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: First-Principles Modeling of Excited-State 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 projector-augmented wave methodology, we present a promising low-scaling $GW$ 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 self-energy 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 simple-to-implement 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 2-dimensional systems. A speed-up 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 large-scale GW calculations for 2D materials Weiyi Xia, Weiwei Gao, Yabei Wu, Peihong Zhang Accurate and efficient predictions of excited-states 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 k-point sampling density and the number of bands included in the calculations of the dielectric function and the Coulomb-hole self-energy, aiming at shedding some light on accurate and efficient GW calculations for two-dimensional 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: Many-body 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 self-energy has been shown to yield very accurate crystal band structures [2] and the Bethe-Salpeter equation is known to describe very well the excitons in solids [3]. However, until recently, very little was known about the performance of many-body perturbation theory for atoms, molecules, and clusters. Our in-house 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 open-source: 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 Bethe-Salpeter equation [9]. Recently, we have evaluated self-energies that go beyond the standard $GW$ approximation, the so-called ``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, k-points 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 large-scale GW code WEST[1,2] including accurate k-point 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.west-code.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 Many-Body Perturbation Theory calculations: methodological developments, data collections, validation Marco Govoni, Giulia Galli Green's function based many-body 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 electron-electron and electron-phonon 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.west-code.org. [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F7.00007: Single-particle and two-particle excited states with strong spin-orbit coupling Bradford A. Barker, Steven G. Louie Many materials of interest have strong spin-orbit 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 single-particle excitations at the GW level and two-particle excitations at the GW-plus-Bethe Salpeter-Equation (GW-BSE) 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: Spin-orbit coupling effects in excited-state phenomena: ab initio plane-wave-based GW and GW-BSE studies Meng Wu, Steven G. Louie The ab initio GW and GW-BSE methods based on many-body perturbation theory play an important role in understanding and predicting the electronic and optical properties of materials. And spin-orbit interaction introduces interesting spin physics and relativistic effects in materials such as III-V semiconductors and transition metal dichalcogenides that contain heavy elements. With full-spinor support in a plane-wave-based GW-BSE method, we study the effects of spin-orbit 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 electron-hole screening in CH$_3$NH$_3$PbI$_3$ solar cell materials using the Bethe-Salpeter equation Joshua Leveillee, Andre Schleife Hybrid organic-inorganic perovskite materials have emerged as promising next generation thin-film solar cells. While many working devices have been developed, the weak electron-hole interaction and low exciton binding energy have not been fully explained. Methods beyond ground-state calculations are required to fully understand the excited state properties of these materials. In this work, the excitonic spectrum of CH$_3$NH$_3$PbI$_3$ is simulated using the Bethe-Salpeter Equation. We describe contributions to dielectric screening, such as electronic, free-carrier, lattice, and polaron, through the use of model dielectric functions for the electron-hole Coulomb interaction. The calculated optical properties are directly compared, qualitatively and quantitatively, to experimental results. We find that the contributions of lattice and free carrier screening are highly influential on the optical spectrum. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F7.00010: Abstract Withdrawn
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Tuesday, March 14, 2017 1:39PM - 1:51PM |
F7.00011: GW/Bethe-Salpeter calculations for charged and model systems from real-space DFT David A. Strubbe GW and Bethe-Salpeter (GW/BSE) calculations use mean-field input from density-functional theory (DFT) calculations to compute excited states of a condensed-matter system. Many parts of a GW/BSE calculation are efficiently performed in a plane-wave basis, and extensive effort has gone into optimizing and parallelizing plane-wave GW/BSE codes for large-scale computations. Most straightforwardly, plane-wave DFT can be used as a starting point, but real-space 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 real-space grid also allows convenient calculations on non-atomic model systems. I will discuss the interfacing of a real-space (TD)DFT code (Octopus, www.tddft.org/programs/octopus) with a plane-wave 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 many-body 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 Time-Dependent Density Functional Theory is used to study local electron flow in heterogeneous material junctions and two-dimensional 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: Speed-up of GW Full-Frequency 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 excited-state properties of materials. However, the successful application of the method to large systems with thousands to tens-of-thousands 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 speed-ups of an order of magnitude can be achieved without significant loss of accuracy. [Preview Abstract] |
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