Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F7: FirstPrinciples Modeling of ExcitedState Phenomena II: Computational AdvancesFocus

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Sponsoring Units: DCOMP DCP DMP Chair: William Huhn, Duke University Room: 266 
Tuesday, March 14, 2017 11:15AM  11:27AM 
F7.00001: Cubic scaling $GW$: towards fast quasiparticle calculations Peitao Liu, Merzuk Kaltak, Ji\v{r}\'{\i} Klime\v{s}, Georg Kresse Within the framework of the full potential projectoraugmented wave methodology, we present a promising lowscaling $GW$ implementation. It allows for quasiparticle calculations with a scaling that is cubic in the system size and linear in the number of $k$ points used to sample the Brillouin zone. This is achieved by calculating the polarizability and selfenergy in real space and imaginary time. The transformation from the imaginary time to the frequency domain is done by an efficient discrete Fourier transformation with only a few nonuniform grid points. Fast Fourier transformations are used to go from real space to reciprocal space and vice versa. The analytic continuation from the imaginary to the real frequency axis is performed by exploiting Thiele's reciprocal difference approach. Finally, the method is applied successfully to predict the quasiparticle energies and spectral functions of typical semiconductors (Si, GaAs, SiC, and ZnO), insulators (C, BN, MgO, and LiF), and metals (Cu and SrVO$_3$). The results are compared with conventional $GW$ calculations. Good agreement is achieved, highlighting the strength of present method. [Preview Abstract] 
Tuesday, March 14, 2017 11:27AM  11:39AM 
F7.00002: Speeding up GW Calculations for Large Scale Quasiparticle Predictions Weiwei Gao, Weiyi Xia, Xiang Gao, Peihong Zhang Although the GW approximation is recognized as one of the most accurate theories for predicting materials excited states properties, scaling up conventional GW calculations for large systems remains a major challenge. We present a powerful and simpletoimplement method that can drastically accelerate fully converged GW calculations for large systems, enabling fast and accurate quasiparticle calculations for complex materials systems. We demonstrate the performance of this new method by presenting the results for bulk and 2dimensional systems. A speedup factor of nearly two orders of magnitude is achieved for large systems. [Preview Abstract] 
Tuesday, March 14, 2017 11:39AM  11:51AM 
F7.00003: Efficient largescale GW calculations for 2D materials Weiyi Xia, Weiwei Gao, Yabei Wu, 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 developed a powerful method [1] that can drastically improve the speed of GW calculations for large systems. In this work, we apply this newly developed method to study the quasiparticle band structure of recently synthesized layered material C2N [2] which contains 18 atoms for the single layer system. We will discuss the convergency behavior of the calculated quasiparticle band structure with respect to the kpoint sampling density and the number of bands included in the calculations of the dielectric function and the Coulombhole selfenergy, aiming at shedding some light on accurate and efficient GW calculations for twodimensional materials. [1] W. Gao, W. Xia, X. Gao, and P. Zhang, in press, Scientific Reports (2016). [2] J. Mahmood et al, Nat Commun. 6, 6486 (2015). [Preview Abstract] 
Tuesday, March 14, 2017 11:51AM  12:27PM 
F7.00004: Manybody perturbation theory for excited electrons: from materials to molecules Invited Speaker: Fabien Bruneval The description of excited states is most easily understood in terms of Green's functions. The working approximations to obtain the Green's function have mostly been developed aiming at condensed matter systems. For instance, the $GW$ approximation [1] to the electron selfenergy has been shown to yield very accurate crystal band structures [2] and the BetheSalpeter equation is known to describe very well the excitons in solids [3]. However, until recently, very little was known about the performance of manybody perturbation theory for atoms, molecules, and clusters. Our inhouse code named MOLGW [4] addresses the efficient and accurate calculations of electronic excitations for finite systems. This code, based on standard quantum chemistry Gaussian basis sets, is conceptually simple, since it does not require any other convergence parameter besides the initial choice of the basis set. The code works efficiently in parallel and is opensource: it can be freely downloaded on the web [5]. With this unique tool, we have demonstrated the concavity error of the $GW$ approximation [6] and we have explored the accuracy of the quasiparticle energy calculations within the $GW$ approximation for organic molecules as compared to photoemission spectroscopy or to high level quantum chemistry references [7,8]. We have also measured the quality of the optical excitations obtained from the BetheSalpeter equation [9]. Recently, we have evaluated selfenergies that go beyond the standard $GW$ approximation, the socalled ``vertex corrections'', giving insight about how to deal with them in practice. [1] L. Hedin, Phys. Rev. 139, A796 (1965). [2] M.S. Hybertsen and S.G. Louie, Phys. Rev. B 34, 5390 (1986). [3] G. Onida, L. Reining, and A. Rubio, Rev. Mod. Phys. 74, 601 (2002). [4] F. Bruneval, T. Rangel, S.M. Hamed, M. Shao, C. Yang, and J.B. Neaton, Computer Phys. Comm. 208, 149 (2016). [5] http://www.molgw.org [6] F. Bruneval, J. Chem. Phys. 136, 194107 (2012). [7] F. Bruneval and M.A.L. Marques, J. Chem. Theory Comput. 