Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session E19: Real Space Methods for the Electronic Structure Problem: Dynamics and ApplicationsFocus Live

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Sponsoring Units: DCOMP Chair: Angel Rubio, Max Planck Inst Structure & Dynamics of Matter; Center for Computational Quantum Physics Flatiron Institute, Simons Foundation NY, USA 
Tuesday, March 16, 2021 8:00AM  8:36AM Live 
E19.00001: Real space and real time electron dynamics simulations for attosecond physics in solids Invited Speaker: Shunsuke Sato Realspace and realtime electron dynamics simulation based on the timedependent density functional theory is a powerful tool to analyze highlynonlinear interactions of light with solids. To investigate laserinduced nonequilibrium electron dynamics in solids, we developed a numerical technique to simulate pumpprobe experiments [1]. Recently, we applied the numerical pumpprobe simulations to the attosecond transient absorption spectroscopy and studied the ultrafast electron dynamics in solids [2,3]: First, we investigated laserinduced electron dynamics in GaAs with the firstprinciples simulations. As a result, we found an essential role of the lightinduced intraband transition in transient optical properties of opticallydriven semiconductors [2]. Then, we investigated ultrafast electron dynamics in Titanium, and the firstprinciples simulations provided microscopic insight into laserinduced electronlocalization dynamics in transition metals [3]. 
Tuesday, March 16, 2021 8:36AM  8:48AM Live 
E19.00002: A Real Space Approach to Uniqueness in Polarization Shoham Sen, Yang Wang, Timothy Breitzman, Pradeep Sharma, Kaushik Dayal A fundamental issue in the atomic and quantum scale modeling of dielectric materials is the question of defining the macroscopic polarization. In a periodic crystal, the usual definition of the polarization as the dipole of the charge in a unit cell depends on the choice of the unit cell. We examine this issue using a rigorous approach based on the framework of twoscale convergence. Starting with a periodic charge density on a compact domain, we examine the continuum limit of lattice spacing going to zero. We prove that accounting for the boundaries consistently provides a route to uniquely compute electric fields and potentials, despite the nonunique polarization. Specifically, there are partial unit cells at the boundary which, not being chargeneutral, give rise to a surface charge. Different choices of the unit cell in the interior of the body leads to different partial unit cells at the boundary; the net effect is that these changes compensate each other. We also explain how the aforementioned polarization is connected to the “Free Energy Density” and the “Modern Theory of Polarization”/“Berry Phase” definitions of polarization. We show that using both these definitions, the potentials and bound charges are the same. 
Tuesday, March 16, 2021 8:48AM  9:00AM Live 
E19.00003: A Gridbased Oneelectron Model for Describing Nonvalence Correlation Bound Anions in Molecules and Clusters Devin Mulvey, Tae Hoon Choi, Vamsee K. Voora, Tijo Vazhappilly, Kenneth Jordan Nonvalence correlation bound (NVCB) anions are bound by dispersiontype correlation between the excess electron and the electrons of a molecule or cluster. The computationally demanding, O(N^{56}), electron attachment equationofmotion (EAEOM) method is typically used to describe these species. An alternative, oneelectron models, treat only the excess electron explicitly and use classical polarization potentials to capture the molecular density's response. We describe models developed in our group for characterizing NVCB anions and their implementation in our oneelectron model code PISCES. To solve the oneelectron Schrödinger equation sine discrete variable representation (DVR) grid bases are employed. A mixed real and momentum space method is combined with Fast Fourier transforms. A dual grid approach has been developed too, where an affordable coarse DVR grid is refined via interpolation. We report binding energies in agreement with ab initio EOM calculations at a fraction of the computational time. We are currently working to extend this to the image potential states of graphene using finite model systems. 
Tuesday, March 16, 2021 9:00AM  9:12AM Live 
E19.00004: Extended Lagrangian BornOppenheimer Molecular Dynamics with Numeric AtomCentered Orbitals Konstantin Lion, Mariana Rossi, Anders M. N. Niklasson, Matthias Scheffler, Claudia Draxl Stable trajectories from direct BornOppenheimer molecular dynamics (BOMD) require very accurate forces and, as such, a tightly converged selfconsistent field optimization in each time evolution step. Contrary to direct BOMD, only a single optimization step is necessary in Extended Lagrangian BornOppenheimer molecular dynamics (XLBOMD) [1] prior to each force evaluation, thus greatly reducing computational cost. We discuss a new implementation of XLBOMD in the all electron, numeric atomcentered orbital code FHIaims [2]. The extended electronic system is represented by the density matrix within KohnSham densityfunctional theory. We ensure efficiency by implementing core aspects of the scheme within the Electronic Structure Infrastructure [3]. The stability of the implementation for diverse types of material critically depends on the kernel K that determines the acceleration in the integration of the extended electronic system. We address the stability and performance of different K for several types of systems (isolated molecules, metals, semiconductors). 
