This restricted sub-space approximation has a negligible effect on the quality of the description of the core excitations. Furthermore, restricting the orbital sub-space was found to affect the valence excited state energies by only less that 0.1 eV, with respect to the excitation energies obtained without any orbital restriction. For simulations of RIXS, the restricted sub-space approximation to TD-DFT with a limited number of valence orbitals together with the core orbitals in the donor space can be employed pragmatically to allow for the simultaneous description of core excitations and valence excitations require to generate a RIXS spectrum.