A novel equation-of-motion coupled-cluster framework for robust modeling X-ray nonlinear spectroscopies*

Nonlinear X-ray spectroscopies such as resonant inelastic X-ray scattering (RIXS) are powerful probes to investigate the structure of matter and dynamics with unprecendented resolution, especially when augmented with reliable theoretical tools. However, modeling nonlinear X-ray spectra is challenging due to the difficulties in describing the physics of underlying core-electron processes. In particular, the presence of the valence ionization continuum in which core-level states are embedded renders standard theoretical methods, designed for modeling valence-electron processes, inadequate. The ionization continuum is also responsible for the non-convergent behavior of the response states in standard ab initio calculations for modeling nonlinear spectra. We will present a novel equation-of-motion coupled-cluster framework based on the core--valence separation scheme that provides convergent calculations of X-ray response calculations and facilitates robust modeling of RIXS spectra for both closed- and open-shell species. We will illustrate the capabilities of our novel approach by measuring its performance in modeling the RIXS spectrum of the transient aqueous OH radical formed in the ionization of water against experiments.