Dissociative model of water clusters

A model of water is introduced that dissociates into either ionic or covale nt molecular fragments. The model is based on a semiempirical calculation e mbedded in a molecular mechanics approach. It contains electronic degrees o f freedom that allow the calculation of partial charges that vary smoothly from the bound state to the dissociated states. The hydrogen bond is modele d classically with Coulomb interactions, and shows significant improvement in predicting cluster behavior by adding directional character via an overl ap between a lone pair orbital on oxygen and the hydrogen of an adjacent mo lecule. An implicit bond polarization is present due to stretching bonds an d charge transfer when the hydrogen bonds are formed. The energetic and geo metric properties of water dissociation products, molecular clusters, and i onic clusters are presented. (C) 1999 American Institute of Physics. [S0021 -9606(99)51418-9].