Based on our alternative definition for the double d -shell effect, the apparent need of a double d -shell active space can be explained. As it is known since the late 70's, electronic configurations of first-row transition metal atoms with distinct d -occupancies lead to small variations of d -orbital sizes that drastically affect their energies. Therefore, if one aims to minimize the total energy of two configurations with distinct d -occupancies, the space must be flexible enough to optimize dorbitals with sizes optimal for both d -occupancies. Such a flexibility can be obtained with a double d -shell active space. However, for a qualitative description of wave functions, it is not necessary to minimize the total energy of all states, but only to have a set of states described with similar accuracy. This is, for instance, the primary idea of the state-averaged complete active space selfconsisten field method (SA-CASSCF). Thus, to consider the multi-d -occupancy character (or double d -shell effect) it is only necessary that the configurations with distinct d -occupancies are described with similar accuracy, not necessarily via individual energy minima. We have shown that such similar accuracy is possible using a single d -shell active space if only two d -occupancies are necessary to describe the system. Similar results were obtained in previous calculations for the molybdenum-copper model system, where the authors have shown that a single d -shell active space can be used to obtain good results, while the MMCT state could not been found using a double d -shell active space. However, a deeper explanation for the quality of these results is lacking. For instance, it is unclear whether the results rely on some sort of fortuitous error cancellation, or if their approach can be extended to other studies. In either case, it becomes apparent that the double d -shell effect may be treated, or at least the problems associated to it mitigated, without the second set of d -orbitals. This is consistent with our attempt to redefine the double d -shell effect without considering one specific solution.