In this article, we will present and discuss results for the molybdenum-copper model originally proposed by Graaf et al. using our approach, the partially fixed reference space (PFRS). It is a protocol for orbital optimization designed to impose that the d -orbitals converge to a size that is able to describe, simultaneously and with similar accuracy, configurations with distinct occupancies. This is made in a two-step procedure, where the ligand orbitals are optimized without allowing changes in the size of the metal d -orbitals, that have been previously optimized. This is in line with a recent series of articles that report good results when alternative orbital optimization schemes are used in a smaller CASCI expansion, when compared to the usual SA-CASSCF approach, as compiled by Levine et al. In particular it has potential to fill the gap that exists for orbitals to model charge transfer processes in transition metal containing compounds, as pointed out by the mentioned compilation work. We will show that this procedure, together with our proposed definition for the double d -shell effect, can explain Graaf et al. results, 8 based on the multi-d -occupancy character of the wave function. For this purpose, as the double d -shell effect is substantially larger for the first-row transition metals, we will first focus on the electron affinity of the copper subsystem. This reduces the computational cost but maintais the difficulties due to the 3d transition metal.