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

TL;DR

This paper introduces a new damped response framework for calculating resonant inelastic X-ray scattering using EOM-CC methods, with implementation details, comparisons, and preliminary applications.

Contribution

The paper presents an implementation of a damped response framework for RIXS at the EOM-CCSD and CC2 levels, enabling future extensions to higher excitation methods.

Findings

CC2 provides comparable RIXS spectra to CCSD for valence excited states.

The framework addresses convergence issues using core-valence separation techniques.

Preliminary results demonstrate applicability to solvated molecules.

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.