Influence of the Drude charge value on the performance of polarisable water model: a test for microscopic and macroscopic parameters

We report a molecular dynamics simulation of liquid water using the SWM4-NDP polarizable water model that belongs to the class of Drude oscillator type models. Generally, parameterisation protocols currently used for this type of models implicitly assume the validity of point-dipole approximation for the induced dipole (where the response of the dipole to the external field should depend only on the value of polarizability but not on the actual values of the charge or spring constant of the Drude oscillator). Therefore, possible dependency of the model results from the Drude charge/spring constant is often neglected. However, we show in this paper that the point-dipole approximation condition is actually not satisfied for a range of Drude oscillator parameters; that leads to a significant dependency of the obtained results from the particular choice of the Drude charge/spring constant. We analyzed the influence of the Drude charge value on the main macroscopic properties of water given by the SWM4-NDP water model (including density, static dielectric constant, self-diffusion constant, and surface tension). Our results show that (in general) the properties of model water do depend on the Drude charge value. We also provide an estimate for the Drude charge value (- 10e) above which the point-dipole limit is achieved with good accuracy. In this paper we also discuss the differences in the convergence behaviour of different model water properties. Based on the obtained results we developed a new re-parameterised version of the SWM4-NDP model (SWM4-NDP10e) where the point-dipole limit condition is satisfied. We show that the model reproduces well important macroscopic as well as microscopic properties of water at ambient conditions.