Using Graaf et al. molybdenum-copper model complex 6 as a study case, in this article we propose that there is an unified definition for the double d -shell effect that contemplates the results for both single and multireference methods: The double d -shell effect is the mixing of two or more configurations with distinct d -occupancies in a wave function. It is the wave function's multi-doccupancy character. Similarly to the multireference character, the multi-d -occupancy character is necessary to qualitatively describe the wave function. However, the latter does not require a multi determinantal expansion, differently from the former, because the multi-d -occupancy character can be encoded in the molecular orbitals. This encoding was discussed in our previous work and will be further explored in the present article. We point out that hints of the multi-d -occupancy character in the first-row transition metal containing compounds electronic structure have been present in the literature for a long time. For instance Chong et al. in 1986 explored the first-row transition metal hydrides ground states based on their 3d n 4s 2 and 3d n+1 4s character mixing. Moreover, Bauschlicher Jr. in 1988 has shown, for TiH, that when compared with full configuration interaction (FCI) results (for a basis set that contains three d -shells), CASSCF/MRCISD+Q calculations using a single d -shell active space have a better performance for the pure d 2 -occupancy state than the single reference SCF/CPF. On the other hand, the single reference approach had better agreement with the FCI values for a mixed 3d 2 4s 2 /3d 3 4s state.