Dispersion-relation methods are used to derive a form-factor representation for the retarded dispersion energy of two hydrogen atoms that are described by relativistic electron theory. By expressing the electromagnetic form factors in terms of atomic transition matrix elements the complete multipole expansion of the interatomic dispersion energy is obtained. The long-range asymptotic limit of the successive multipole interactions is given explicitly.
Michels, M. A. J., & Suttorp, L. G. (1977). Form-factor representation and multipole expansion of the retarded interatomic dispersion energy. Molecular Physics, 33(1), 245-254. https://doi.org/10.1080/00268977700103191