Computational study on small water clusters using a semiempirical valence bond approach

Small clusters of water (H2O)(n), n = 3-8, are studied using a semiempirical valence bond approach to investigate the bonding energy contribution and hydrogen-bonding structure in the most stable conformation. The energy contribution was decomposed into electron pair-pair interactions and valence-bond energy for each water monomer. Our study shows that there is significant bonding difference between small clusters (n less than or equal to 5) of water and larger clusters (n > 5). In the larger clusters, there are structures containing tetravalent oxygen centers, which is impossible in the small clusters. The contribution to the binding energy from each H-bond varies from -4.7 kcal/mol to -7.3 kcal/mol in the water clusters considered here. The contribution of -5.9 kcal/mol per H-bond in the cubic octamer is comparable to the experimental value (-6.7 kcal/mol) of the binding energy in ice.