where S and I represent the two spins. If the coupling between the two spin centres can be approximated as a purely secular interaction, then the static Hamiltonian of the system in an external magnetic field is 𝐻 9 Under the assumption of weak coupling (𝜔 )* ≪ |𝜔 ) -𝜔 * | ≪ 𝜔 ) , 𝜔 * ), the thermal equilibrium state of this system is given by: