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 contribution to the interaction energy are computed for different water trimer arrangements at the second- to the fourth-order many-body perturbation (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 origin was quantitatively accounted for in terms of the difference between the two- and three-body energy terms for the two configurations. We have found that the three-body term is attractive and accounts for 18% of the binding energy of the 'cyclic' trimer but its contribution is less significant (6%) and repulsive for the 'open' trimer. It is also repulsive but smaller (4%) for an intermediate trimer configuration which has the arrangement double acceptor, double donor, donor-acceptor for the three water molecules. Many-body energy terms higher than two are, however, less significant for configurations resembling bulk water which have, on the average, two hydrogen bonds per water molecule. The MP4 results were found to differ only slightly (less than 2%) from the corresponding MP2 results.