Quantum mechanical perturbation expansion of the second virial coefficient and the Ursell‐Mayer function. II. The Heitler‐London linear polarizability model

By a modification of a previous method of quantum mechanical perturbation expansion, the Ursell‐Mayer function f(R) is obtained in the form 1 + f(R) = exp(−ΔE)(1 + βU₁ + β²U₂ + ⋯) where ΔE is the Heitler‐London energy and U_n is at least of order 2n in the Coulomb interaction energy of the pair of molecules. The U_n's depend on electron overlap integrals and distorted charge distributions. When electron overlap and charge distribution distortion are neglected the U_n's become products of long range dispersion interaction potentials. Furthermore when the effect of nonlinear polarizabilities is neglected one obtains $Un = U1n /n!$⁠. The results are restricted to temperatures sufficiently low compared to the excitation energies of the two molecules.