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

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 eT. 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.