Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J39: First-principles modeling of excited-state phenomena in materials VI: GW+BSE Theory DevelopmentFocus
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Sponsoring Units: DCOMP DMP DCP Chair: Myrta Grüning, Queens Univ Belfast Room: 703 |
Tuesday, March 3, 2020 2:30PM - 3:06PM |
J39.00001: Ground and excited states of open-shell system from a spin-flip Bethe-Salpeter approach Invited Speaker: David Strubbe Open-shell systems, including molecules and defects, are interesting platforms for spin physics and quantum information. Their multi-determinantal states are difficult to handle with conventional first-principles calculations. A solution from quantum chemistry is the spin-flip (SF) approach: the ground and excited states are considered as spin-flipping excitations of a single-determinant high-spin reference state. The SF approach was originally used with wavefunction-based approaches such as configuration interaction, then later extended to time-dependent density-functional theory (TDDFT), showing some successes. Standard TDDFT approximations, however, have well-known deficiencies including treatment of charge-transfer excitations and condensed phases. Therefore, we introduce a spin-flip approach to the GW/Bethe-Salpeter approach (GW/BSE), taking advantage of a similar structure of the equations to TDDFT, but with an ab initio long-ranged and non-local interaction kernel, which enables more accurate calculations of excited states. I will discuss the theory of the spin-flip BSE method and our implementation with the Octopus and BerkeleyGW codes, and show our investigation of the critical issues of spin contamination and convergence with number of states in BSE. We have demonstrated success of SF-BSE on some simple problems. In the torsion of the ethylene molecule (C2H4), which transitions between singlet and triplet ground states, we found excellent agreement with higher-level and more expensive calculation methods, and a consistency at the equilibrium geometry with standard BSE calculations. We find good results also for the Si atom’s singlet-triplet splitting, which has been problematic for TDDFT. Given these successes on small systems, I will discuss prospects for application of our approach to problems in condensed phases. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J39.00002: All-electron Implementation of Bethe-Salpeter Equation Method: Development and Application Yi Yao, Dorothea Golze, Chi Liu, Patrick Rinke, Volker Blum, Yosuke Kanai A new all-electron implementation of the Bethe-Salpeter Equation (BSE) method for optical excitations is presented. We benchmarked the accuracy of our implementation1 on low lying optical excitations of organic molecules in the Thiel benchmark set2 for validation, and the basis-set dependence is carefully analyzed. We then integrated the core-GW approach by Golze et al.3 into our BSE implementation for X-ray absorption spectroscopy (XAS). We compare our results for K-edge XAS spectra of small organic molecules to earlier EOM-CCSD work by Peng et al4. We found the accuracy of our BSE approach is comparable to or better than EOM-CCSD, which gives the errors within 0.5 eV of experimental value for the test set. The implementation for extended periodic systems is discussed lastly with examples of liquid water and crystalline silicon. |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J39.00003: Efficient Construction of 4-point Green’s Function in Real-Space Representation using Permutation Sampling Monte Carlo method Nicole Spanedda, Peter McLaughlin, Arindam Chakraborty A principle challenge in constructing the 1-particle Green’s function is the steep scaling of computational cost with increasing system size. We address this problem by transforming the Green’s function into real-space representation and calculating all required integrals using the Permutation Sampling Monte Carlo method (PSMC). We started with the frequency-domain representation of the self-energy and performed Laplace transformation to obtain a 4-point representation of the self-energy in position basis. The main benefit of PSMC is that it avoids the steep scaling associated with the traditional methods of constructing the self-energy. Specifically, we avoided AO-to-MO integral transformation and explicit representation of the self-energy in particle-hole basis. In this work we demonstrated the linear scaling of computational cost with the number of molecular-orbital basis functions that was achieved by the PSMC method. Consequently, PSMC can be applied to systems that are computationally impractical using conventional methods. We used PSMC to calculate the ionization potentials of PbS quantum dots (Pb4S4-P140S140). |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J39.00004: Equation of motion coupled-cluster approach for multi-electron excitations in x-ray spectra John Rehr, Fernando Vila, Joshua Kas, Karol Kowalski, Bo Peng We present a real-time equation of motion coupled cluster approach [1] for calculations of multi-electron excitations in core-level x-ray absorption and emission spectra. Integration of the equations to leading order in particle-hole excitations yields a cumulant representation of the core-hole spectral function. Absorption and emission spectra are then calculated in terms of a convolution of an effective one-body x-ray spectrum and the core-hole spectral function. Comparisons with determinantal approaches and with the Delta-SCF approximation are also discussed. Illustrative calculations are discussed separately [2]. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J39.00005: Real-time EOM-CCS Green's function method for the core spectral functions Fernando Vila, John Rehr, Bo Peng, Karol Kowalski X-ray photoemission spectra (XPS) typically exhibit satellite peaks associated with many-body excitations that have proved difficult to simulate from first principles. We address this problem using a real-time equation-of-motion coupled-cluster-singles (RT-EOM-CCS) approach for the core spectral function. RT methods provide a versatile approach to electronic response, but have not been widely applied using CC theory. In RT-EOM-CCS, the Green's function (GF) for an N-electron system is computed from the overlap between the initial core-excited |N-1> wavefunction and its time-propagated form |N-1,t>. The latter is approximated using a CC ansatz with time-dependent amplitudes. The EOM of the overlap is solved using a centered predictor-corrector approach. We show that the form of the overlap is analogous to that in the static CC equations, and that the form of the GF is related to that in the cumulant approach.1 Finally, we present results for low N systems. