used our code to create a database containing QP bandstructure and BSE absorption spectra of ~1000 materials with diverse chemical compositions. |
Thursday, March 18, 2021 9:00AM - 9:12AM Live |
R22.00006: Random Phase Approximation for gapped systems: the role of vertex corrections and applicability of the constrained random phase approximation Erik van Loon, Malte Roesner, Mikhail I. Katsnelson, Tim Wehling The many-body theory of interacting electrons is an intrinsically difficult problem that requires simplifying assumptions. The Random Phase Approximation (RPA) provides such a simplification, it is a computationally feasible approach for determining the screening properties of materials. An important application is the constrained Random Phase Appromixation (cRPA), which is the state-of-the-art scheme for the ab-initio calculation of effective interactions in low-energy models. Here, we show explicitly that the distance to the Fermi level and the resulting short electronic propagation length justifies the use of the (c)RPA. Our analysis of electron-electron interactions provides a real space analogy to Migdal's theorem on the smallness of vertex corrections in electronphonon problems. An essential finding is that an RPA calculation based on Kohn-Sham states and the Kohn-Sham gap already includes the leading (excitonic) vertex correction in insulators. |
Thursday, March 18, 2021 9:12AM - 9:24AM Live |
R22.00007: Higher order many-body perturbation theory applied to atomic systems Simone Vacondio, Daniele Varsano, Alice Ruini, Andrea Ferretti While the GW method has been the subject of an intense work of validation, higher-order many-body perturbation theory (MBPT) methods have received much less attention. Here we investigate the performance of beyond-GW MBPT approaches in atomic systems described within the spherical approximation. By using a dedicated numerical treatment based on a B-spline/spherical harmonics representation [1], we obtain benchmark results avoiding many commonly adopted approximate procedures (including complete basis set extrapolations, and frequency description). We present a variety of results, including static polarizabilities, ionization potentials, and Kohn-Sham exchange and correlation potentials as obtained from the solution of the Sham-Schlüter equation (SSE) for a number of MBPT schemes, including GW, 2nd Born, and SOSEX [2] self-energies. |
Thursday, March 18, 2021 9:24AM - 9:36AM Live |
R22.00008: Direct Optimization of Quasi-2D Atomic Structures with Diffusion Monte Carlo Jaron Krogel, Juha Tiihonen, Hyeondeok Shin Quasi-2D materials display great optical sensitivity to lattice strain, simultaneously with large lattice flexibility. Predictive characterization of the optical properties of these materials therefore requires precise determination of lattice parameters. We present a novel parallel line search method to simultaneously determine multiple structural parameters of 2D materials with sparse sampling of the diffusion Monte Carlo (DMC) potential energy surface (PES). The method relies on density functional calculations as a surrogate to guide the search along the most rapidly converging directions in parameter space. Resampling techniques are used on the surrogate model to predict and further minimize the computational cost of the DMC PES search. We present examples of the method as applied to 2D GeSe and flake-like 2D molecules. |
Thursday, March 18, 2021 9:36AM - 9:48AM Live |
R22.00009: Fano-Feshbach approach for calculation of Auger decay rates with equation-of-motion coupled-cluster wave functions Wojciech Skomorowski, Anna Krylov X-ray absorption creates electron vacancies in a core shell. These highly excited states often relax by the Auger decay - an autoionization process in which one valence electron fills the core hole and another valence electron is ejected into the ionization continuum. Despite the important role of Auger processes in many experimental settings, their first-principle modeling remains challenging, mainly due to the necessity to describe many-electron continuum. We present a novel approach to calculate Auger decay rates by combining Fano-Feshbach resonance theory with equation-of-motion coupled-cluster (EOM-CCSD) framework and core-valence separation (CVS) scheme. The continuum many-body decay states are represented by products of an appropriate EOM-CCSD state and a continuum orbital describing the outgoing electron. The Auger rates are expressed in terms of two-body Dyson amplitudes (reminiscent of two-particle transition density matrix) contracted with two-electron bound-continuum integrals. Two approximations to the state of the outgoing electron are considered: a plane wave and a Coulomb wave with an effective charge. Numerical results are provided for core-ionized and core-excited benchmark systems (Ne, H2O, CH4, and CO2), and compared with available experimental spectra. |
