Long- and intermediate-range interaction between two ground state hydrogen atoms

Perturbation theory in the polarization approximation has been employed, without using the multipole expansion for the interaction Hamiltonian, to calculate the second- and third-order interaction energies of two ground state hydrogen atoms for internuclear separations R - 4(2) 12 a.u. Using recently computed variational energies for H-H and H-H̄, the values of higher order corrections to the interaction energy have been estimated. The accuracy of several approximations commonly used in calculations of inter-molecular interaction energies has been tested.