Abstract: | Very often only “diagonal” second order energies, varying as an even power of R?1, occur in the multipole expansion of the interaction energy. However for many molecular interactions important “nondiagonal” second order energies, varying as an odd or even power of R?1 can arise. This point is emphasized by a general discussion and by a detailed specific example, the interaction of an ionized dipolar molecule with a nondegenerate atom. Also some useful theorems, on the orientation average of various types of second order energies, are derived. |