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 YY08: V: Electronic Structure |
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Sponsoring Units: DCOMP Chair: Magdalena Waleska Aldana Segura, Universidad de San Carlos de Guatemala Room: Virtual Room 8 |
Wednesday, March 22, 2023 10:00AM - 10:12AM |
YY08.00001: High-Accuracy, High-Performance DFT Calculations on Exascale CPU-GPU Architectures with the RMG code Emil Briggs, Wenchang Lu, Jerzy Bernholc The promise of Exascale computer architectures is close to being realized, with the first system operational in 2022. However, applying their computational power to the study of large-scale scientific problems requires careful attention to the interplay between system architecture and computational algorithms. RMG is a software package for electronic structure calculations designed from inception for massively parallel platforms, which has been further optimized for pre-exascale machines such as Summit at ORNL. We describe the development of algorithms to further improve accuracy, convergence, and performance, as well as the implementation of advanced features such as hybrid functionals, semi-local pseudopotentials, and spin-orbit coupling. Tests on the Crusher platform at ORNL show excellent performance and scaling, which should translate well to the exascale Frontier that has identical nodes. RMG source code and build scripts for the pre-exascale Summit, Cray XE-XK, clusters, Linux, and Windows workstations are at www.rmgdft.org, together with help files and examples. A web interface for generating RMG input from standard atomic structure files or other commonly used electronic structure codes is also available, as is an interface for analysis of RMG output. |
Wednesday, March 22, 2023 10:12AM - 10:24AM |
YY08.00002: Nearly ab-initio MLFT applied to Kα XES of 3d transition metal oxides Charles Cardot, Gerald Seidler, Joshua J Kas, Fernando D Vila, John J Rehr Accurate first-principles predictions of the excited states and spectra of 3d transition metals have historically been challenging due to their highly correlated nature. Model based approaches such as multiplet ligand field theory (MLFT) have been successful in describing the spectra of such systems, but have limited predictive capability due to the large number of free parameters. Recent work combines density functional theory (DFT) with MLFT to obtain many of these parameters, and has been been applied to the x-ray absorption and valence to core emission of a wide variety of materials. However, there are few applications of this approach to core-to-core emission. Here we present an extension of the DFT plus MLFT approach of Haverkort et al. [1] within the code Quanty, to achieve a nearly ab-initio calculation of core-to-core X-ray Emission Spectroscopy (XES) of these materials. In this approach, a tight binding (TB) Hamiltonian describing the crystal field splitting as well as coupling to the ligand states is extracted from the DFT calculation. The 2-particle Coulomb interaction (Slater-Condon) parameters are also calculated within DFT, taking into account the nephelauxatic reduction. We demonstrate promising agreement between experiment and theory across a range of transition metals (Cr, Mn, and Ni), as well as the ability to reproduce key spectral trends which are dependent on oxidation state, spin state, and coordination geometry. Finally, we investigate the dependence of the spectra on the remaining free parameters to demonstrate the merits and limitations of the approach. |
Wednesday, March 22, 2023 10:24AM - 10:36AM |
YY08.00003: All-electron plane-wave electronic structure calculations Francois Gygi We demonstrate the use of the plane wave basis for all-electron electronic structure calculations. The approach relies on the definition of an analytic, norm-conserving, regularized Coulomb potential, and a scalable implementation of the plane wave method capable of handling large energy cutoffs (up to 80kRy in the examples shown). The method is applied to the computation of electronic properties of isolated atoms as well as diamond, silicon, MgO, solid argon, and a configuration of 64 water molecules extracted from a first-principles molecular dynamics simulation. The computed energies, band gaps, ionic forces and stress tensors provide reference results for the validation of pseudopotentials and/or localized basis sets. A calculation of the all-electron band structure of diamond and silicon using the SCAN meta-GGA density functional allows for a validation of calculations based on pseudopotentials derived using the PBE exchange-correlation functional. In the case of (H2O)64, the computed ionic forces provide a reference from which the errors incurred in pseudopotentials calculations and in localized gaussian basis sets calculations can be estimated. All calculations are performed using the Qbox code (http://qboxcode.org). |
Wednesday, March 22, 2023 10:36AM - 10:48AM |
YY08.00004: Optimizing the Integration of Multireference Electronic Structure Methods with Non-Equilibrium Green's Functions Erik P Hoy, Andrew Sand A quantum revolution is underway bringing with it a new generation of novel nanoscale sensors and electronic devices with non-classical charge transport characteristics. Describing these fundamentally quantum devices requires a fully quantum transport approach that consistently captures both dynamic and multireference electron correlation effects. This remains a major challenge for many transport methodologies due to limitations in their underlying electronic structure methods. We previously introduced a unique methodology [J. Chem. Phys. 155, 114115 (2021)] for integrating multiconfigurational electronic structure methods within a non-equilibrium Green’s function (NEGF) formalism. I will discuss improvements to this methodology and analyze the relative importance of multireference correlation in characterizing non-classical charge transport effects. |
Wednesday, March 22, 2023 10:48AM - 11:00AM |
