THE POLARIZATION APPROXIMATION AND THE AMOS-MUSHER INTERMOLECULAR PERTURBATION THEORIES COMPARED TO INFINITE-ORDER AT FINITE SEPARATIONS

We argue that the most common form of intermolecular perturbation theo ry, the polarization approximation (PA), gives at finite separations, for nearly all systems, no usable, exact relationship between the lowe st unperturbed energy and the physical ground state energy. This would be true even if the PA energy expansion converged. in contrast, the A mos-Musher (AM) theory does give an exact relationship between the low est unperturbed energy and the physical ground state energy. We find n o theoretical justification for using the PA theory beyond second orde r in the energy, but the AM theory may be extended to arbitrarily high order.