Effects of the Ewald sum on the free energy of the extended simple point charge model for water

Abstract

The effects of including the Ewald sum on several key system properties for the extended simple point charge (SPC/E) model of water, in the temperature range of 140–300 K have been investigated using molecular dynamics simulations. The original SPC/E parameters are used without alteration since this study is intended to determine whether the Ewald sum affects the model sufficiently to warrant reparameterization. Overall comparison to experiment has been improved for the liquid phase as evidenced by several factors. The effect of temperature on the self-diffusivity coefficient has improved in the range from 300 to 200 K. There is a shift in the temperature at which the density maximum occurs (260 K from 255 K in previous simulations of SPC/E water without the Ewald) and the value of ρmax has decreased to 1.0003 g/ml (versus 0.99997 g/ml for experiment at 277 K) from 1.0082 g/ml for the SPC/E model. However, the shape of the ρ(T) profile is worse than without the Ewald sum in comparison to experiment, but is qualitatively similar to Monte Carlo data from Harrington et al. for SPC/E modeled water including a reaction field method. For the solid, while the ρ(T) profile is unchanged, values are worse than without an Ewald sum in comparison to experiment (≈3% higher than experiment without the Ewald and ≈5% with the Ewald). Calculation of free energies of various low pressure ices and liquid water using thermodynamic integration has produced the major finding, the determination of a melting point for low pressure ices. The melting points of proton-disordered ices Ih and Ic were found to be 279±5 K and 291±5 K, respectively. Melting points of their proton-ordered counterparts are 284±5 K (Ih) and 281±5 K (Ic)