United Atom Treatment of the Behavior of Potential Energy Curves of Diatomic Molecules for Small R

It is shown that the potential energy function V (R) of any state of a diatomic molecule can be expanded for small internuclear distances, in the from ${\mathrm{V(R)=[(Z}}_a{Z}_b{\mathrm{)/R]+W}}_u{\mathrm{+E}}_2{\mathrm{·R}}^2{\mathrm{+E}}_3{\mathrm{·R}}^3\mathrm{+···}$ , where W_u is the energy of that state of the united atom to which the molecular state under consideration connects. Explicit expressions for the coefficients E₂, E₃ are given in terms of the electronic charge density of the united atom. Applications of the derived energy expansion are discussed.