Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session A59: First-Principles Simulations of Excited-State Phenomena: GW Method and Beyond IFocus
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Sponsoring Units: DCOMP Chair: Xiao Zhang, University of Michigan; Eva Zurek, State Univ of NY - Buffalo Room: Room 301 |
Monday, March 6, 2023 8:00AM - 8:36AM |
A59.00001: GW in Gaussian Bloch Orbitals for Solids - relativistic effects, pseudopotentials, and impacts of different self-consistency cycles Invited Speaker: Dominika Zgid I will present recent developments in GW with Gaussian Bloch Orbitals for Solids that happened in my group. First, I will present a formulation of relativistic self-consistent GW for solids based on the exact two-component formalism with one-electron approximation (X2C1e) and non-relativistic Coulomb interactions. Our theory allows us to study scalar relativistic effects, spin-orbit coupling, and the interplay of relativistic effects with electron correlation without adjustable parameters. The simplicity of X2C1e enables the construction of higher order theories, such as embedding theories, on top of perturbative calculations. |
Monday, March 6, 2023 8:36AM - 8:48AM |
A59.00002: Mixed Stochastic-Deterministic Approach for GW Calculations Aaron R Altman, Sudipta Kundu, Felipe H da Jornada Many-body perturbation theory in the form of first-principles GW calculations is an established method of obtaining accurate quasiparticle properties of materials. However, traditional approaches based on both the Sternheimer equation and sum-over-bands approaches scale quarticly in system size and can be notoriously difficult to converge. Fully stochastic methods which scale linearly can handle very large systems, but often use real-time and real-space propagation that requires custom codes and efficient evaluation of the Kohn-Sham Hamiltonian's action on a trial vector. Here we present a combined stochastic-deterministic approach to reciprocal-space GW calculations that achieves quasi-quadratic scaling while incurring negligible error, <100 meV in quasiparticle energies. Our method displays smooth convergence, and we benchmark on a variety of systems spanning dimensionality, screening, and size. Implementation is straightforward in existing reciprocal-space GW codes and allows the calculation and convergence of large systems without a fully stochastic, real-time formalism. |
Monday, March 6, 2023 8:48AM - 9:00AM |
A59.00003: BerkeleyGW on the Path to Exascale Mauro Del Ben, Steven G Louie, Jack Deslippe On the path to exascale, graphics processing units (GPUs) dominate the HPC landscape, requiring developers to adapt and redesign their core implementations to effectively leverage the novel hardware paradigms. Here we present the various strategies employed to tackle these portability efforts for the BerkeleyGW software package. BerkeleyGW is a materials science software package employed to study the excited state properties of electrons in materials by using the GW and the GW plus Bethe-Salpeter Equation (GW-BSE) methods, and beyond. It is capable of scaling to hundreds of thousands of CPU cores and effectively utilizes strong scaling to tens of thousands of GPUs. We achieved over 100 PFLOP/s of sustained performance and over 50% of peak of Summit@OLCF, making GW calculations for thousands of atoms systems feasible within minutes. We discuss our software design to achieve true performance portability across various GPU vendor architectures by analyzing the performance of vendor specific programming models (CUDA and HIP) and the directives based portable counterparts (OpenACC and OpenMP-target). We highlight the challenges we encountered as well as the practices we found useful in the porting pipeline. |
Monday, March 6, 2023 9:00AM - 9:12AM |
A59.00004: Estimating the accuracy of pseudopotential based GW method at different levels of self consistency using Gaussian orbitals. Vibin Abraham, Ming Wen, Gaurav Harsha, Gaurav Harsha, Dominika Zgid Many body perturbation theory provides an efficient and qualitatively cheap approach for estimating spectroscopic properties and charged excitation energies in solids and molecules. In this work we present the finite temperature GW method, which is one of the simplest approximations to the self-energy, at different levels of self consistency. In particular, we study the one shot G0W0, quasiparticle self consistent GW (qpGW) and fully self consistent GW (scGW) approaches. We assess the accuracy of commonly used pseudopotential based GW methods by comparing them to the all electron spin free X2C based GW approach which captures scalar relativistic effects efficiently. We present data showing that the use of ECP gives relatively good results for DFT while for correlated calculations, the errors are larger compared to the all electron calculations. |
Monday, March 6, 2023 9:12AM - 9:24AM |
A59.00005: Charge Transfer Screening and Energy Level Alignment at Complex Organic–Inorganic Interfaces: A Tractable Ab Initio GW Approach Nicholas Lin Quan Cheng, Catalin D Spataru, Fengyuan Xuan, Su Ying Quek The energy level alignment (ELA) at organic-inorganic interfaces quantifies charge injection barriers in organic and molecular electronic devices. Many-body perturbation theory in the GW approximation enables the quantitative prediction of ELA in many systems but can be computationally challenging for large interfaces. We have developed an approach [1] to perform GW calculations on large interfaces, which involves the eXpansion of the polarizability (chi) matrix from a unit cell to the supercell, the Addition of chi from the two subsystems, and the use of wavefunctions from the Full interface to compute the self-energies. This XAF-GW method has been shown to work even in the presence of interface hybridization to form bonding and anti-bonding orbitals. Here, we modify the XAF-GW approach to specifically account for many-body interactions due to charge transfer effects and obtain excellent agreement with benchmark GW calculations with significantly reduced computational cost [2]. We show that many-body interactions lead to gate-tunable molecular HOMO-LUMO gaps in a F4TCNQ/graphene interface. By comparison with a two-dimensional electron gas model, we also show the importance of explicitly accounting for intraband transitions in determining the charge transfer screening in organic-inorganic interfaces. |