Thursday, March 18, 2021 9:48AM - 10:00AM Live |
R22.00010: Optimally-Tuned Range-Separated Hybrid Functional Starting Points for One-Shot GW Calculations Stephen E Gant, Jonah Haber, Dahvyd Wing, Guy Ohad, Leeor Kronik, Marina Filip, Jeffrey Neaton Accurate quasiparticle energies and subsequently fundamental band gaps can be obtained using ab initio many-body perturbation theory (MBPT) within the GW approximation. Frequently, so-called one-shot G0W0 calculations are carried out to reduce computational expense and complexity. However, one-shot G0W0 can be sensitive to the Kohn-Sham eigensystem that is used as a starting point for constructing the Green’s function and RPA screened Coulomb interaction, limiting the predictive power of this approach. Here, we present a one-shot G0W0 approach that uses a Wannier-localized, optimally-tuned screened range-separated hybrid (WOT-SRSH) functional as a starting point. The WOT-SRSH functional is tuned for each system to satisfy an approximate extension of the IP theorem, a procedure which has recently been shown to predict excellent band gaps. In this work, we perform G0W0@WOT-SRSH for several bulk semiconductors and insulators and find that it can significantly reduce starting point dependence. We compare our calculations with experiment and discuss the potential predictive power of this approach for one-shot G0W0. |
Thursday, March 18, 2021 10:00AM - 10:36AM Live |
R22.00011: Subspace Embedding and Downfolding Techniques for Solving the Bethe Salpeter Equation: Interplay of Localized and Continuum Excitons in Complex Systems Invited Speaker: Diana Qiu Ab initio many-body perturbation theory methods, like GW and GW plus Bethe Salpeter Equation (GW-BSE), are well-established and highly-accurate techniques for calculating the quasiparticle and optical properties of moderate-sized systems. There remain, however, a number of challenges when it comes to scaling up these techniques to address systems with a large number of heterogeneous atoms, various forms of aperiodicity, and large energy scales well-outside the optical regime. In this talk, I will discuss our newly developed subspace embedding and downfolding techniques for GW-BSE calculations on low-dimensional, nanostructured and amorphous systems that exemplify these challenges. In particular, we apply GW-BSE to study optical properties of heterostructures, defects, and molecular functionalization of quasi two-dimensional (quasi-2D) materials. I will also discuss the effect of electron-hole interactions on core-level spectra of quasi-2D materials and amorphous water, including dynamical effects due to scattering with the electron-hole continuum, where we find that electron-hole interactions play an essential role in the scattering of core-level excitations with excitations from the valence band. The calculations are made possible through a combination of physically motivated approximations and algorithms, including non-uniform spatial sampling, low-rank approximations, and subspace embedding and matrix downfolding techniques. |
Thursday, March 18, 2021 10:36AM - 10:48AM Live |
R22.00012: Group representations of exciton states and their derivation from first principles Jiawei Ruan, Zhenglu Li, Chin Shen Ong, Steven G Louie Excitons play an essential role in the optical properties of semiconductors, especially in reduced-dimensional systems. Their symmetry characters are important ingredients that are relevant to selection rules for optical transition and other interactions. Here, we present a method to derive group representations of exciton states directly from ab initio GW-Bethe-Salpeter-equation calculations without any assumptions on the characters of the envelope functions. This method can be applied to study symmetry properties of Wannier and Frenkel excitons, as well as excitons arising from Mexican-hat quasiparticle bands or parallel valence and conduction bands (e.g. the C exciton in monolayer MoS2). The method gives definitive conclusion on the exciton-state splitting and degeneracy, mitigating uncertainties from numerical noises. |
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