Engineering a simple polarizable model for the molecular simulation of water applicable over wide ranges of state conditions

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Abstract
We perform a systematic analysis of the relationship between the molecular geometry, the force-field parameters, the magnitude of the induced dipoles, and the resulting site–site microstructure of a model for water consisting of simple point charges plus a self-consistent point dipole polarizability. We constrain the model to represent the experimental values of the pressure and the configurational internal energy of water at ambient conditions, while keeping a permanent dipole moment of 1.85 D. The resulting force fields are then used to perform additional simulations at high temperature to determine the effect of polarizabilities on the site–site structure, and to make contact with neutron scattering experiments as well as ab initio simulation results. We show that the parameterization of the model is possible for 0≤ROM≤0.25 Å, where ROM is the oxygen-to-negative charge distance along the bisectrix of the H–O–H angle, resulting in total dipole moments from 2.88 to 3.03 D, with polarization energies accounting for 40%–57% of the total configuration internal energy of water. These results, in conjunction with the behavior of the short range site–site correlation functions, highlight the shortcomings of the simple point charge approximation for the polarization behavior at short intermolecular distances, and give a meaningful reference from which we can attempt to overcome these defects.