While Curie and Larmor susceptibilities were well understood from experimental measurements, J.H. Van Vleck noticed that the calculation above was incomplete. If H is taken as the perturbation parameter, the calculation must include all orders of perturbation up to the same power of H . As Larmor diamagnetism comes from first order perturbation of the H 2 , one must calculate second order perturbation of the B term: