At first the S 2+ ground state follows a similar behavior, but associated to a d 9 configuration. Its HF wave function converges to a d 9 -occupancy, that is, a Cu 2+ center, in agreement with what is chemically expected. However, when part of the correlation energy is added via the CCSD method, the painted picture changes. The final wave function has large contribution of single excitations, with T 1 and D 1 diagnostics of 0.023 and 0.18, respectively. Two amplitudes are particularly large, with absolute values of 0.20 and 0.05, and excite a ligand electron to the singly occupied d -orbital, originating d 10 configurations. The only other amplitude higher than 0.05 in absolute value (considering both singles and doubles) is of a 3d →3d excitation. That is, differently from the HF wave function, that describe the S 2+ ground state as a pure d 9 -occupancy state, the CCSD wave function can be better described as a multi-d -occupancy wave function composed of a d 9 and d 10 mixture.