Reaction field simulation of water

Two molecular dynamics simulations of 216 molecules interacting by the ST2 water potential were carried out. Apart from technical improvements, the first simulation was identical to that reported by Stillinger and Rahman 4. In the second, however, the reaction field method was applied and the influence of long range interactions was carefully studied. 7500 time steps (= 7·5 ps) were generated. The temperature given by the average kinetic energy was 120°C. The atom-atom pair correlation functions and the average dipole energy are rather insensitive to the reaction field. The average cosine between two molecular dipoles, however, is strongly influenced and corresponding curves are of different sign in the second and third coordination shell. For the integral of the average cosine, namely the Kirkwood g-factor g K, the situation is even worse : Large errors occur if the long range interactions are neglected. On the other hand, a convergence of g K was achieved by the inclusion of the reaction field. The predicted g-factor, g K = 2·3-3, as well as the dielectric constant, ε = 45-60, are in good agreement with experiment (g K = 2·5, ε = 51). Finally, the single particle dynamics is analysed with the aid of the mean squared displacement and the reorientational cosine cos ϑ(t). For short times, corresponding curves are close together, but for longer times a maximum deviation of 20 per cent is observed.