MNDO M AND PM3 STUDY OF SMALL WATER CLUSTERS, (H2O)(N) N=2 TO 16/

For the clusters (H2O)(n), n=2 to 16, minimum energy structures and st epwise hydration enthalpies have been calculated at MNDO/M and PM3 lev els and compared with available ab initio and experimental data: The M NDO/M has been found to provide better description of intermolecular p otential of water trimer than PM3, consistent with state-of-the-art ab initio analysis. It has been shown that, for the water tetramer at le ast, PM3 method predicting the nonadditive many-body contributions to the total interaction energy is in good agreement with high level-ab i nitio calculations. Discontinuous drops in the plots of Delta H-n-1,H- n vs. n indicating the formation of unusually stable water configurati ons have been observed. Cooperativity in hydrogen bonding, i.e. additi onal enhancement of interaction energy due to mutual polarization of t he associated O-H groups, has been discussed. Possible formation of ic e crystallites during cluster growth has been proposed.