Tuesday, March 16, 2021 9:12AM  9:24AM Live 
E19.00005: Optical excitations of α–RuCl_{3} using realspace quantum Monte Carlo Abdulgani Annaberdiyev, Guangming Wang, Lubos Mitas α–RuCl_{3} has been an intensely studied material in recent years due to its exotic electronic and magnetic properties. It is a quasi2D system with an insulating ground state which exhibits a few energetically close magnetic states and conjectured to realize Kitaev's spinliquid. It has been suggested that the inclusion of spinorbit coupling effects of the Ru atom and an accurate description of electron correlations is critical for the proper description of the electronic and magnetic properties of α–RuCl_{3}. We study the ferromagnetic and nonmagnetic phases of this material with a highly accurate diffusion Monte Carlo (DMC) method to investigate these aspects. In particular, we aim to calculate the optical gaps using spinorbit averaged potentials with fixednode DMC, as well as using explicit spinorbit potentials with fixedphase DMC and 2component formalism. Our goal is to shed light on the significance of spinorbit effects and electron correlations on the key quantities such as band excitations and cohesive energy. 
Tuesday, March 16, 2021 9:24AM  9:36AM Live 
E19.00006: Dielectric Screening in Large Silicon Nanocrystals from RealSpace Pseudopotential Calculations Timothy Liao, KaiHsin Liou, James Chelikowsky The nature of dielectric screening in semiconductor nanocrystals (NCs) is an continuing topic of interest. We examine the screening of a point charge in hydrogenterminated Si NCs containing up to 5,400 atoms. Under the framework of pseudopotentialdensityfunctional theory, we solve the KohnSham equation in realspace using the PARSEC code. We compute the dielectric properties of a Si NC by replacing a Si nucleus at the center of the NC with an Al or P nucleus while maintaining the number of electrons. We consider NCs of sufficient size to converge the dielectric properties to the bulk limit. We contrast these results with previous studies on smaller NCs. The ability to calculate dielectric properties of a large confined system allows us to consider charged defects in bulk Si in a straight forward manner without invoking a compensating background. 
Tuesday, March 16, 2021 9:36AM  9:48AM Live 
E19.00007: Superconductivity in borondoped crystalline and amorphous carbon Yuki Sakai, James Chelikowsky, Marvin L Cohen We investigate the superconducting properties of borondoped carbon materials composed of sp^{3}^{ }hybridized atoms: cubic diamond, hexagonal diamond, and body centered tetragonal C_{4} (bct C_{4}). We find that a high density of states (DOS) at the Fermi energy result in a high superconducting transition temperature (T_{c}) in all three materials. The high DOS is particularly realized when boron dopants are not placed at nearest neighbor sites. A T_{c} above 60 K can be obtained for cubic diamond, although other crystalline allotropes can exhibit a T_{c} of 4050 K at 25 % borondoping. We also consider superconductivity in borondoped amorphous carbon. 
Tuesday, March 16, 2021 9:48AM  10:00AM Live 
E19.00008: Using realspace simulations of noncontact atomic force microscopy to distinguish functional groups, atomic species and molecular geometries in organic molecules Dingxin Fan, Yuki Sakai, James Chelikowsky Imaging the internal chemical structure of molecules remains an ongoing challenge. Noncontact atomic force microscopy (ncAFM) with a CO functionalized probe tip is a powerful tool for molecular structure characterizations. For many organic molecules, the visualization of individual atoms is a real possibility using ncAFM, save for the complexity of interpreting the measured images. In order to gain a better understanding of such images, we employ realspace pseudopotentials constructed within density functional theory code, PARSEC, to simulate ncAFM images. We are able to discriminate functional groups (such as C≡C, CH_{2} and C=O groups) and heteroatoms (such as O, N and S atoms) in organic molecules by mapping our simulated images to experimental images. Also, we find that ncAFM is capable of directly visualizing the orientation of organic molecules at varies adsorption sites on metal substrates. 
