On the nature of the Hamiltonian for the interaction of radiation with atoms and molecules: (e/mc)p⋅A, −μ⋅E, and all that

In nonrelativistic quantum electrodynamics there is a class of equivalent Hamiltonians that describe a system of radiation and atoms or molecules in interaction. Three specific examples, namely, the minimal coupling, the multipolar, and the space‐translated forms are analyzed here in detail. Their equivalence is demonstrated. The Lagrangian approach has also been examined to give further insight into the dynamics. The equations of motion are compared. Finally, the process of two‐photon absorption by an atom is used as an application of the various interaction Hamiltonians in a discussion of their computational differences.