It states that the 3d -electron correlation is dominated by a 3d → 4d radial correlation. In such a case, this effect should be accounted almost entirely by any post-CAS methods that considers 3d to 4d double excitations, what is not true even for small diatomic cases. This inconsistency in this explanation is not ignored in the literature, for instance in Pierloot's words (reference page 126): "[...] such an effect should be classified as dynamic rather than nondynamic, [...] this is one of the exceptional cases where an accurate description of dynamic correlation effects really benefits from a reoptimization of the orbitals involved, i.e., from a multireference treatment.". This justification of an exceptional type of dynamic correlation, that must be treated similarly as a nondynamic one, gives rise for more questions than answers. For instance, if an exceptional behavior exists in the first-row transition metals, what prevents it to affect heavier transition metals, such as 4d metals, or other atoms as the lanthanides?