Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F46: Real Space Methods for the Electronic Structure ProblemFocus Recordings Available

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Sponsoring Units: DCOMP Chair: Mehmet Dogan, University of California, Berkeley Room: McCormick Place W470A 
Tuesday, March 15, 2022 8:00AM  8:36AM 
F46.00001: Stochastic Density Functional Theory: Real and EnergySpace Fragmentation for Noise Reduction Invited Speaker: Ming Chen

Tuesday, March 15, 2022 8:36AM  8:48AM 
F46.00002: DFTFE — a massively parallel realspace density functional theory code using higherorder adaptive spectral finiteelement discretization, and its largescale application to study dislocation core energetics in crystalline materials Sambit Das, Phani Motamarri, Vikram Gavini KohnSham density functional theory (DFT) calculations have been instrumental in providing many crucial insights into materials behavior and occupy a sizable fraction of world's computational resources today. However, the stringent accuracy requirements in DFT needed to compute meaningful material properties, in conjunction with the asymptotic cubicscaling computational complexity with number of electrons, demand huge computational resources. Thus, these calculations are routinely limited to material systems with at most few thousands of electrons. In this talk, we present a massively parallel realspace DFT framework (DFTFE), which is based on a local realspace variational formulation of the KohnSham DFT energy functional discretized with higherorder adaptive spectral finiteelement, and handles pseudopotential and allelectron calculations in the same framework. We will present the efficient and scalable numerical algorithms in conjunction with mixed precision strategies for the solution of KohnSham equations, that has enabled computationally efficient, fast and accurate DFT calculations on generic material systems reaching ~100,000 electrons, on both manycore and hybrid CPUGPU architectures. DFTFE achieves an order of magnitude performance advantage over widely used planewave codes both in CPUtimes and walltimes. Further, we will present our recent work on developing a robust and efficient preconditioner for selfconsistent field iterations in KohnSham DFT. Finally, we demonstrate the successful application of DFTFE to accurately study the core energtics of pyramidal dislocations in magnesium, which have so far been out of reach as large system sizes containing thousands of atoms are required to accurately resolve the relevant physics in the dislocation core. 
Tuesday, March 15, 2022 8:48AM  9:00AM 
F46.00003: Exploring Ion Dynamics in Charged Systems using RealSpace Pseudopotential Density Functional Theory KaiHsin Liou, James R Chelikowsky Ions are prevalent in Nature. Their dynamics is key to understanding various physical processes and chemical reactions. Proton hopping, for instance, is an important mechanism to explain the exceptionally high diffusivity of protons in water. Studies have demonstrated such proton transfer mechanisms may be vital to biochemical systems. To study proton hopping by firstprinciples approaches, it is common to use a supercell. These boundary conditions can add artificial correlated motion. Moreover, charged systems in supercells often adopt a uniform compensating background charge to avoid the computation of divergent electrostatic energies. However, it is not clear how well this approach reproduces the physical boundary conditions felt by the proton. We will compare supercell methods with a confined domain method where there is no need of a compensating charge within the same framework of realspace pseudopotential density functional theory. 
Tuesday, March 15, 2022 9:00AM  9:36AM 
F46.00004: The Power of TDDFT in RealTime and RealSpace: From Light Harvesting to Photoemission Invited Speaker: Stephan Kuemmel Solving the timedependent equations of KohnSham or generalized KohnSham theory by propagation in real time on a grid in real space is a powerful approach for calculating electronic excitations. The technique is ideally suited for studying large systems and longrange chargetransfer phenomena due to its excellent parallelization. We here discuss two example of electron dynamics that can excellently be described with the realtime and realspace technique. First, we discuss how to simulate photoemission as a process in real time. When a system is excited by an electromagnetic field of sufficient frequency, it can emit electrons. We simulate the emission of the outgoing wave packet in real time and calculate the corresponding kinetic energy spectrum. In this way, angular resolved photoemission signals can be calculated. Challenging phenomena, like pumpprobe photoemission mapping excited states and circular dichroism in the angular distribution of the emitted electrons are thus obtained in close agreement with experiments. Second, we discuss how the energytransfer processes that occur in natural lightharvesting systems can be studied via realspace and realtime techniques. We demonstrate in calculations for the antenna complexes of photosynthetic bacteria how one can extract just from the timedependent density information about how energy is transported between the chromophores and thus efficiently collected. 
