Similarly, using our approach it is possible to fix the 3d -orbital, as the optimal for a d 10 configuration, and vary the ligand orbitals by changing the relative weight (w) between the S + and S 2+ ground states during the orbital optimization, that is represented in box III of Figure . Doing this we can measure how the interaction among ligand electrons and the copper center is impacted if the ligand orbitals are optimized around a Cu 2+ center or a Cu + center. The energy variation of the S + ground state with the ligand orbitals is shown in Figure ) and it is similar to the one seen for the d -orbital size change in Figure ). Therefore, going from ligand orbitals shaped around the Cu + center to those shaped around Cu 2+ provokes the same energy variation on the S + ground state as changing the internal metal orbitals from an optimal 3d 9 to an optimal 3d 10 .