YY08.00005: Convergence of quantum transport calculations with respect to atomic basis sets: Application to graphene-based DNA sequencing Jaeeun Kim, Han Seul Kim, Hyeonwoo Yeo, Seunghyun Yu, Yong-Hoon Kim In the past decade, first-principles quantum charge transport calculations based on density-functional theory (DFT)-based non-equilibrium green’s function (NEGF) formalism have become one of the most powerful and routine computational tools in modern nano-scale research. The DFT-NEGF approach by its nature starts from the Hamiltonian matrices constructed based on spatially localized atomic-like bases sets. However, despite the critical role of the quality of localized basis sets on the accuracy of DFT-NEGF calculations, the correlations between the basis sets and quantum charge transport calculations has been insufficiently discussed. Specifically, a scheme to assess the numerical convergence of the transmission with respect to localized basis sets is still absent. In this presentation, by carrying out DFT-NEGF calculations for a DNA nucleobase placed within the nanogap between two graphene nanoribbon electrodes we systematically test the number and extension of atomic basis sets and find that unconverged or overcomplete atomic basis sets can produce significantly misleading quantum transport calculation data. We will finally present the practical guidelines of adopting localized basis sets to carry out accurate yet cost-effective ab initio quantum transport calculations. |
Wednesday, March 22, 2023 11:00AM - 11:12AM |
YY08.00006: Automated Wannierizations Junfeng Qiao, Giovanni Pizzi, Nicola Marzari Maximally localized Wannier functions (MLWFs) are widely used in computational condensed matter physics. The standard approach to construct MLWFs often requires initial guesses which are based on chemical intuition and some measure of trial and error. Here, we first introduce an algorithm based on "projectability disentanglement" that provides reliably and automatically atom-centered Wannier functions describing both occupied and empty states. Then, we show how to mix these again into target subspaces; e.g., to describe valence and conduction bands separately. We test these algorithms on 200 representative materials (77 insulators), showing that the final MLWFs are very well localized and can accurately interpolate band structures at the meV scale. Such approaches enable automated Wannierizations for both metals and insulators, promoting further applications of MLWFs in physical applications and high-throughput calculations. |
Wednesday, March 22, 2023 11:12AM - 11:24AM |
YY08.00007: New Approach to Calculating Electronic Band Structure of Bi2Te3: Parametrized Tight-Binding Corrections to Density Functional Theory Wannier Hamiltonian Shima Sharifi Najafabadi, Stephen B Fahy, Ivana Savic
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Wednesday, March 22, 2023 11:24AM - 11:36AM |
YY08.00008: Real-space Green's function approach for intrinsic losses in x-ray spectra John J Rehr, Joshua J Kas Intrinsic losses in x-ray spectra originate from excitations in a system due to a suddenly created core-hole. These losses give rise to many-body effects such as satellites and edge singularities in x-ray photoemission spectra (XPS) and x-ray absorption spectra (XAS). As shown by Langreth, these effects can be treated within linear response and the RPA in terms of a cumulant Green’s function in momentum space. Here we present a complementary ab initio real-space Green’s function (RSGF) approach in terms of the dynamically screened core-hole Wc(ω) and the independent particle response function. We show that the cumulant kernel β(ω), which characterizes the excitation spectrum, is analogous to XAS, but with the transition operator replaced by the core-hole potential and monopole selection rules. Assuming spherical symmetry about each site, the calculations can be carried out using a discrete site-radial coordinate basis, in parallel with RSGF calculations of XAS. The behavior of Wc(ω) and β(ω) reflect the analytic structure of the loss function, with peaks in β(ω) arising from the zeros of the dielectric matrix. This behavior is consistent with the interpretation of these quasi-boson peaks as plasmons or charge-transfer excitations. The approach further simplifies when Wc(ω) is localized and spherically symmetric. Illustrative results are presented for several systems and compared with other methods. |
Wednesday, March 22, 2023 11:36AM - 11:48AM |
YY08.00009: Unraveling the origin of chiroptical properties in 2D achiral-chiral hybrid lead perovskites from first-principles calculations Pranab Sarker, Hao Li, Deyu Lu, Tao Wei, Qiuming Yu Chiral hybrid organic-inorganic perovskites are an emerging class of non-magnetic spintronic materials. Alloying organic achiral cations with the chiral ones creates new avenues to tune the chiroptical properties, e.g., circular dichroism (CD) response, to a greater extent. However, how the achiral-chiral cation mixing contributes to engineering the electronic and chiroptical properties is far from understood. We provide insights into the structure-property relationship in the achiral-chiral mixed system from first-principles calculations. We demonstrate that the achiral-chiral cation mixing leads to stronger asymmetric hydrogen bonding between chiral organic cations and inorganic sublattices, resulting in larger spin-splitting and enhanced CD signals. Our results shed light on the origin of electronic and chiroptical properties in the 2D mixed achiral-chiral hybrid organic-inorganic perovskites. |
Wednesday, March 22, 2023 11:48AM - 12:00PM |
YY08.00010: Exploring gradient-approximated functionals for strongly coupled light-matter systems Cankut Tasci, Johannes Flick Recent theoretical advances have made it possible to use ab initio methods to describe strong-light matter systems from first principles. One of these methods, quantum-electrodynamical density functional theory (QEDFT) is a promising candidate with a low computational cost to allow for computationally efficient simulations with high accuracy. Recently, we have introduced a computational efficient density functional [1] that only depends on the electron density and its gradients for QEDFT. In this work, we explore the accuracy of this functional for realistic multi-mode setups described by macroscopic QED. We study systems, such as interacting molecules strongly coupled to spherical cavities and nanoplasmonic layered systems. In addition, we demonstrate pathways to correctly capture multi-photon processes and anisotropy. |
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