Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B44: RealSpace Methods for the Electronic Structure Problem IIFocus

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Sponsoring Units: DCOMP Chair: Amir Natan, Tel Aviv University Room: 704 
Monday, March 2, 2020 11:15AM  11:51AM 
B44.00001: 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 complex and highlynonlinear interactions of light with solids. To investigate laserinduced ultrafast electron dynamics in solids, we developed a numerical technique to simulate pumpprobe experiments [1]. Recently, we applied the numerical pumpprobe simulations for the attosecond transient absorption spectroscopy and studied the lightinduced 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 important 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]. 
Monday, March 2, 2020 11:51AM  12:03PM 
B44.00002: Speeding up realspace based firstprinciples methods for excitedstates properties using interpolative separable density fitting James Chelikowsky, Weiwei Gao Density fitting methods are a class of algorithms that provide lowrank approximations for products of orbital pairs. However, most of its applications in firstprinciples codes are based on localized basis sets. A recently proposed interpolative separable density fitting (ISDF) method does not rely on predefined auxiliary basis and is formulated in real space representation. We employ the 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. 
Monday, March 2, 2020 12:03PM  12:15PM 
B44.00003: Ab initio stochastic timedomain formulation of the BetheSalpeter equation Chenchen Song, Roi Baer, Daniel Neuhauser, Jeffrey B Neaton, Eran Rabani We present a reduced scaling formulation of the BetheSalpeter equation (BSE) through a combination of a timedependent approach and stochastic representations, which allows efficient ab initio calculations of optical absorption spectra for semiconductor nanoparticles. The linear response of the system to external fields is simulated by propagating quasiparticle orbital dynamics in real time, followed by Fourier transforming the dipoledipole correlation function to obtain the absorption spectrum. The spatially dependent screening of the system is described within the random phase approximation (RPA) and combines several efficient stochastic techniques, including factorization with stochastic representations, and timedependent Hartree propagation of stochastic occupied orbitals. The computational cost of the new BSE formulation is quadratic scaling with respect to system size, which is a significant improvement compared with the conventional symplectic eigenvalue representation of the BSE. We discuss preliminary results from the applications of the method to silicon and CdSe nanocrystals. 
Monday, March 2, 2020 12:15PM  12:27PM 
B44.00004: Fast realtime timedependent density functional theory calculations using higherorder finite element methods Bikash Kanungo, Vikram Gavini We present a computationally efficient approach to solve the timedependent KohnSham equations in real time using higher order finiteelement spatial discretization, applicable to both pseudopotential and allelectron calculations. To this end, we develop an apriori mesh adaption technique, based on the semidiscrete (discrete in space but continuous in time) error estimate on the timedependent KohnSham orbitals, to construct an efficient finiteelement discretization. Subsequently, we obtain the fulldiscrete error estimate to guide our choice of the time step. Importantly, for the allelectron case, we present an efficient mixed basis, termed as enriched finite element basis, that combines the efficiency of atomicorbitalstype basis to capture the sharp variations of the electronic fields closer to the atoms along with the completeness and spatialadaptivity of the finite element basis. We demonstrate significant savings afforded by our approach over the finitedifference and Gaus 
Monday, March 2, 2020 12:27PM  12:39PM 
B44.00005: Recent Advances in the Development of the Octopus Code for GPUs Sebastian Ohlmann Since the end of Moore's law, using stateoftheart supercomputers has become more and more difficult because simulation codes need to expose more and more parallelism to benefit from larger machines. Moreover, supercomputers have become heterogeneous as the increase in CPU performance has slowed down: all of the first three exascale machines to be built in the US will heavily rely on GPUs to achieve their compute performance. Thus, simulation codes in all areas, also realspace DFT codes, will need to utilize GPUs efficiently in order to use these nextgeneration machines. In this contribution, we report on recent advances in the development of the realspace DFT code Octopus for GPUs. Those developments include porting more algorithms to GPUs, both for groundstate and timedependent calculations, improving the data handling when copying data to and from the GPU and improving the overlap of communication with computation. We will show how these advances improve the speed of the code and its scalability and apply the code to some larger problem sizes on GPUs. 
