An approximate formula for the intermolecular Pauli repulsion between closed shell molecules

The exchange repulsion formula proposed by Murrell and co-workers (Proc. Roy. Soc. (Lond.), 1965, A284, 566; J. chem. Phys., 1967, 47, 4916) is considered in detail. Potentially important terms missing in the formalism of Murrell and co-workers are identified and evaluated for the water dimer using several basis sets. Insights into the contributing terms are obtained by using localized molecular orbitals. The results point towards a relatively simple expression for intermolecular exchange repulsion, based on the isolated wavefunctions of the two overlapping species.