The role of long ranged forces in determining the structure and properties of liquid water

Abstract

We have performed molecular dynamics calculations for liquid water using the revised central force model potential truncated at various distances and using two sizes of system, in order to study the effect of system size and range of the potential on the calculated thermodynamic and structural properties and to compare the results with those obtained by Ewald summation. All calculations were performed for a cubic system using periodic boundary conditions. Provided the side of the cube is equal to or greater than twice the range of the potential, the thermodynamic properties and distribution functions, including an orientational distribution function, are insensitive to the size of the system, for fixed range. Provided the range of the potential is equal to or greater than 6 Å, the thermodynamic energy and pressure are only slightly dependent on the range of the potential, to an extent that is, however, larger than that observed for dipolar hard spheres and the Stockmayer potential which do not have the tetrahedral structure similar to water. For potentials with ranges of 6 Å or greater, the atom–atom distribution functions are very insensitive to the range, but the orientational correlations are very sensitive to the range, as had been observed many times in studies of dielectric properties of simulated fluids. A potential with a range of 6 Å has thermodynamic and structural properties very similar to those of a longer ranged potential and similar to those obtained by Ewald summation. Use of such a model, which correctly describes interactions between nearest neighbors and next nearest neighbors but has no longer ranged forces, lead to significant increases in the speed of simulations.