In the usual double d -shell active space approach, this mixing of configurations with distinct doccupancies is achieved with an active space that is capable to recover the optimal 3d -orbital size of multiple d -occupancies, together with a large amount of the d -d dynamic correlation. However, we have shown, previously for small diatomic systems and in the present work for a molybdenumcopper model, that a qualitatively correct wave function can be achieved using an intermediate set of orbitals. Moreover, as the mixing of configurations in a given wave function depends on the relative energies among them, our proposed definition also contemplates the reason why the qualitative description of excited states looks to suffer from the double d -shell effect. In this case, even if the ground state has no d -occupancy mixing, that is, does not have a substantial double d -shell effect, some of the excited states eventually will have, what can cause large errors in relative energies. For instance, this has been shown to be the problem for the definition of the triplet/quintet gap in the cobalt monofluoride diatomic molecule and it is likely to be the reason for the CCSD(T)/MRCI discrepancy for the NiF ground state. In addition to the proposed definition for the double d -shell effect, we have shown that the stabilization of configurations with distinct d -occupancies is dependent on the orbitals spatial location. That is, the optimization of a large active space of local orbitals is not necessarily enough to describe multiple configurations simultaneously. Here we used the electronic affinity of the copper subsystem as example, where both the metal and ligands orbitals have similar relevance for the stabilization d 10 -occupancy configurations. For the d 9 configurations, on the other hand, variations in the metal orbitals have almost no impact in the energy, but the ligand orbitals are twice more relevant than for the d 10 configuration. That way, a minimum active space for the ligands will be optimized toward the d 9 -occupancy optimal orbitals, as it leads to the larger energy reduction.