Therefore, we describe the S 2+ ground state as a single-reference system, but with a multi-doccupancy character. Despite of this single-reference character, a single Slater determinant wave function does not converge to the correct d -occupancy mixing unless a state-average calculation that destabilizes the d 9 -occupancy configuration and stabilizes the d 10 one is used. Therefore, this approach does not optimize the states energy-wise, but allows the optimization of relative energies among distinct occupations. As a consequence, both (i) the relative energies among states with distinct d -occupancy profiles, and (ii) the mixing among occupancies within a given state, can be obtained with higher accuracy.