Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A29: Firstprinciples Modeling of ExcitedState Phenomena in Materials I: Method DevelopmentFocus

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Sponsoring Units: DCOMP DMP DCP DCMP Chair: Adrienn Ruzsinszky, Temple University Room: LACC 406A 
Monday, March 5, 2018 8:00AM  8:36AM 
A29.00001: Advancing accurate and scalable electronic structure formalisms for lightharvesting materials Invited Speaker: Volker Blum Lightharvesting applications (photovoltaics, photocatalysis, or photoelectrocatalysis) rely on physical processes that are directly amenable to firstprinciples simulations, facilitating detailed understanding and predictions of new materials at the very scale that matters in experiment. Major challenges for simulations include system size and complexity, accuracy of the underlying numerical descriptions, and accuracy of the physical description of ground and excited state phenomena. This talk outlines our current reach based on a highaccuracy, scalable framework of numeric atomcentered basis functions (the FHIaims code) and a general, opensource software infrastructure ELSI (http://elsiinterchange.org) that connects seamlessly to electronic structure solvers for different scales and problem classes. Specific application areas addressed in this talk include: (i) New multinary chalcogenide semiconductors Cu_{2}BaSn(S,Se)_{4} and, more generally, I_{2}IIIVVI_{4} (I=Cu,Ag; II=Ba,Sr; IV=Ge,Sn; VI=S,Se) for photovoltaics, designed to overcome limits of the Cu_{2}ZnSn(S,Se)_{4} kesterites; (ii) Predicting the structural, electronic and optical properties of new crystalline layered organicinorganic perovskites with large, electronically active organic functionalities such as oligothiophenes, allowing one to tune the detailed carrier properties by varying both the organic and the inorganic components. We particularly highlight the power of largescale hybrid densityfunctional calculations (here covering crystalline materials with over 400 atoms per unit cell) for predictions of carrier properties with excellent qualitative accuracy, providing a reliable means to identify promising new materials for future experimental syntheses. 
Monday, March 5, 2018 8:36AM  8:48AM 
A29.00002: Improved Method for Efficient Largescale GW Calculations for 2D Systems Weiyi Xia, Yabei Wu, Weiwei Gao, Peihong Zhang Accurate and efficient predictions of excitedstates properties of complex materials remain a major challenge due to complication of the convergence issue and the unfavorable scaling of the computational cost with respect to the system sizes. GW calculations for 2D materials pose additional challenges due to the analytical behavior of the 2D dielectric function. Recently we have come up with an improved scheme that can significantly reduce the kpoint density required in GW calculations for 2D materials. This method, when combined with our recently developed method [1] that greatly alleviates the burden of including a large amount of empty states in conventional GW calculations, can drastically reduce the computational costs for GW calculations for complex 2D materials. We have applied our new method to calculate the GW quasiparticle band structures of several 2D materials, including recently synthesized C_{2}N and C_{3}N. We will discuss the convergency behavior of the calculated quasiparticle band structure with respect to various sampling and cutoff parameters and compare our results with previous work, aiming at shedding some light on accurate and efficient GW calculations for twodimensional materials. [1] W. Gao, W. Xia, X. Gao, and P. Zhang, Scientific Reports 6, 36849 (2016). 
Monday, March 5, 2018 8:48AM  9:00AM 
A29.00003: Ab initio Green'sFunction Approach for 3 and 4Particle Correlated Excitations: Trions and Biexcitons Felipe da Jornada, Andrea Cepellotti, Steven Louie The ab initio GW plus BetheSalpeter equation (GWBSE) approach is a quantitatively accurate formalism to describe neutral excitations in systems of various electronic structure and dimensionality. However, higherorder excitations involving more than two quasiparticles are not readily obtained from this formalism. These excitations include trions and biexcitons. The latter can be exploited, for instance, to design solarcell devices that go beyond the ShockleyQueisser limit. In this work, we describe a new approach based on the interacting Green’s function formalism to compute these excitations from first principles. We describe how we cast the problem of finding trion and biexciton excitations rigorously into a Dysonlike equation. We also present firstprinciples calculations of trions and biexcitons on singlewall carbon nanotubes. 
