where J ij is the coupling constant. S i and S j represent the spin operators for each spin site. Positive J corresponds to FM spin coupling and a HS ground state, while negative J indicates a preferred AF spin coupling and a LS ground state. The magnitude of J indicates the energy gap between two spin states. Experimentally, J is obtained using techniques such as electron paramagnetic resonance (EPR). Using computation, J can be estimated by relating it to the energy difference between HS and LS states described by the Heisenberg Hamiltonian, according to