center interacting with a positively charged ligand ([Cu + L + ] 2+ ). However, in the two first cases, Figure ) and b), this reduction in the relative energy gap is not enough to cause a mixing between configurations. As a consequence, in both cases the final ground state wave function (black line in the qualitative scheme) is always a pure S 2+ (3d 9 ) configuration. On the other hand, when the effect of changing both groups of orbitals is considered, the gap is reduced at the point where there Although the CASCI results discussed up to this point are from single Slater determinant wave functions, they are able to recover the mixing between d -occupancies because it can be encoded within the S 2+ singly occupied orbital. This orbital, denoted here as ϕ s , is a mixture of the copper 3d z 2 -orbital, in the CN bridge direction, with a mainly nitrogen ligand orbital ϕ L (as shown in the top row of Figure ):