The retarded energy shift and pair polarizabilities of interacting atoms in an external field

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Abstract
The interaction energy of two atoms in the presence of an external electrostatic field is evaluated up to sixth order with the help of covariant field-theoretical perturbation theory. The divergent Feynman diagrams entering the calculation are resummed according to a systematic procedure that has been developed in a previous paper. The pair polarizability and first hyperpolarizability that give the response of the interacting atoms to the external field are subsequently obtained from the energy shift by virtue of a Hellmann-Feynman theorem. The retardation effects showing up in the pair polarizability are discussed in detail.