## Quantum theoretical calculations of van der Waals interactions between molecules. Anisotropic long range interactions

Presenting a relatively simple ab initio method to calculate full van der Walls interaction potentials between molecules, rules are given for the optimization of basis functions which permit the reliable evaluation of second order long range interactions. Closed expressions for the long range interaction energy are derived in which the orientational dependence is simplified to the utmost. Calculations show that even for molecules which have no dipole moment, such as ethylene, the strongly anisotropic electrostatic interactions are of the same magnitude as the dispersion interactions, but also that the anisotropic (''cross'') terms in the dispersion energy are about equal inmore »