9, 324 (2013). [8] T. Rangel, S.M. Hamed, F. Bruneval, and J.B. Neaton, J. Chem. Theory Comput. 12, 2834 (2016). [9] F. Bruneval, S.M. Hamed, and J.B. Neaton, J. Chem. Phys. 142, 244101 (2015). [Preview Abstract] 
Tuesday, March 14, 2017 12:27PM  12:39PM 
F7.00005: Verification and Validation of GW calculations for solids Ikutaro Hamada, Marco Govoni, Giulia Galli Many body perturbation theory based on the GW approximation is a well established approach to compute quasiparticle energies of solids. Yet, systematic convergence tests as a function of basis sets, kpoints and other numerical parameters entering the calculation are still lacking. We present a systematic convergence study of quasiparticle energies using a new release of the largescale GW code WEST[1,2] including accurate kpoint sampling of the Brillouin zone[3]. We also discuss comparisons with experiments.\\[4pt][1] M. Govoni and G. Galli, J. Chem. Theory Comput. 11, 2680 (2015); www.westcode.org\\[0pt][2] P. Scherpelz, M. Govoni, I. Hamada and G. Galli J. Chem. Theory Comput. 12, 3523 (2016).\\[0pt][3] I. Hamada, M. Govoni and G. Galli (to be published). [Preview Abstract] 
Tuesday, March 14, 2017 12:39PM  12:51PM 
F7.00006: Large Scale ManyBody Perturbation Theory calculations: methodological developments, data collections, validation Marco Govoni, Giulia Galli Green's function based manybody perturbation theory (MBPT) methods are well established approaches to compute quasiparticle energies and electronic lifetimes. However, their application to large systems  for instance to heterogeneous systems, nanostructured, disordered, and defective materials  has been hindered by high computational costs. We will discuss recent MBPT methodological developments leading to an efficient formulation of electronelectron and electronphonon interactions, and that can be applied to systems with thousands of electrons. Results using a formulation that does not require the explicit calculation of virtual states, nor the storage and inversion of large dielectric matrices will be presented. We will discuss data collections obtained using the WEST code [1], the advantages of the algorithms used in WEST over standard techniques, and the parallel performance. Work done in collaboration with I. Hamada, R. McAvoy, P. Scherpelz, and H. Zheng.\\ [4pt][1] M. Govoni, and G. Galli, "Large scale GW calculations", J. Chem. Theory Comput. 11, 2680 (2015); www.westcode.org. [Preview Abstract] 
Tuesday, March 14, 2017 12:51PM  1:03PM 
F7.00007: Singleparticle and twoparticle excited states with strong spinorbit coupling Bradford A. Barker, Steven G. Louie Many materials of interest have strong spinorbit coupling, which necessitates the use of spinor wavefunctions in the calculation of their electronic and optical properties. We have implemented such spinor functionality in the BerkeleyGW code package to calculate from first principles singleparticle excitations at the GW level and twoparticle excitations at the GWplusBethe SalpeterEquation (GWBSE) level. We present example calculations of benchmark materials with computational scaling details on the NERSC and TACC machines. [Preview Abstract] 
Tuesday, March 14, 2017 1:03PM  1:15PM 
F7.00008: Spinorbit coupling effects in excitedstate phenomena: ab initio planewavebased GW and GWBSE studies Meng Wu, Steven G. Louie The ab initio GW and GWBSE methods based on manybody perturbation theory play an important role in understanding and predicting the electronic and optical properties of materials. And spinorbit interaction introduces interesting spin physics and relativistic effects in materials such as IIIV semiconductors and transition metal dichalcogenides that contain heavy elements. With fullspinor support in a planewavebased GWBSE method, we study the effects of spinorbit coupling in the quasiparticle and excitonic properties of several materials of current interest, including reduced dimensional systems. [Preview Abstract] 
Tuesday, March 14, 2017 1:15PM  1:27PM 
F7.00009: Exploring various sources of electronhole screening in CH$_3$NH$_3$PbI$_3$ solar cell materials using the BetheSalpeter equation Joshua Leveillee, Andre Schleife 
Tuesday, March 14, 2017 1:27PM  1:39PM 
