  Resonant Inelastic X-ray Scattering (RIXS), which probes the occupied and
unoccupied electronic subspaces in an interrelated fashion, is one of the most
detailed, complex and information-rich experimental techniques employed in the
investigation of electronic structure across physics, chemistry and materials
science. We introduce an ab-initio, accurate and efficient computational
framework for simulation and analysis of RIXS spectra, which combines two
diverse and established approaches to modeling electronic excited states. The
Core-Hole Linear-Response RIXS (CleaRIXS) method not only ensures accurate
incorporation of the interaction of electrons with core and valence holes, but
also automatically maps the salient RIXS features to the relevant electronic
excitations and de-excitations. Through direct comparison with previous
methanol C K-edge RIXS measurements [J. Phys. Chem. A 120, 2260 (2016)], we
show the efficacy of the formalism in modeling different regions of the RIXS
spectrum and in gaining physical insight regarding their origins. The
importance of including the valence electron-hole interactions is explored, in
addition to the connection between CleaRIXS and determinant-based approaches
for simulating X-ray absorption and non-resonant X-ray emission. A qualitative
discussion of additional RIXS features arising from possible
vibrationally-mediated processes is also presented. CleaRIXS provides a robust
and extendable framework for prediction and interpretation of RIXS processes
and for the simulation of complex electronic excited states in general.
