An expression for the non-additive, three-body interaction energy of three neutral atoms is deduced, by the methods of quantum electrodynamics. Non-relativistic reduction and dipole approximation result in an expression which for interatomic separations R small compared to the wavelengths of typical atomic transitions reduces to the triple-dipole energy varying as R –9 (Axilrod & Teller), whereas at distances large compared with these wavelengths, the three-body interaction energy becomes proportional to R –10 , owing to the influence of retardation.
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