F7.00010: Abstract Withdrawn The inclusion of excitonic effects in semiconductors with the BetheSalpeter equation leads to good agreement of the optical spectra with experimental measurements. However, this approach requires in general very fine meshes of wavevectors in the Brillouin Zone in order to obtain wellconverged spectra, with very heavy computational load, preventing access to numerous derived quantities, as e.g. Raman intensities [1]. We present a new methodology that allows to decrease the work load to reach a given accuracy [2]. This technique is based on a trilinear interpolation technique within the Brillouin zone, combined with the Lanczos algorithm and doublegrid technique, to achieve efficient speedup and memory use. The technique is benchmarked in terms of accuracy on selected test cases. The scaling has also been studied from low to veryhigh density of points in the Brillouin zone, showing a much better scaling than a complete BetheSalpeter calculation. This approach might be used in the future for more complex calculations of optical properties. [1] Y. Gillet, M. Giantomassi, X. Gonze, Phys. Rev. B 88, 094305 (2013). [2] Y. Gillet, M. Giantomassi, X. Gonze, Comput. Phys. Commun. 203, 83 (2016). 
Tuesday, March 14, 2017 1:39PM  1:51PM 
F7.00011: GW/BetheSalpeter calculations for charged and model systems from realspace DFT David A. Strubbe GW and BetheSalpeter (GW/BSE) calculations use meanfield input from densityfunctional theory (DFT) calculations to compute excited states of a condensedmatter system. Many parts of a GW/BSE calculation are efficiently performed in a planewave basis, and extensive effort has gone into optimizing and parallelizing planewave GW/BSE codes for largescale computations. Most straightforwardly, planewave DFT can be used as a starting point, but realspace DFT is also an attractive starting point: it is systematically convergeable like plane waves, can take advantage of efficient domain parallelization for large systems, and is well suited physically for finite and especially charged systems. The flexibility of a realspace grid also allows convenient calculations on nonatomic model systems. I will discuss the interfacing of a realspace (TD)DFT code (Octopus, www.tddft.org/programs/octopus) with a planewave GW/BSE code (BerkeleyGW, www.berkeleygw.org), consider performance issues and accuracy, and present some applications to simple and paradigmatic systems that illuminate fundamental properties of these approximations in manybody perturbation theory. [Preview Abstract] 
Tuesday, March 14, 2017 1:51PM  2:03PM 
F7.00012: Study of local currents in 2D materials and junctions using a point source and TDDFT Shenglai He, Kalman Varga The performance of nanoscale electronic device depends both on the property of junctions and conducting channels. An investigation of local electron pathway can help us better understand the relation between structure and transport property [1]. In this research, a combination of source potential and TimeDependent Density Functional Theory is used to study local electron flow in heterogeneous material junctions and twodimensional materials such as graphnene and transition metal dichalcogenides. By injecting a conducting state at a single point and propagating the system in time, the wavefunction of the system in this specific state can be obtained. The local current can then be calculated from the wavefunction.$\backslash $pard$\backslash $pard[1] Shenglai He, Arthur Russakoff, Yonghui Li, and Kalman Varga, . Appl. Phys. 120, 034304 (2016) [Preview Abstract] 
Tuesday, March 14, 2017 2:03PM  2:15PM 
F7.00013: Speedup of GW FullFrequency Calculations using the Static Dielectric Matrix Subspace Approximation Mauro Del Ben, Felipe H. da Jornada, Jack Deslippe, Steven G. Louie, Andrew Canning Over the last several decades the GW method has been established as a quantitatively accurate approach for predicting the quasiparticle and excitedstate properties of materials. However, the successful application of the method to large systems with thousands to tensofthousands of atoms is a challenge due to the computational complexity associated with the evaluation of the dielectric matrix $\epsilon$ and its frequency dependence. We describe the implementation in traditional GW calculations based on the expression of the frequency dependent part of $\epsilon$ on the subspace generated by selected eigenvectors of the static dielectric matrix. We validate the method with several benchmark calculations on molecules, slabs and bulk systems. We show that the overall accuracy of the method is solely determined by the threshold on the eigenvalues of the static $\epsilon$ and that excellent time to solution and speedups of an order of magnitude can be achieved without significant loss of accuracy. [Preview Abstract] 
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