with 𝑖𝑖𝑖𝑖 representing site indices with the heaviside function Θ(𝑑𝑑) forcing causality by excluding negative values of 𝑑𝑑. By taking time derivatives of both sides of the equation and Fourier transforming, an equation of motion for the Green's function can be developed and hence related directly back to the neutron scattering cross section by taking the imaginary part, as outlined in previous studies. The approach followed in previous works applying the Green's functions to neutron spectroscopy is to divide the magnetic Hamiltonian into two parts -a local single-ion part for the magnetic ion of interest (including terms like the crystalline electric field, spin-orbit coupling, and molecular fields), and an interion part describing interaction between magnetic ions (usually modelled in terms of a Heisenberg exchange). Here, we simplify this by considering 1 as a single ion entity with the eigenstates of the total magnetic Hamiltonian defined by {|n⟩} with corresponding energy eigenvalues {𝜔𝜔 n }. The Green's function is then reduced to the single-ion term,