The methodology used here to decompose ferromagnetic and antiferromagnetic contributions to the overall observed exchange coupling constant relies on the fundamental equations eq. 2 and eq. 3. We propose that for a qualitative picture of these contributions in realistic complexes beyond simplified models with two electrons in two orbitals, CASSCF/NEVPT2 calculations can be used. In principle, any active space size and composition can be chosen, as long as the magnetic orbitals and the orbitals contributing to the superexchange mechanism are included. Pairwise orbital energy differences are central to the fitting procedure.