where S i(j) are the local spin operators acting on the sites i(j) and J ij is the exchange cou-pling constant between the sites i and j. The symbol < i, j > indicates that the sum refers to nearest-neighbor interactions only. A positive J value indicates ferromagnetic exchange interaction with high-spin ground state, conversely a negative sign of J indicates an antiferromagnetic exchange between the spin centers and hence low-spin ground state. For a diradical with two S = 1/2 spin centers, only one coupling constant appears in the above Hamiltonian, which can be shown to be related to singlet-triplet energy splitting (∆E ST )