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J39.00006: Neutral Excitation Energies of Crystalline Solids from Periodic Equation-of-Motion Coupled-Cluster Theory Xiao Wang, Timothy Berkelbach There have been increasing interests in the development of high-accuracy, wavefunction-based quantum chemistry methods, such as coupled-cluster theory, with periodic boundary conditions for electronic structure problems of crystalline solids. We present an ab initio study on electronically excited states of solids using Gaussian-based periodic equation-of-motion coupled-cluster theory with single and double excitations (EE-EOM-CCSD). EE-EOM-CCSD provides a quantitative treatment of singly excited states, such as excitons and plasmons, and a qualitative treatment of doubly excited states, such as biexcitons. Results of optical band gaps, exciton binding energies, and exciton dispersions will be presented for a variety of inorganic insulators and semiconductors. |
Tuesday, March 3, 2020 4:06PM - 4:42PM |
J39.00007: Fingerprints of dynamical correlation in electron spectroscopies Invited Speaker: Matteo Gatti One of the great challenges of condensed-matter physics is the description, understanding, and prediction of the effects of the Coulomb interaction on materials properties. In electronic spectra, the Coulomb interaction causes a renormalization of excitation energies and a transfer of spectral weight. Most importantly, it can lead to qualitatively new structures, such as satellites in photoemission or double-plasmon resonances in energy-loss spectra. Being a genuine signature of dynamical correlation, they are absent in a non-interacting picture but can be understood in terms of the coupling between different elementary excitations. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J39.00008: Biexcitons and exciton dynamics in low-dimensional systems from an ab initio interacting Green’s function formalism Felipe Da Jornada, Andrea Cepellotti, Steven Louie The synthesis of quasi low-dimensional materials, such as the monolayer transition metal dichalcogenides (TMDs), opened the door to studying new classes of systems with nanoscale dimensionality confinement and weak electronic screening, leading to strongly enhanced electron interactions. Many of these systems host a variety of charged and neutral multiparticle excitations – such as excitons, trions, and biexcitons. We present here a first-principles formalism based on the interacting Green’s function to compute and understand these excitations and their dynamics. We apply our formalism and its associated code on high performing computers to the monolayer TMDs, predicting a diversity of multiparticle excitations with large binding energies (~20 meV) and complex valley and spin textures. We also show how this formalism can be employed to investigate other exciton-exciton interactions to understand challenging phenomena involving dynamics from first principles. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J39.00009: Electron-hole attraction effect under time-dependent electric field within a generalized Landau-Zener model Yasushi Shinohara The exchange term is a crucial term for first-principles electronic structure theories, leading to accurate prediction within hybrid-functional for density-functional theory and the most simple many-body effect as a term in the Hartree-Fock (HF) equation. This term is also crucial for the description of the electronically excited state, namely excitonic influence on dielectric function either due to bound- or unbound-excitons. In spite of its importance in photoabsorption spectra, the effect of the (screened) exchange term for nonlinear phenomena has not been investigated well because of severe calculation cost. |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J39.00010: First principle excited state calculations using a frequency–dependent geminal–screened electron-hole interaction kernel Peter McLaughlin, Arindam Chakraborty A primary computational limitation for excited state methods is the inclusion of virtual or unoccupied states in the calculations. The inclusion of these states increases the calculation cost for excited state calculations. We present the frequency–dependent geminal–screened electron–hole interaction kernel (FD–GSIK) method for describing electron–hole correlation in electronically excited many–electron systems. FD–GSIK avoids using unoccupied orbitals for kernel construction by performing infinite–order summation of particle–hole excitation and representing it as a compact real–space operator. The central idea of our approach is to use Löwdin partitioning technique to construct a frequency–dependent and r12–explicitly correlated operator for treating electron–hole correlation for the excited state wave function that is derived from first principles and is parameter–free. Evaluation of all integrals were performed in real space using stratified Monte Carlo, which avoided the steep computational cost of evaluation, storage and transformation of the atomic orbitals to molecular orbitals. The FD–GSIK was applied to large nanoparticles including Pb140S140, Pb140Se140, and Cd144Se144, to obtain excitation and electron–hole binding energies. |
Tuesday, March 3, 2020 5:18PM - 5:30PM |
J39.00011: Atomic relativistic approximation for ab initio prediction of excited-state potential energy surfaces and X-ray spectra Tianyuan Zhang, Joseph M Kasper, Xiaosong Li The atomic relativistic approach is a formally simple and linear scaling ansatz that exploits the locality of the relativistic effect. Studies have shown that this approximation introduces moderate error on ground state absolute energy in the presence of short bonds between heavy nuclei. However, scientists have not investigated its accuracy on computing excited states for absorption spectra and excited-state potential energy surfaces. In this work, we demonstrate atomic exact two-component (X2C) predictions of the L2,3-edge X-ray absorption spectra of five representative heavy-atom-centered molecules and the excited-state potential energy curves of the platinum dimer (Pt2). Surprisingly, not only the errors to full X2C results in core excitation energies are negligible on the order of 0.01 eV, but the oscillator strengths also agree well with those from full X2C computations. Meanwhile, the atomic X2C potential energy curves and crossings of Pt2 are almost indistinguishable from the full X2C ones. |
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