Monday, March 2, 2020 12:39PM  12:51PM 
B44.00006: Excitonic effects from realtime parameterfree hybrid functions Nicolas TancogneDejean, Angel Rubio

Monday, March 2, 2020 12:51PM  1:03PM 
B44.00007: Abinitio photoionization dynamics without continuum states Umberto De Giovannini Photoionization underpins a range of spectroscopies central to the study of structural and dynamical properties of matter in the gas and solid phases. In solid state physics angular resolved photoelectron spectroscopy (ARPES) and timeresolved (tr) ARPES are the most prominent techniques. Leveraging the flexibility offered by realspace methods we developed a technique, based on the realtime formulation of timedependent density functional theory (TDDFT), to simulate ARPES and trARPES abinitio without explicit reference to continuum states [1]. I will present the theory, the algorithm involved in the implementation and some of the most representative applications and predictions. 
Monday, March 2, 2020 1:03PM  1:15PM 
B44.00008: Efficient electromagnetic potential calculations within ground state and time dependent density functional theory. Dor Gabay, Ali Yilmaz, Vitaliy Lomakin, Amir Boag, Amir Natan Electromagnetic potentials play a fundamental role in ground and excited state calculations of Density Functional Theory (DFT). One method for the calculation of such potentials is to directly solve their corresponding differential equations. In this work, the equivalent integral expressions of those potentials are evaluated within their spectral representation. Such integral expressions play a critical role in handling problems ranging from hybrid functionals to the calculation of retarded potentials within the timedependent KohnSham equation. The simplest of these are FFT procedures used to evaluate the static Poisson integral within groundstate calculations. We extend the use of these integral expressions to timedependent retarded electromagnetic potentials in both the Lorenz and Coulomb gauges and demonstrate the efficiency of the approach. We use this method for the calculation of several electronic structures using a realtime realspace TDDFT approach. Finally, the various gaugefixing conditions are compared to assure alignment in the limit of small magnetic fields and the advantages of the Lorenz gauge are outlined. 
Monday, March 2, 2020 1:15PM  1:27PM 
B44.00009: Realspace approach to the calculation for ionization potentials, exciton, and biexciton binding energies in quantum dots using explictlycorrelated electronhole interaction kernel method Peter McLaughlin, Nicole Spanedda, Arindam Chakraborty Inclusion of unoccupied states is the leading computational bottleneck for calculation of excited states for large chemical systems. In this work, we present the geminalscreened electronhole interaction kernel (GSIK) method to address this problem. The GSIK is a realspace r12 method that avoids unoccupied orbitals for constructing the electronhole interaction kernel by performing an infiniteorder diagrammatic summation of particlehole excitations and deriving a renormalized realspace electronhole correlator operator. The GSIK method also bypasses the computational expensive AOtoMO integral transformation by computing all integrals directly in the realspace numerically using permutation sampling Monte Carlo method. These two features allow GSIK method to be used for chemical systems where inclusion of a large number of unoccupied orbitals will be computationally prohibitive. In this work, the GISK method was applied to investigate exciton binding energies, biexciton binding energies, and ionization potentials for large semiconductor (Pb_{140}S_{140}, Pb_{140}Se_{140}, Cd_{144}Se_{144}) nanoparticles. The results from these calculations demonstrate the efficacy of the GSIK method for capturing electronhole correlation in large clusters and nanoparticles. 
Monday, March 2, 2020 1:27PM  1:39PM 
B44.00010: Recent developments in the Octopus code for strong lightmatter coupling Micael Oliveira The Octopus code [1,2] is a finitedifferences realspace code designed to fully take advantage of the flexibility and versatility of realspace grids and provide developers with a framework to easily implement and test new ideas and methods in the field of electronic excited states properties and dynamics, while ensuring optimal execution performance and parallelization. 
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