As in the subsystem case, the orbitals can be divided in groups, via the PFRS approach, to provide a better picture of the energies dependence with the convergence of the molecular orbitals. However, in this case there are three sets of orbitals, one for each metal center (copper and molybdenum) and one for the ligands. This increases the number of parameters, and thus increases the number of calculations needed to obtain a reasonable number of points. As the double d -shell effect is larger for first-row transition metals, we choose to focus on the copper weight and use same weight for the molybdenum, that is, a single weight for both metals. iii It is relevant to note that heavier transition metal atoms can have multi-d -occupancy character. However, their configurations' energies have smaller dependence with the optimal 4d -orbital size. That way, the multid -occupancy can be reproduced with a larger range of d -orbitals. For instance, the Mo 4+ →Mo 5+ 4d -orbitals size goes from 1.595 to 1.635 a 0 , but the variation in the relative energy is of around 1 eV, at the CASSCF level of theory (see Figure ).