The spin hamiltonian

The author derives a spin hamiltonian V(S) for the g fold degenerate level of a system perturbed in first order by a physical perturbation V; the derivation explicitly relates the coefficients a, b, c,... of V(S) to the matrix elements VMM' of V between the zero order physical basis functions psi M. He shows how the symmetry of V and time reversal affect V(S), and considers three often treated examples as an illustration of the power and unity of his method.