  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.
