Post-CASCI calculations were performed using the n-electron valence perturbation theory of second order (NEVPT2) level of theory, where core orbitals were not correlated (1s for the representative atoms, from 1s to 3p for copper and 1s to 4p for molybdenum). For the molybdenumcopper model system, results from the partially contract version, pc-NEVPT2, presented in some punctual cases large total energy variations with the orbitals choice. On the other hand, due to the use of a single Slater determinant reference wave function, the strongly and partialy contracted versions of NEVPT2 for the copper subsystem result in the same total energies. Thus, although conclusions based on the pc-NEVPT2 results, after ignoring these non-physical values, are the same as for the sc-NEVPT2, the results for NEVPT2 calculations shown and discussed in this text are for the strongly contracted version, sc-NEVPT2. Results using both contractions can be found in the Support Information.