2ND-ORDER EXCHANGE EFFECTS IN INTERMOLECULAR INTERACTIONS - THE WATER DIMER

A new method of deriving explicit formulas for the calculation of second-order exchange contributions (induction as well as dispersion) within the framework of symmetry-adapted perturbation theories is presented. It is shown how exchange contributions can be expressed as a combination of electrostatic interaction energies between suitably generalized charge distributions (overlap intermolecular charge distributions). Each of these contributions are derived within the Hartree-Fock approximation (neglect of all electron correlation effects within the noninteracting molecules) and by considering only single-electron exchange between interacting molecules. Numerical calculations for the interaction of two water molecules are presented. In the region of the equilibrium geometry, it is found that the complete second-order exchange contribution accounts for about 20% of the total intermolecular interaction energy. This contribution is essentially dominated by the exchange induction component which is found to represent approximately 1 kcal/mol (using a basis set containing 94 orbitals). To our knowledge, this is the first example of calculation of exchange induction interaction energy for a molecular system. Concerning the less important, but non-negligible, exchange dispersion component, our result is found to agree with a very recent calculation for the water dimer. © 1990 American Institute of Physics.