A b i n i t i o valence-bond calculations of the van der Waals interactions between π systems: The ethylene dimer

Abstract

A multistructure valence-bond method for the calculation of van der Waals forces is presented which includes in one consistent formalism the electrostatic, induction, and dispersion forces and takes exchange correctly into account. The application of this method to the ethylene dimer leads to the following main conclusions: (1) The ’’first order’’ electrostatic forces are comparable in magnitude to the ’’second order’’ forces even though the molecules possess no permanent dipole moments. Dispersion forces are much larger than induction. Second order interactions are more isotropic than first order forces. (2) In the multipole expansions of the long range forces, the inclusion of the first term only is not sufficient for a good approximation to the interaction. (3) Exchange effects become nonnegligible at approximately 12 bohr, while the van der Waals minimum between two perpendicular molecules is at 9.4 bohr. At about 6 bohr, penetration effects make the multipole expansion meaningless. Possible simplifications for future calculations are suggested. A seemingly good van der Waals minimum which is calculated by the ab initio SCF method is shown to be caused in part by the (mathematical) effect of basis set enlargement. CNDO results disagree completely with the ab initio calculations.