Monday, March 6, 2023 9:24AM - 9:36AM |
A59.00006: Second-harmonic generation in 2D semiconductors via the time-dependent adiabatic GW method: the role of electron-hole interactions Marcos Menezes, Felipe H da Jornada, Zhenglu Li In this work, we use a first-principles, interacting Green’s functions approach within the time domain to study the non-linear optical response of 2D semiconductors. We employ the recently developed time-dependent adiabatic GW (TD-aGW) approach, which correctly captures quasiparticle excitations within the fully dynamical GW approximation at equilibrium and produces the same linear response given by the GW-BSE approach with a statically screened electron-hole interaction kernel [1, 2]. Beyond the linear regime, this approach also allows us to naturally obtain higher-harmonic responses and including excitonic effects. As an application, we compute the second harmonic generation (SHG) coefficient , associated with the second-order susceptibility tensor, and higher-order responses. We survey the SHG in a few 2D semiconductors of interest, such as transition metal dichalcogenides (TMDCs) of different band topologies, and discuss the role of the electron-hole interaction in the spectra. Considering its importance on the linear response of low-dimensional systems, we expect excitonic effects to be relevant to higher-order responses functions, as suggested in earlier calculations [1, 3, 4]. Besides promoting a deeper understanding of the optical properties of known materials, these calculations suggest methods to tune many-body interactions and the resulting nonlinear optical properties of low-dimensional materials. |
Monday, March 6, 2023 9:36AM - 9:48AM |
A59.00007: Speeding-up GW calculations for 2D materials Bimal Neupane, Yuanxi Wang GW approximation can properly describe the quasiparticle band structure of semiconductors and insulators beyond semi-local or hybrid density functional theory (DFT). For 2D materials, one of the computationally expensive part of a properly converged GW calculation is to finely sample the Brillouin zone in obtaining the non-interacting RPA polarizability, due to sharp dielectric features in 2D semiconductors. This difficulty in achieving convergence in k-point sampling becomes even more computationally expensive in modeling defects inside supercells at the GW level. We can speed up the GW-level defect calculations by using the polarizability of pristine 2D materials as an approximation, obtained from a unit-cell calculation and folded into a supercell Brillouin zone. Combining this method with the extrapolar technique [1] will further accelerate those calculations. We expect this method to be applied to high-throughput GW calculations for point defects in 2D materials with improved defect level placements and band gaps compared to DFT-level calculations. |
Monday, March 6, 2023 9:48AM - 10:00AM |
A59.00008: Hilbert-Space Separation Schemes in Energy-Space and Real-Space for Excited-State Calculations Diana Y Qiu, Victor Chang Lee, Marina R Filip, Felipe H Jornada, Jack McArthur We discuss two new Hilbert-space separation schemes for GW and GW plus Bethe Salpeter equation (GW-BSE) calculations. In the first, we look at a technique for accelerating calculations on layered metal halide perovskites. In recent years, a number of techniques have been developed to separately calculate the polarizability of systems where the wavefunction overlap is small, such as in layered van der Waals materials or molecules adsorbed on surfaces. Here, we develop a generalized scheme to extend these techniques to systems, such as halide perovskites, where the two subsystems are ionically bonded. We show that this technique can decrease the computational cost of calculations on a single perovskite unit cell by as much as an order of magnitude and is trivially extendable to calculations over larger supercells. In the second scheme, we develop a Hilbert-space downfolding technique for systems where subspaces are well-separated in energy space. We apply this to study shallow core-level excitations in bulk and monolayer transition metal dichalcogenides (TMDs). We find that the typical truncation of the Hilbert space into core and valence levels can give rise to spurious plasmon-like features, whose origin we analyze. |
Monday, March 6, 2023 10:00AM - 10:12AM Author not Attending |
A59.00009: Periodic Coupled-Cluster Green's Function for Photoemission Spectra of Realistic Solids Tianyu Zhu, Katelyn Laughon, Jason M Yu We present an efficient implementation of the coupled-cluster Green's function (CCGF) method for simulating photoemission spectra of periodic systems. We formulate the periodic CCGF approach with Brillouin zone sampling in the Gaussian basis at the coupled-cluster singles and doubles (CCSD) level. To enable CCGF calculations of realistic solids, we propose an active-space self-energy correction scheme by combining CCGF with the cheaper many-body perturbation theory (GW) and implement the model order reduction (MOR) frequency interpolation technique. We find that the active-space self-energy correction and MOR techniques significantly reduce the computational cost of CCGF while maintaining the high accuracy. We apply the developed CCGF approaches to compute spectral properties and band structure of silicon (Si) and zinc oxide (ZnO) crystals and find good agreement with experimental measurements. |
