Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session K46: Excited State II: Method developmentquantum embedding and Xray SpectroscopyFocus Recordings Available

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Sponsoring Units: DCOMP DMP Chair: Sahar Sharifzadeh, Boston University Room: McCormick Place W470A 
Tuesday, March 15, 2022 3:00PM  3:36PM 
K46.00001: Absorption Spectra of Solids from Periodic EquationofMotion CoupledCluster Theory Invited Speaker: Xiao Wang As an important technique to study the interaction between light and materials, absorption spectroscopy has been widely used in the areas of photovoltaics, photocatalysis, lightemitting diodes, and more. The ability to simulate optical absorption spectra from first principles is of great importance both to complement experiments and to predict the properties of new materials. On the other hand, recent years have seen rapid development of wavefunctionbased quantum chemistry methods incorporating periodic boundary conditions, where accurate predictions have been made for periodic solids. Here, we present ab initio absorption spectra of threedimensional solids calculated using Gaussianbased periodic equationofmotion coupledcluster theory with single and double excitations (EOMCCSD). The spectra are calculated efficiently by solving a system of linear equations at each frequency, giving access to an energy range of tens of eV without explicit enumeration of excited states. We assess the impact of Brillouin zone sampling, for which it is hard to achieve convergence due to the cost of EOMCCSD. Our results show that the excitonic effects that dominate the lowenergy part of optical spectra of semiconductors and insulators are well described by EOMCCSD, and our most converged spectra exhibit lineshapes that are in good agreement with experiment. Detailed discussions on various costsaving approximations associated with incomplete basis sets, frozen orbitals, and partitioned EOM ansatz will be provided. 
Tuesday, March 15, 2022 3:36PM  3:48PM 
K46.00002: Ground and ExcitedState energies of copper oxide molecules and anions from first principles via the SpinFlip BetheSalpeter Equation approach Bradford A Barker, David A Strubbe This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, CTC and CPIMS Programs, under Award DESC0019053. 
Tuesday, March 15, 2022 3:48PM  4:00PM 
K46.00003: Efficient Treatment of Molecular Excitations in the Liquid phase using stochastic manybody theory Guorong Weng, Vojtech Vlcek I will present an efficient method to compute charge excitation in structurally inhomogeneous and disordered systems based on stochastic formulation of manybody perturbation theory. Our approach effectively separates the problem into separate subspaces corresponding to a molecule and the environment, and yields reconstructed molecular states. The electronic correlations are stochastically sampled in each subspace and can treat giant systems with thousands of electrons. I will exemplify the method on molecular quasiparticle energies in the liquid environment, representing systems of wide interest. Our method is first tested on three solutesolvent systems: phenol, thymine, and phenylalanine in water. Our computed results are in excellent agreements with photoemission experiments and the resulting environmental effects are on par with highlevel quantum chemistry calculations. I will comment on the interactions between molecule and the environment: the screening of the environment accounts for $\sim40\%$ of the correlation energy, while secondary solutesolvent feedback can also cause up to 0.6 eV destabilization of the quasiparticle energy. Our method is further applied to molecules in nonaqueous solvents and the calculated solvent effects are analyzed upon their electric polarizability. 
Tuesday, March 15, 2022 4:00PM  4:12PM 
K46.00004: An Exact Double Counting Scheme for Quantum Defect Embedding Theory Nan Sheng, Christian W Vorwerk, Marco Govoni, Giulia Galli We recently introduced a manybody embedding scheme, called quantum defect embedding theory (QDET), to describe strongly correlated defect states in solids [1,2,3]. In QDET, an effective Hamiltonian for the localized states of defects in solids is derived within manybody perturbation theory, and the effect of the environment is included within the constrained randomphase approximation. Here, we present an exact diagrammatic double counting correction scheme for QDET which allows for a systematic convergence of the electronic structure of defects as a function of the size of the active space. We demonstrate the wide applicability of our formalism by presenting results for molecules and spin defects in widebandgap semiconductors. 
Tuesday, March 15, 2022 4:12PM  4:24PM 
K46.00005: Benchmarking the Quantum Chemical Methods to Examine Ground and Excited States Electronic Structure of Diatomic Molecules Deepak K Rai, B. R. K. Nanda The predictive ability of various quantum chemistry based ab initio methods were examined for the ground and excited states electronic structure of highly functional diatomic molecules, formed by the elements of groups XV, XVI, and XVII of the periodic table. The bond length and dissociation energy of these diatomic molecules in their singlet and triplet states as well as their optical absorption spectra are obtained for benchmarking. Our study reveals that out of the methods examined, namely, HF, SCI, SDCI, MRSDCI, QCI, CCSD, and TDDFT, none of them cannot be universally employed to accurately predict the above properties of these molecules. However, through a comprehensive analysis we proposed the suitability each of these methods for individual properties of individual molecules. The present study reveals new optical absorption peaks within the ionization limit for N_{2}, S_{2}, and I_{2}. The affirmation objectives are significant because of the rapidly growing interest in the functionalization of molecules on the solid surface and 2d materials. 