Monday, March 5, 2018 9:00AM  9:12AM 
A29.00004: A Finite Field Algorithm for GW Calculations Beyond the Random Phase Approximation He Ma, Marco Govoni, Giulia Galli, Francois Gygi The GW approximation is widely used to compute electronic structure of molecules and materials. Most GW calculations are performed within the random phase approximation (RPA). In this talk I will present our recent development of a finite field algorithm to evaluate density response functions beyond the RPA. The algorithm does not require explicit evaluation of empty electronic states and leads to highly scalable calculation on high performance architectures. The algorithm was implemented by coupling the Qbox [qboxcode.org] and WEST [westcode.org] codes. I will present results for molecules, solids and solidsolid interfaces and discuss vertex corrections in GW calculations. 
Monday, March 5, 2018 9:12AM  9:24AM 
A29.00005: Imaginary time, shredded propagator method for largescale GW calculations Minjung Kim, Subhasish Mandal, Eric Mikida, Kavitha Chandrasekar, Eric Bohm, Nikhil Jain, Qi Li, Laxmikant Kale, Glenn Martyna, Sohrab IsmailBeigi The GW method is one of the most accurate ab initio methods for the prediction of electronic band structures. Despite its power, the GW method is not routinely applied to large scale materials physics or chemistry problems due to its unfavorable computational scaling: standard implementations scale as O(N^{4}) where N is the number of electrons in the system. To develop largescale GW software, we have implemented algorithms that work in real space for the canonical planewave pseudopotential approach to electronic structure calculations. One benefit is that the realspace polarizability matrix method requires substantially fewer fast Fourier transforms compared to the standard reciprocal space methods. In addition, use of realspace allows us to create a cubic scaling algorithm which utilizes Laplace transform over imaginary time with GaussLaguerre quadrature. The use of energy windows maximizes the efficiency of the quadrature integration. For the GW selfenergy, we are also able to use energy windows and quadrature to achieve cubic scaling. In this presentation, we will describe these methods, their accuracies and their efficiencies compared to other available GW methods. 
Monday, March 5, 2018 9:24AM  9:36AM 
A29.00006: Realspace Methods for Calculating Excitedstates Properties of Nanoscale Systems: The NanoGW Package Weiwei Gao, James Chelikowsky We will illustrate some recent developments and applications of the NanoGW code, a parallelized code designed for calculating excitedstate properties of nanoscale systems with the GW approximation and BetheSalpeter equation (BSE). We discuss the advantages of using the realspace wave functions and working in the space of singleparticle transitions for GW/BSE calculations of nanoscale systems. In addition, we will present performance measure and verification of NanoGW by benchmarking it with the GW100 set of molecules. 
Monday, March 5, 2018 9:36AM  9:48AM 
A29.00007: Large Scale BetheSalpeter Equation Calculations Ngoc Linh Nguyen, He Ma, Marco Govoni, Francois Gygi, Giulia Galli We present an efficient method to solve the BetheSalpeter equation (BSE) to compute electronhole excitations and optical spectra of systems with hundreds of electrons, including molecules and solids. The method does not require the explicit evaluation of virtual electronic states, nor the inversion of dielectric matrices; its computational workload scales as the third power of the number of electrons in the system. We computed density response functions in finite field, and used localized KohnSham orbitals, as obtained by the bisection method [1]. The approach is implemented by coupling the Qbox [http://qboxcode.org] and WEST [http://www.westcode.org] codes. Results for the excitonic binding energies and optical absorption spectra of the Thiel molecular set, and for condensed systems of unprecedented size, including water and ice samples with hundreds of electrons, will be discussed. Ref. [1] François Gygi, PRL 102, 166406 (2009). 
Monday, March 5, 2018 9:48AM  10:00AM 
A29.00008: Visualizing electronic excitations with the particlehole map: orbital localization and metric space analysis Edward Pluhar, Carsten Ullrich The particlehole map (PHM) is a computational tool to visualize electronic excitations (calculated using timedependent densityfunctional theory), based on representations in canonic molecular orbital transition space. Beyond the standard canonical representation, transformation to localized orbitals is a common technique in electronic structure theory. We analyze the PHM as a visualization tool for both canonical and localized molecular orbital representations in various inorganic and organic chargetransfer complexes. We show that the canonical and localized PHMs can be used to construct simple descriptors of the chargetransfer character of an excitation through metric space analysis. 
