Hydrogen bonding in water clusters: Pair and many-body interactions from symmetry-adapted perturbation theory

This paper contains a study of the pair and many-body interactions in cycli c water clusters: trimer, tetramer, and pentamer. Symmetry-adapted perturba tion theory (SAPT) is applied to compute the pair- and three-body interacti ons directly and to analyze the individual electrostatic, induction, disper sion, and exchange contributions. The total interaction energies are also o btained by supermolecule coupled-cluster calculations including single, dou ble, and noniterative triple excitations, CCSD(T). The three-body interacti ons contribute up to 28% of the total interaction energy in these water clu sters in their equilibrium geometries and up to 50% of the barriers for dif ferent tunneling processes investigated in the trimer. The main three-body contribution is due to second-and third-order induction effects, but also t hree-body exchange effects are substantial. Dispersion contributions are on ly significant in the pair energy. The four-body effects are relatively sma ll, and the five-body effects were found to be negligible. Furthermore, we tested the quality of various density functional methods for describing the se many-body interactions.