Cooperativity and hydrogen bonding network in water clusters

Clusters of water molecules are held together by hydrogen bonding networks. These networks are differentiated by the participation of the individual water molecules in the hydrogen bonds either as proton donors (d), proton accepters (a) or their combinations, It has long been assumed that the stability of clusters is determined by the dominant two-body interactions between the water molecules. We have found that homodromic hydrogen bonding networks, i.e. those exhibiting donor-acceptor (da) arrangements between all water molecules, are associated with the largest nonadditivities among other networks present in low lying minima of small water clusters. This finding offers a novel explanation for the stability of homodromic rings that are the global minima for the clusters trimer through pentamer. Among the non-additive terms, three-body terms are mainly responsible for determining the relative stabilities between the various trimer through pentamer isomers. This suggests that purely two-body pairwise additive potentials will result in errors exceeding 20% for clusters larger than the pentamer. Among all higher order components, the three-body term was found to be the most important. (C) 2000 Published by Elsevier Science B.V. All rights reserved.