As in the PFRS case, the usual approach has a weight where both CAS and NEVPT2 relative energies are the same. Therefore, we may suppose that this weight generates CASCI wave functions with similar accuracy, as they have the same amount of missing correlation energy. Such crossing occurs around 70%, as shown in the zoom of Figure ). At this point the 3d z 2 -orbital that composes the molecular orbitals is similar to the optimal one for the isolated Cu + (3d 10 ), as shown in Figure ). In terms of our proposed approach, this is similar to defining the metal weight as 100% of the d 10 -occupancy and around 70% of the S + ground state for the ligand one. Comparing these values with the blue points in Figure , the leftmost point was 100% of the metal weight and 65% of the ligand with a relative energy of 8.89 eV. That is, our proposed protocol results in values close to the usual SA-CASSCF, with the advantage to control the bias for each group of orbitals.