Isomerization, melting, and polarity of model water clusters: (H2O)(6) and(H2O)(8)

Energetics, structural features, polarity, and melting transitions in water clusters containing up to eight molecules were studied using ab initio met hods and empirical force field models. Our quantum approach was based on de nsity functional theory performed at the generalized gradient approximation level. For the specific case of (H2O)(6), we selected five conformers of s imilar energy with different geometries and dipolar moments. For these case s, the cyclic arrangement was found to be the only nonpolar aggregate. For (H2O)(8), the most stable structures corresponded to nonpolar, cubic-like, D-2d and S-4 conformers. Higher energy aggregates exhibit a large spectrum in their polarities. The static polarizability was found to be proportional to the size of the aggregates and presents a weak dependence with the numb er of hydrogen bonds. In order to examine the influence of thermal fluctuat ions on the aggregates, we have performed a series of classical molecular d ynamics experiments from low temperature up to the melting transition using two different effective pseudopotentials: the TIP4P and MCY models. Minimu m energy structures for both classical potentials were found to reproduce r easonably well the results obtained using ab initio methods. Isomerization and phase transitions were monitored by following changes in dipole moments , number of hydrogen bonds and Lindemann's parameter. For (H2O)(6) and (H2O )(8), the melting transitions were found at T-m approximate to 50 and 160 K , respectively; for both aggregates, we observed premelting transitions bet ween well differentiated conformers as well. (C) 1999 American Institute of Physics. [S0021-9606(99)30916-8].