Département de chimie moléculaire

This paper contains a study of the pair and many-body interactions in cyclic water clusters: trimer, tetramer, and pentamer. Symmetry-adapted perturbation theory (SAPT) is applied to compute the pair- and three-body interactions directly and to analyze the individual electrostatic, induction, dispersion, and exchange contributions. The total interaction energies are also obtained by supermol. coupled-cluster calcns. including single, double, and noniterative triple excitations, CCSD(T). The three-body interactions contribute up to 28{%} of the total interaction energy in these water clusters in their equil. geometries and up to 50{%} of the barriers for different tunneling processes investigated in the trimer. The main three-body contribution is due to second- and third-order induction effects, but also three-body exchange effects are substantial. Dispersion contributions are only significant in the pair energy. The four-body effects are relatively small, and the five-body effects were found to be negligible. Furthermore, we tested the quality of various d. functional methods for describing these many-body interactions. [on SciFinder(R)]