Second-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.