Hi When looking at the interaction between an EM-wave/field E with an isolated atom, we know that they interact via the time-dependent dipole interaction V = dE. This can be derived by looking at an electron bound to an atom, and it is used in systems, where the atom can be rightfully approximated as a two-level atom. However, how can we always say that the atom only has a single electron attached to it? Who says that there can't be two/three/... electrons attached to our approximate two-level atom? In that case I don't believe the dipole interaction is valid. Any help is appreciated. Best wishes, Niles.