SIGNIFICANCE OF HIGHER-ORDER MANY-BODY INTERACTION ENERGY TERMS IN WATER CLUSTERS AND BULK WATER

The magnitudes of the two- and three-body energy terms and their contr ibution to the interaction energy are computed for different water tri mer arrangements at the second- to the fourth-order many-body perturba tion (MP2 and MP4) levels of theory. Configurations in which the water molecules act as proton donor-accepters, double accepters and double donors were considered. The energy separation between the 'cyclic' and 'open' water trimer minima was estimated at 42 kcal mol(-1). Its orig in was quantitatively accounted for in terms of the difference between the two- and three-body energy terms for the two configurations. We h ave found that the three-body term is attractive and accounts for 18% of the binding energy of the 'cyclic' trimer but its contribution is l ess significant (6%) and repulsive for the 'open' trimer. It is also r epulsive but smaller (4%) for an intermediate trimer configuration whi ch has the arrangement double acceptor, double donor, donor-acceptor f or the three water molecules. Many-body energy terms higher than two a re, however, less significant for configurations resembling bulk water which have, on the average, two hydrogen bonds per water molecule. Th e MP4 results were found to differ only slightly (less than 2%) from t he corresponding MP2 results.