The retarded energy shift and pair polarizabilities of interacting atoms in an external field. Application of resummed field-theoretical perturbation theory

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 the authors developed in a previous paper (1979). 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 Hellman-Feynman theorem. The retardation effects showing up in the pair polarizability are discussed in detail.