A knowledge of the interaction energy between two atoms as a function of their separation is important in many aspectsofradiation damage theory. Recent calculations of this energy from first principles are reviewed in this article, particular attention being paid to calculations based fin the Thomas-Fermi and Thomas-Fermi-Dirac statistical theories of the atom. The advantages and limitations of these theories are discussed from the radiation damage point of view. The energy ranges over which experimental results or theoretical calculations are more useful at present are also discussed.