Hi Say I want to describe the interaction between a free atom atom and a classical EM field. The full Hamiltonian for the problem must be [tex] H = (H_0 + p^2/2m) + (H_V + H_E) + H_D [/tex] where H0 denotes the internal levels of the atom, HV the vacuum field, HE the energy density of the classical field and HD the dipole interaction. My question is regarding HE. I have never seen it written explicitly and have not been able to find a reference where they do so. How would one write this? Is it QM-version of something similar to Poyntings vector? Best regards, Niles.