Local structure and work of formation of water clusters studied by molecular dynamics simulations

Small clusters composed of 64, 94, 125, 190, 256, and 512 water molecules have been studied by molecular dynamics simulations using the ST2 water model. Radial profiles of the local density, energy, electric potential, and components of the pressure tensor were calculated. The work of formation was derived for the different cluster sizes on the basis of the normal pressure tensor component P-N and was related to the curvature-dependent surface tension of the clusters. Our calculations show that the surface tension gamma increases with the cluster radius R in the size range investigated, to beyond the limiting value for the flat interface. This course of the gamma(R) function is consistent with the corresponding surface energy function which was obtained in a more direct manner from the energetic parameters. In addition, it indicates, however, that the ST2 water model yields surface entropy values which are much lower than anticipated for real water. We have also elucidated the surface effect on the self-diffusion coefficient and on the reorientational relaxation time. (C) 1996 Academic Press, Inc.