The reaction coordinate of MECI-2 is reported to be along a C-C bond elongation. The predicted C-C distance is 1.64 and 1.60 Å (Table in the Supporting Information) using SA-CASSCF and all CASPT2 variants, respectively, such that there is only a small difference between the methods. However, MECI-1 is reported along the reaction coordinate of O-O elongation, and the predicted bond lengths are 2.47 and 2.60 Å (Table in the Supporting Information). Hence, in this case, the perturbative correction is more significant, resulting in bond distances differing by more than 0.1 Å.