Monday, March 5, 2018 10:00AM  10:12AM 
A29.00009: Stochastic methods for manybody perturbation theory Daniel Neuhauser, Vojtech Vlcek, Eran Rabani, Roi baer We present the stochastic approach to formulation of the manybody perturbation theory and to calculations of the quasiparticle energies in the G_{0}W_{0} approximation . General principles of the stochastic methods in TDDFT, GW and BSE will be discussed. We demonstrate how our approach overcomes the deficiencies of the conventional schemes and that with increasing system size it becomes effectively less expensive leading to approximately linear scaling of the algorithm. This is a fundamental nature of our method, which does not rely on particular character of the electronic structure (e.g. sparsity of the density matrices). In addition we demonstrate that the scaling prefactor is small and the stochastic method is thus a method of choice for all systems from few tenths to thousands of electrons. 
Monday, March 5, 2018 10:12AM  10:24AM 
A29.00010: Quasiparticle spectra from molecules to bulk with stochastic manybody methods Vojtech Vlcek, Eran Rabani, Daniel Neuhauser I will present new developments in the stochastic approaches to electronic structure and manybody perturbation theory, which overcome steep scaling of conventional deterministic schemes. I will exemplify on calculations for systems with more than 5000 valence electrons and show evolution of photoemission spectra, quasiparticle energies, lifetimes and emergence of collective excitations (plasmons and shakeup) for systems ranging from molecules to bulklike nanoparticles. The results demonstrate how the surface plasmon resonances affect the frequency of the collective excitation and position of the satellites in the spectra for small and low dimensional systems. 
Monday, March 5, 2018 10:24AM  10:36AM 
A29.00011: Densityfunctional perturbation theory for excited states from constrained DFT David Strubbe Constrained DFT (cDFT) is a crude but computationally cheap approach for modeling excited states, in which nonequilibrium occupations are assigned to the KohnSham states. Despite the availability of more sophisticated approaches, cDFT remains useful in contexts such as chargetransfer excitations, thermalized excited electron populations in solids, and TDDFT for excitedstate absorption. Within ordinary DFT, vibrational and dielectric linearresponse calculations have become routine with densityfunctional perturbation theory (DFPT), which needs only the occupied states. This technique works at zero temperature or with an effective temperature described through a smearing function, and can be used in both static and TDDFT calculations. However, the formalism has not been available for calculation of states with arbitrary occupations, as can arise in the cDFT description of excited states. I derive a simple modification to extend DFPT to arbitrary fractional occupations and show example applications for systems with nonequilibrium occupations. 
Monday, March 5, 2018 10:36AM  10:48AM 
A29.00012: Efficient Iterative TDDFT: Applications to QMD Simulations of Optical Spectra Peter Koval, Marc Barbry, Daniel SanchezPortal Here, we present an efficient algorithm to compute the absorption spectra within linear response timedependent density functional theory (TDDFT). The algorithm utilizes the locality of underlying basis functions and Krylovsubspace techniques; it is useful with any compact set of localized basis functions such as atomic orbitals, Wannier functions or a general tightbinding representation. The current realization, implemented in our opensource code pySCF_nao (https://github.com/cfmmpc/pyscf/tree/nao), employs numerical atomic orbitals and assumes the use of pseudopotentials to describe inner electrons. Thus, it is compatible with popular DFT codes like SIESTA, OpenMX or Fireball. The efficiency of our algorithm allows combining it with molecular dynamics (MD) simulations, where spectra must be averaged over hundreds of configurations to obtain relevant information, e.g., including the effects of temperature on optical properties. In spite of the widespread use of DFTbased MD simulations, its combination with TDDFT calculations has remained scarce to date due to the computational cost of such calculations. However, here we present some examples of such combination thanks to the efficiency of our iterative TDDFT method. 
Monday, March 5, 2018 10:48AM  11:00AM 
A29.00013: Approximate spectral decomposition of densitydensity response functions Han Yang, Marco Govoni, Giulia Galli Recently, an implementation of the G_{0}W_{0} method that does not require any explicit summation over empty electronic states has been proposed. The implementation uses a spectral decomposition of densitydensity response functions [1]. To accelerate many body perturbation theory calculations, we propose a method to obtain approximate spectral decompositions of densitydensity response functions, which do not compromise the accuracy of quasiparticle energies obtained in G_{0}W_{0} calculations. The performance of this approximation for molecules, solids and heterogeneous interfaces will be discussed. 
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