The Hartree-Fock-Roothaan ab initio self-consistent average field method has been used in calculating the pair interaction potentials for atoms in the P2 and GaP molecules. The gallium and phosphorus atoms are considered in their ground states. The calculations agree well with experiment.When phosphorus evaporates from solution in molten gallium under Langmuir conditions, the direct molecular beam contains only P2 molecules and gallium atoms [1], which has not been given a suitable theoretical explanation, in spite of its practical importance. It is therefore necessary to simulate the interactions of the atoms and molecules in the vapor above the liquid. If one knows the corresponding pair interaction potentials, the simulation can be based on molecular dynamics [2,3], and the method is based on selecting the pair interaction potential, for which one often uses a Morse potential:( 1) in which D is the molecular dissociation energy, R0 the equilibrium interatomic distance, R the distance between the atoms, and a a parameter.Information on pair interaction potentials can be obtained by measuring virial coefficients, critical gas and vapor parameters, saturation vapor pressures, and other properties affected by the interaction, i.e., by experiment. The set of such data provides atomic interaction potential curves, with the distance between the atoms the independent parameter.Our purpose has been to use nonempirical quantum-chemical calculations on the molecular orbitals MO for the diatomic molecules P2 and GaP to determine the parameters of the pair interaction potential of (1) for Pe and GaP.We used the Hartree-Fock-Roothaan self-consistent field method [4,5] in a nonempirical calculation on the electronic structures of Pe and GaP. The molecular orbitals were represented as linear combinations of the atomic orbitals AO. We considered the singlet states of the molecules ( l + Zg state for P2 and 1 ]~+ state for GAP). We used the one-determinant and adiabatic approximations to determine the dependence of the total energy for each molecule on the interatomic distance. The one-determinant approximation is applicable for small changes from the equilibrium value in the interatomic distance. The calculations were performed in a gaussian basis. The Hartree-Fock-Roothaan method approximates the complete multielectron wave function for the system as an antisymmetrized product of one-electron functions q0p, which are called molecular orbitals [4,5]. It is in which N is the number of electrons.The following is the mean total electron energy (neglecting the nuclear interaction):Voronezh State Technical University, Voronezh.