Monday, March 6, 2023 10:12AM - 10:24AM |
A59.00010: Quasiparticle self-consistent band structure and excitons of V2O5 and LiV2O5 Claudio Garcia, Santosh K Radha, Walter R Lambrecht V2O5 is a layered material of interest as Li battery cathode. Recent cathodoluminescence measurements by Walker et al. [J. Mater. Chem 8, 11800 (2020)] show effects as function of Li content which are not yet understood, in particular in view of the recent finding that exitonic effects are huge in V2O5 [Gorelov et al. npj Comput. Mater. 8, 94 (2022)]. We present calculations of the band structure and optical properties of V2O5 and LiV2O5 in the α and γ-structures. The self-energy is calculated using the quasiparticle self-consistent QSGW method with W calculated either with or without ladder diagrams as recently implemented in the Questaal-code [Cunning- ham et al. arXiv:2106.05759]. The quasiparticle gap is found to be much higher than the optical gap and is only slightly reduced by including ladder diagrams, while exciton binding energies exceed 1.5 eV. The gap and exciton binding energies are further enhanced in monolayer V2O5. In both α and γ-LiV2O5 we find an antiferromagnetic ordering of the magnetic moments along the zig-zag chain direction, induced by half-filling of the V-dxy-splitoff band. We investigate the effects of this Li induced band filling on the quasiparticle band structure and excitons. |
Monday, March 6, 2023 10:24AM - 10:36AM |
A59.00011: A spin-dependent first-principles investigation of the optoelectronic properties of rare earth zirconates via GWA and the Bethe-Salpeter equation Ryan Grimes, Shunshun Liu, Prasanna V Balachandran We have applied many-body perturbation theory to investigate the optoelectronic properties of Ln2Zr2O7 (Ln = La, Ce, Gd, Sm) pyrochlore zirconates starting from spin-dependent density functional theory at the generalized gradient approximation level. Results are intended to explore the relative influence of each lanthanide on the electronic band gap and dielectric properties of pure pyrochlore zirconates. Trivalent lanthanides are modeled with optimized 4f-valence Gaussian-type pseudopotentials to insure the presence of a gapped mean-field ground state. Going beyond DFT, we performed single-shot G0W0 quasi-particle corrections and calculated the excitonic Hamiltonian via the Bethe-Salpeter equation (BSE) to obtain the real and imaginary dielectric function components for each structure using the YAMBO software suite. A double k-grid procedure was applied in order to (1) improve convergence of the optical calculations with respect to k-point sampling and (2) resolve and characterize ground state bound excitons. G0W0 band gap energies of 5.74, 2.65, 4.84, and 4.21 eV were obtained for La2Zr2O7, Ce2Zr2O7, Gd2Zr2O7, and Sm2Zr2O7, respectively. Our BSE results for Sm2Zr2O7 indicate the presence of one or more bound excitons with an energy signature of 1.2 eV. |
Monday, March 6, 2023 10:36AM - 10:48AM |
A59.00012: Ab-initio simulation of many-body effects in high-harmonic generation in monolayer WS2 Victor Chang Lee, Diana Y Qiu, Yang-hao Chan In materials where the nonlinear susceptibility is substantial, n photons with the same frequency can be combined to generate a new photon with n times the initial frequency in a process known as harmonic generation. However, the simulation of high-order nonlinear susceptibilities as a perturbation problem can become very complex as the order increases. In recent years, time-dependent simulations have been applied to the simulation of higher order susceptibilities as a method to avoid the exponential scaling in complexity with the inclusion of many-body effects. In our work, we applied a real-time nonequilibrium Green’s function method within the adiabatic GW approximation to the study of nonlinear optical susceptibilities under a continuous wave illumination in monolayer WS2. Our results show a good agreement with previous experimental measurements and calculations of second-order susceptibilities and higher orders can be simulated without an additional increase in computational cost. |
Monday, March 6, 2023 10:48AM - 11:00AM |
A59.00013: Accurate defect levels in semiconductors and insulators from Koopmans spectral functionals Riccardo De Gennaro, Nicola Colonna, Nicola Marzari Point defects in insulating crystalline materials lead to the formation of impurity levels within the forbidden gap that play an essential role in determining the properties of electronic devices. Despite its computational feasibility, density-functional theory lacks a proper description of spectral properties and does not provide an accurate prediction of the defect binding energy, and more accurate formulations are often hindered by the size of the supercells required to damp the spurious interactions of the defect state with its periodic replicas. Here, we compute the position of defect levels using Koopmans spectral functionals [1], a novel orbital-density-dependent approach that captures the physics of the electron addition/removal process. Recent works have proven the accuracy of Koopmans functionals in predicting the band structure of semiconductors and insulators [2,3,4], laying the groundwork for a correct description of defect states. |
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