A molecular dynamics study of polarizable water

We have investigated the effect of adding a point polarizability to a SPC like rigid water model in molecular dynamics simulations. A new algorithm for calculating the induced dipole moments based on a predictive, instead of an iterative scheme, is presented. The predictive scheme considerably reduces the demand on computer time. Both schemes gives identical results for energy, structure and dynamics. In the liquid phase simulations of the polarizable water model the total dipole moment is enhanced from its static gas phase value of 1·85 D to 3·2 D. The radial distribution functions indicate an increased structure as compared to SPC water. Dynamic properties are slower than for the original SPC model. Further adjustments of the hard core of the polarizable SPC model to yield a better energy estimate reduces the total dipole moment to 2·9 D. This model, the polarizable SPC (PSPC) water, enhances the agreement with the original SPC structure and improves the dynamical properties of the model. In order to single out the effects of the many-body forces, simulations with non-polarizable water models were also performed. Nonpolarizable water with gas phase dipole moment shows much less structure and much faster dynamics whereas non-polarized water with enhanced (2·9 D) dipole moment seems to freeze. These results indicate that polarizability is important to the properties of model water, but its proper treatment needs careful consideration.