Tuesday, March 15, 2022 4:24PM  4:36PM 
K46.00006: Model analysis of multiplet excitation of RE ions using QSGW Katsuhiro Suzuki, Hirofumi Sakakibara, Takao Kotani, Kazunori Sato The multiplet property of rareearth (RE) atoms is one of the most important phenomena. However, it is difficult to discuss the properties of multiplet states using density functional theory (DFT). The reason for this difficulty comes from underestimation of the electron correlations in LDA/GGA correlation functional which is usually used in DFT calculation. Generally, LDA+U is used in strongly correlated systems like the forbital system. However, this "U" is an empirical parameter and it is difficult to define uniquely. In order to solve this problem, we focus on quasiparticle selfconsistent GW (QSGW) approximation, and try to reproduce multiplet states of RE ions in the framework of DFT. We focus on free RE ions from Ce to Yb, and obtain electronic properties of them using QSGW. After that, we determine the parameters of the manybody Hamiltonian for the local forbitals of RE ions by comparing the mean field Hamiltonian of this one with QSGW Hamiltonian. Using these parameters, we obtain multiplet excited states and compare them with previous studies. Finally, we discuss the evolution of multiplet properties of trivalent Eu ions in materials using this framework. In this presentation, we discuss the trends in the multiplet states of each RE ion and the influence of the crystal field of Eu ions in materials. 
Tuesday, March 15, 2022 4:36PM  4:48PM 
K46.00007: Transition from Lorentz to Fano Spectral Line Shapes in NonRelativistic Quantum Electrodynamics Davis M Welakuh, Prineha Narang Spectroscopic investigations have played a key role in understanding the microscopic properties of matter, and its interaction with electromagnetic radiation. The associated spectroscopic signatures that are involved in such investigations are symmetric Lorentz and asymmetric Fano line shapes [1]. The latter has found several applications in photonics such as optical switching, sensing, lasing and nonlinear and slow light devices [2]. This led to investigation of Fano resonances in a large variety of different systems [3]. Thus, control over their generation from initially Lorentzian line shapes becomes necessary. To this end, we demonstrate through ab initio simulations of coupled lightmatter systems, a transition from an initially Lorentzian line shape into a Fano resonance when we couple the matter subsystem strongly to the electromagnetic continuum [4]. We show this for the case involving bare electronic as well as polaritonic excitations of a coupled lightmatter systems. In addition to the control over the generation of Fano resonances, we have access to the Purcell enhancement of spontaneous emission together with the observation of electromagnetically induced transparency which is a special case of Fano resonance. Our findings has potential applications for realizing tunable Fano resonances of different matter systems strongly coupled to the electromagnetic continuum as well as it presents an alternative way to realize a Purcell enhancement of the spontaneous emission process. 
Tuesday, March 15, 2022 4:48PM  5:00PM 
K46.00008: Realspace Green's function approach for the Langreth cumulant John Rehr, Joshua J Kas The effect of intrinsic excitations on the xray photoemission spectra (XPS) can be described by the corehole Green’s function G_{c}(t). As shown by Langreth, due to the linkedcluster theorem, G_{c}(t) can be expressed exactly in cumulant form, G_{c}(t) = G_{c}^{0}(t)exp[C(t)], where the independent particle corehole Green’s function is G_{c}^{0}=exp(iε_{c}t) and C(t) is the cumulant. In an interacting electron system, the cumulant C(t) is given to linear order by the densityresponse to a suddenly turnedon corehole. In previous work, a realtime timedependent density functional theory approach (RTTDDFT) has been developed for the cumulant. Here we develop a realspace Green's function approach for the Langreth cumulant C(t). Our formulation starts from the observation that in frequency space, the cumulant kernel β(ω) has the same form as the expression for the atomic polarizability in the TDDFT approach of Zangwill and Soven, but with the corehole potential V(r) replacing the dipole interaction. As a consequence, one can immediately derive a realspace Green's function approach, in analogy with the BSETDDFT formulation of Ankudinov et al. (TDDFTBSE). In particular, the screened dipole matrix elements of XAS are replaced by screened monopole transition elements in response to a suddenly turnedon corehole: M_{cL} = <cV_{sc}L>, where the dynamically screened corehole potential is V_{sc}(r) = ε^{1}(ω) V_{c}(ω), and the dielectric matrix is ε = [1K^{L}χ^{0}]. In contrast to XAS, V_{c}(r) and V_{sc}(r) are spherically symmetric, so the selection rules preserve angular momentum L=L_{c}. Thus, the calculations with the RSGF approach are similar to XAS, except for modified matrix elements and selection rules. Also in analogy with the XAS, one can define a fine structure in the cumulant kernel due to nearneighbor scattering. 