Tuesday, March 16, 2021 10:00AM  10:12AM Live 
E19.00009: PyQMC: an allPython realspace quantum Monte Carlo code William Wheeler, Shivesh Pathak, Jo?o N. B. Rodrigues, Cooper Lorsung, Yueqing Chang, Yiqing Zhou, Brian Busemeyer, Kiel T Williams, Alexander Munoz, Lucas Wagner PyQMC is a new, easytouse implementation of realspace quantum Monte Carlo (QMC) for highlyaccurate simulations of correlated electron systems. The allPython code enables fast development of new techniques and flexible, complex workflows, such as the recent work from our group on QMC excited states [1]. Integration with the electronic structure package PySCF [2, 3] leverages available tools and facilitates direct comparison with many other ab initio methods. The wide availability of Python libraries offers additional flexibility. With PyQMC's parallelization implementation, cloud resources or HPC can be used with the same code. The vectorized architecture ensures good performance and is GPU ready. PyQMC includes variational Monte Carlo, wave function optimization, and diffusion Monte Carlo on molecules and solids, and is under active development. The code is freely available at https://github.com/WagnerGroup/pyqmc. 
Tuesday, March 16, 2021 10:12AM  10:24AM Live 
E19.00010: Cumulant Green’s function calculations of the asymmetry of 4f_{7/2 }peaks in the XPS of Au and Pt Charles Cardot, Joshua Kas, John Rehr, Fernando Vila, Joe Woicik Measurements of the 4f_{7/2} quasiparticle peaks in the XPS of Au and Pt exhibit asymmetries which have been attributed to edge singularity effects in the spectral function. The goal of this work is to model these asymmetries using realtime TDDFT calculations of the low energy particlehole excitation spectrum. In our approach a cumulant Green’s function is used to calculate the shift in spectral weight from the quasiparticle peak to the satellite features that account for the asymmetry. The cumulant is calculated using a realtime TDDFT formalism using an extension of the SIESTA code base. The computational workflow is carried out using Corvus, a recently developed pythonbased workflow management tool designed to integrate multiple scientific software packages. This implementation is efficient and allows for a postprocessing analysis appended to the original workflow. Our calculations fit to DoniachSunjic line shapes are in good agreement with the experimental XPS data for Au and Pt. 
Tuesday, March 16, 2021 10:24AM  10:36AM Live 
E19.00011: Speeding up excitedstates calculations using interpolative separable density fitting Weiwei Gao, James Chelikowsky We employ a recently proposed interpolative separable density fitting (ISDF) method to significantly reduce the cost of linear response timedependent density functional theory (LRTDDFT) and GW calculations. In our implementation, we exploit the symmetry property of a system to effectively reduce the number of auxiliary basis and thus the computational cost. Our benchmarks show the cost for constructing auxiliary basis and interpolation coefficients are negligible compared to the total computational cost. Compared to a conventional “brutalforce” approach, the cost for evaluating all kernel matrix elements in LRTDDFT and GW calculations is reduced by up to three orders of magnitude. The accuracy of our implementation is benchmarked with the GW100 set. 
Tuesday, March 16, 2021 10:36AM  10:48AM On Demand 
E19.00012: Modeling the Opening SARSCoV2 Spike: an Investigation of its Dynamic ElectroGeometric Properties Anna Kucherova, Selma Strango, Shahar Sukenik, maxime theillard The recent COVID19 pandemic has brought about a surge of crowdsourced initiatives aimed at simulating the proteins of the SARSCoV2 virus. A bottleneck currently exists in translating these simulations into tangible predictions that can be leveraged for pharmacological studies. Here we report on extensive electrostatic calculations done on an exascale simulation of the opening of the SARSCoV2 spike protein, performed by the Folding@home initiative. We compute the electric potential as the solution of the nonlinear PoissonBoltzmann equation using a parallel sharp numerical solver. The inherent multiple length scales present in the geometry and solution are reproduced using highly adaptive Octree grids. We analyze our results focusing on the electrogeometric properties of the receptor binding domain and its vicinity. This work paves the way for a new class of hybrid computational and data enabled approaches, where molecular dynamics simulations are combined with continuum modelling to produce highfidelity computational measurements serving as a basis for protein biomechanism investigations. 
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