New Implementation of an Equation-of-Motion Coupled-Cluster Damped-Response Framework with Illustrative Applications to Resonant Inelastic X-ray Scattering

Abstract

We present an implementation of a damped response framework for calculating resonant inelastic X-ray scattering (RIXS) at the equation-of-motion coupled-cluster singles and doubles (CCSD) and second-order approximate coupled-cluster singles and doubles (CC2) levels of theory in the open-source program e T . This framework lays the foundation for future extension to higher excitation methods (notably, the coupled-cluster singles and doubles with perturbative triples, CC3) and to multilevel approaches. Our implementation adopts a fully relaxed ground state and different variants of the core−valence separation projection technique to address convergence issues. Illustrative results are compared with those obtained within the frozen-core core−valence separated approach, available in Q-Chem, as well as with experiment. The performance of the CC2 method is evaluated in comparison with that of CCSD. It is found that, while the CC2 method is noticeably inferior to CCSD for X-ray absorption spectra, the quality of the CC2 RIXS spectra is often comparable to that of the CCSD level of theory, when the same valence excited states are probed. Finally, we present preliminary RIXS results for a solvated molecule in aqueous solution.

Core-Excited States and X-ray Absorption Spectra from Multireference Algebraic Diagrammatic Construction Theory

“…Here, we employ the core-valence separation (CVS) approximation, , which allows us to efficiently compute core-excited states by neglecting their interaction with excitations from the remaining occupied orbitals. CVS has been widely used for simulating core-level excited states and X-ray absorption spectra in combination with a variety of electronic structure theories. ,,,,,,,,− …”

“…Ab initio electronic structure methods − play a crucial role in the analysis of experimental X-ray spectra that are often complex and can be difficult to interpret. Among the widely used theoretical methods for simulating XAS are time-dependent density functional theory (TD-DFT), − equation-of-motion coupled cluster theory, − configuration interaction, − algebraic diagrammatic construction (ADC), − and related approaches. − A common approximation in all of these methods is that the ground-state electronic structure is well described using a single Slater determinant wavefunction (i.e., single-reference).…”

MQM 2022: The 10th Triennial Conference on Molecular Quantum Mechanics

“…Schnack-Petersen and co-workers in the Coriani group at the Technical University of Denmark report a new implementation of the EOM-CC method for resonant inelastic X-ray scattering (RIXS) . They find that the second-order approximate CC singles and doubles (CC2) method produces RIXS spectra that are comparable in quality to the more expensive full CC singles and doubles (CCSD) approach.…”

Efficient Protocol for Computing MCD Spectra in a Broad Frequency Range Combining Resonant and Damped CC2 Quadratic Response Theory

“…This approach is limited by the convergence of the excited state eigenvectors and their derivatives, which becomes increasingly difficult in regions with a high density of states. Damped response theory and the complex polarization propagator framework offer an alternative approach where the spectrum is computed directly as an interpolation of cross-section grid points in any frequency region. − Implementations of this type of approach have been presented for various electronic-structure methods for the computation of linear spectroscopies such as one-photon absorption and electronic circular dichroism (ECD), − of C 6 dispersion coefficients, − and of the nonlinear spectroscopies such as MCD, , nuclear spin-induced circular dichroism (NSCD), magneto-chiral dichroism (MChD) and magneto-chiral birefringence dispersion, as well as resonant inelastic X-ray scattering (RIXS) − and two-photon absorption. − …”