No multipole expansion before perturbation theory

[Clara]

Perturbation theory uses the fact that the perturbation effects are very small compared to the scales of the effects of the unperturbed system. Applying the multipole expansion on the term of the nucleus-electron interaction leads to first a monopole term that is close to energyscales of part of the unperturbed system (related to the Hamiltonian of the electrons). It is only the following terms in the multipole expansion that are relatively small enough compared to the unperturbed system to apply perturbation theory on.

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