Tuesday, March 15, 2022 5:00PM  5:12PM 
K46.00009: Resonant inelastic xray scattering beyond the quasiparticle approximation Keith Gilmore, Joshua J Kas Resonant inelastic xray scattering (RIXS) provides unprecedented experimental access to the fundamental excitations and dynamical responses of correlated materials. Recently, firstprinciple methods, e.g. based on solving the BetheSalpeter equation (BSE), have succeeded in calculating the quasiparticle contribution to RIXS that reflects the underlying bandstructure. However, this neglects the important contribution of secondary excitations that are the signatures of the correlated response of the probed material. We significantly improve the BSE description of RIXS by invoking a quasiboson model to describe these secondary excitations in a manner akin to the classic theory of Mahan, Nozières & Dominicis [1,2]. We generate the quasiboson excitation spectrum (e.g. plasmons and secondary electronhole pairs) through a realtime timedependent density functional theory calculation of the charge density response to the initial corelevel excitation. Our new methodology succeeds at accurately reproducing the experimental RIXS spectrum at the Fe L3 edge of the benchmark correlated metal BaFe_{2}As_{2}, including both quasiparticle and correlated contributions [3]. 
Tuesday, March 15, 2022 5:12PM  5:24PM 
K46.00010: Allelectron BSE@GW method for Kedge Core Electron Excitation Energy Yi Yao, Dorothea Golze, Patrick Rinke, Volker Blum, Yosuke Kanai We present an accurate computational approach to calculate Kedge core electron excitation energies, achieved by combining allelectron GW and BetheSalpeter equation (BSE) methods. We assess the BSE@GW approach for calculating Kedge Xray absorption spectra using a set of small organic molecules and also a mediumsized sulfurcontaining molecule, which was used in a past benchmark of an equationofmotion coupled cluster (EOMCC) method by Peng and coworkers [Peng et al., J. Chem. Theory Comput., 11, 4146 (2015)]. We present the influence of different numerical approximations on the BSE@GW calculations, including the frequency integration scheme for GW, the TammDancoff approximation for BSE, and a relativistic correction scheme. We assess the basis set dependence and convergence with the Gaussian type of Dunning's basis sets and numerical atomiccentered basis sets. We identify the importance of corecorrelation basis functions as well as the augmenting basis functions. As a result, compared to the experimental values of the absolute core excitation energies, the predicted mean absolute error by BSE@GW is as low as 0.60.7 eV. 
Tuesday, March 15, 2022 5:24PM  5:36PM 
K46.00011: Combined Cumulant and Ligand Field Multiplet Theory approach to Xray spectra Joshua J Kas, John Rehr, Thomas P Devereaux Theoretical treatments of electronic correlations in open shell systems are notoriously difficult, especially in condensed matter systems. While ligand field multiplet theory (LFMT) has been extremely successful in treating the effects of these correlations in xray spectra, the theory is in many cases highly parameterized, and although recent progress has been made [1], the approach is not fully ab initio. In addition, multiplet based methods ignore itinerant states and associated excitations, e.g., plasmon excitations. The cumulant expansion for the oneelectron Green's function has been shown to treat these itinerant, quasibosonic excitations exceptionally well, producing near quantitative agreement with experimental results. Here we propose an approximation to combine the cumulant approach with LFMT to treat local and itinerant states in xray spectra. The approach is inspired by previous developments which have used DMFT to treat the effects of correlation [2]. The cumulant is calculated with a realtime TDDFT approach, which relates the quasibosoni excitation spectrum to the density induced by the sudden appearance of the corehole. We calculate the XPS of Fe_{2}O_{3} and find good agreement with experiment, both in the peaks related to multiplet splitting, in shakeup satellite peaks, and in the broad background produced by low energy itinerant excitations. The realtime approach allows an analysis of the shakeup excitations in terms of charge oscillations dominated by transfer between ligand states and the minority spin channel of the metal atoms. [1] M. W. Haverkort, M. Zwierzycki, and O. K. Andersen, Phys. Rev. B 85, 165113 (2012) [2] M. Casula, A. Rubtsov, and S. Biermann, Phys. Rev. B 85, 035115 (2012) 
Tuesday, March 15, 2022 5:36PM  5:48PM 
K46.00012: Finitetemperature selfenergy correction to XANES Tun Sheng Tan, Joshua J Kas, John Rehr Calculation of the complexvalued selfenergy is a bottleneck in the calculation of excited state properties such as xray absorption spectra (XAS). To circumvent this problem, Lu et al. [1] introduced a low order polynomial function to approximate the HedinLundqvist (HL) selfenergy [2, 3] at zero temperature. In this work, we take the same approach to obtain an approximation to the RPA GW quasiparticle selfenergy at finite temperature [4]. Finally, we present calculations of XAS for several materials. 
Tuesday, March 15, 2022 5:48PM  6:00PM 
K46.00013: Predicting Core Electron Binding Energies in 1st Row Transition Metal Elements Using the ΔSelfConsistentField Approach Juhan Matthias Kahk, Johannes C Lischner In the experimental technique Xray Photoelectron Spectroscopy (XPS), core electron binding energies are measured in order to acquire information about the chemical environments that are present in the nearsurface region of a sample. However, the analysis of measured spectra is challenging, and difficulties in assigning detected spectral features to specific structural motifs ("peak assignment") can limit the amount of useful information that XPS can provide. In response to these issues, theoretical methods for predicting core electron binding energies have been developed. Thus far, the vast majority of the computational work has focused on elements of the 2nd and 3rd periods of the periodic table. However, experimentally, core level spectra of all elements of the periodic table (except for H and He) are measured. 
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