MOLECULAR-DYNAMICS STUDY OF WATER CLUSTERS, LIQUID, AND LIQUID-VAPOR INTERFACE OF WATER WITH MANY-BODY POTENTIALS

The molecular dynamics computer simulation technique is used to develo p a rigid, four-site polarizable model for water. The suggested model reasonably describes the important properties of water clusters, the t hermodynamic and structural properties of the liquid and the liquid/va por interface of water. The minimum energy configurations and the bind ing energies for these clusters are in reasonable agreement with accur ate electronic structure calculations. The model predicts that the wat er trimer, tetramer, and pentamer have cyclic planar minimum energy st ructures. A prismlike structure is predicted to be lowest in energy fo r the water hexamer, and a cagelike structure is the second lowest in energy, with an energy of about 0.2 kcal/mol higher than the prismlike structure. The results are consistent with recent quantum Monte Carlo simulations as well as electronic structure calculations. The compute d thermodynamic properties for the model, at room temperature, includi ng the liquid density, the enthalpy of vaporization, as well as the di ffusion coefficient, are in excellent agreement with experimental valu es. Structural properties of liquid water, such as the radial distribu tion functions, neutron, and x-ray scattering intensities, were calcul ated and critically evaluated against the experimental measurements. I n all cases, we found the agreement between the observed data and the computed properties to be quite reasonable. The computed density profi le of the water's liquid/vapor interface shows that the interface is n ot sharp at a microscopic level and has a thickness of 3.2 Angstrom at 298 K. These results are consistent with those reported in earlier wo rk on the same systems. The calculated surface tension at room tempera ture is in reasonable agreement with the corresponding experimental da ta. As expected, the computed average dipole moments of water molecule s near the interface are close to their gas phase values, while water molecules far from the interface have dipole moments corresponding to their bulk values. (C) 1997 American Institute of Physics.