In this equation, Ŝi/j represents the total number of unpaired spins on both metal centers i and j, and J ij is the isotropic exchange coupling constant that quantifies the strength of the interaction. It is by definition positive for ferromagnetic and negative for antiferromagnetic interactions. Using localized magnetic orbitals, the overall J coupling can be decomposed into ferromagnetic (J F,ab ) and antiferromagnetic (J AF,ab ) contributions for each orbital pair (ab):