TD-DFT: RIXS was also simulated with TD-DFT in ORCA within the restricted sub-space approximation . In TD-DFT (and DFT-ROCIS), the corevalence separation is usually employed to reach core excitations without having to describe the vast amount of valence-excitations in between the core-excitations and the ground state. This is implemented in ORCA by restricting the orbital excitation space to just the core orbitals. In restricted sub-space approximation TD-DFT, as described in Ref , the core orbitals are rotated into a previously selected orbitals sub-space already containing a set of valence occupied and unoccupied orbitals, and excitation among all these orbitals are considered to model core-excited and valence-excited states.