Tuesday, March 15, 2022 9:36AM  9:48AM 
F46.00005: Implementation of Ensemble Density Functional Theory for excited states in the realspace Octopus code Uday Panta, David A Strubbe Ensemble Density Functional Theory is a method to calculate excitation energies from an ensemble of ground and excited states, which can resolve some shortcomings – like multiplet splittings and double excitations – of TimeDependent Density Functional Theory (TDDFT) approximations. Whereas many EDFT formulations require selfconsistent ensemble calculations, the computationally efficient Direct Ensemble Correction (DEC) takes the form of a correction to the KohnSham energy differences, as in linearresponse TDDFT [Yang et al., Phys. Rev. Lett. 119, 033003 (2017)]. This theory, in the symmetryeigenstate Hartreeexchange approximation, has already been applied to model systems and atoms, but now we implement DEC in the realspace Octopus code to be able to apply it to larger real systems. Octopus is wellsuited for this study because it can handle model systems as well as real systems, and both finite and periodic boundary conditions. We compare the results from DEC to other standard excitedstate approaches like Configuration Interaction Singles, and both linearresponse and timepropagation TimeDependent HartreeFock and TDDFT. We also analyze the computational cost and accuracy of DEC compared to these different approaches. 
Tuesday, March 15, 2022 9:48AM  10:00AM 
F46.00006: DFT Embedding in Python for Realisticallysized Systems Xuecheng Shao, Michele Pavanello With modern hardware advances, the exascale computing era has begun. Thus, for computational chemistry and physics, multiscale models of materials and biological systems are arguably the most important platforms for tackling the challenges ahead. Nowadays, density functional theory (DFT) is the method of choice. However, DFT's computational scaling hampers its applicability to realistically sized systems. In an effort to retain the predictivity of DFT while at the same time reducing the computational cost, we present eDFTpy, a density embedding Python code. In eDFTpy, the system is split into weakly interacting subsystems treated either at the KohnSham DFT level or at the orbitalfree DFT level. Intersubsystem interactions are evaluated with orbitalfree DFT. This leads to a linearscaling, low prefactor algorithm having essentially KSDFT accuracy that can be used for realisticallysized systems. The parallelization scheme of eDFTpy is stateoftheart: it has low memory cost and small communication time, thus strong parallel scaling to thousands of cores is achieved. 
Tuesday, March 15, 2022 10:00AM  10:36AM 
F46.00007: Magnons from realspace realtime timedependent density functional theory Invited Speaker: Nicolas TancogneDejean In the last couple of years, the first studies investigating magnetization dynamics from first principles in real time have emerged. We recently developed an efficient and nonperturbative scheme to compute magnetic excitations for extended systems employing the framework of realtime timedependent density functional theory, which is an alternative to the linearresponse TDDFT formulation. 
Tuesday, March 15, 2022 10:36AM  10:48AM 
F46.00008: Atomic fingerprinting of heteroatoms using noncontact atomic force microscopy Dingxin Fan, James R Chelikowsky Richard P. Feynman said the following during his talk at the APS annual meeting in 1959: “If you have a strange substance and you want to know what it is, you go through a long and complicated process of chemical analysis.… It would be very easy to make an analysis of any complicated chemical substance; all one would have to do would be to look at it and see where the atoms are. The only trouble is that the electron microscope is one hundred times too poor.” Feynman’s idea has been, partially, realized through the use of noncontact atomic force microscope (ncAFM). NcAFM with a functionalized CO tip made visualization of molecules and atoms possible. This atomic resolution dazzles the scientific world and opens new avenues of science. However, the measured ncAFM images are sometimes hard to interpret. To be specific, using the common CO tip can barely distinguish heteroatoms (such as N, S, etc.) from C atoms. 
Tuesday, March 15, 2022 10:48AM  11:00AM 
F46.00009: A first principles investigation of electronic charge distribution in random alloys Yang Wang, Mariia Karabin, Markus Eisenbach, George M Stocks, Xianglin Liu, Wasim R Mondal, Hanna Terletska, KaMing Tam, WaiGa D Ho, Vladimir Dobrosavljevic, Liviu Chioncel In this presentation, we investigate, from the first principles, the electronic charge distribution in random alloys by applying super cell calculations with a linear scaling real space ab initio method, namely the LSMS method. We revisit the socalled qV relation, a phenomenon observed by previous super cell calculations that the excess charge at each atomic site depends linearly on the long range electrostatic potential, or Madelung potential, at the site. We reveal the underlying mechanism that drives the establishment of the qV relation. We show that this relation can be used to improve the conventional KKRCPA method, or other CPA based ab initio kspace methods, in the selfconsistent field calculations. 
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