Nature and importance of three-body interactions in the (H2O)(2)HCl trimer

The nature and importance of nonadditive three-body interactions in the (H2 O)(2)HCl cluster have been studied by the supermolecule coupled-cluster met hod and by symmetry-adapted perturbation theory (SAPT), The convergence of the SAFT expansion was tested by comparison with the results obtained from the supermolecule coupled-cluster calculations including single, double, an d noniterative triple excitations [CCSD(T)], It is shown that the SAFT resu lts reproduce the converged CCSD(T) results within 3% at worst. The SAFT me thod has been used to analyze the three-body interactions for various geome tries of the (H2O)(2)HCl cluster. It is shown that the induction nonadditiv ity is dominant, but it is partly quenched by the first-order Heitler-Londo n-type exchange and higher-order exchange-induction/deformation terms. This implies that the classical induction term alone is not a reliable approxim ation to the nonadditive energy and that it will be difficult to approximat e the three-body potential for (H2O)(2)HCl by a simple analytical expressio n. The three-body energy represents as much as 21-27% of the pair CCSD(T) i ntermolecular energy.