As a consequence, a double d -shell active space that does not include enough ligand orbitals will likely not be enough to recover the multi-d -character of the wave function, because it only partially stabilizes the d 10 -occupancy configurations. Such a conclusion, based on the multi-d -occupancy definition of the double d -shell effect, may explains why, in some cases, the inclusion of a double d -shell active space is not enough to obtain accurate results without having the active space increased with ligand orbitals as well, often up to infeasible sizes.