This conclusion is reinforced by and give us insights to explain the rather large T 1 and D 1 diagnostics, presented previously, that are however still below the loose threshold proposed for transition metal containing systems. Although the S 2+ HF reference wave function used in the CCSD calculation is biased toward the d 9 -occupancy, such an orbital bias can be partially fixed via the single excitations, what leads to these high diagnostics. However, as observed previously at the MRCISD level for the iron atom excitation energy, the bias fixed via single excitations can be propagated to the correlation energy via double excitations. In this case, the CCSD method is able to mitigate this bias onto the correlation energy due to the presence of triple excitations. These higher excitations recover the correlation for both the reference d 9 -occupancy and d 10 -occupancy configurations formed by single excitations, that is, for instance, missed at the CISD level of theory.