Polarization Corrections and the Hydration Free Energy of Water

Classical nonpolarizable water models play a crucial role in computer simulations due to their simplicity and computational efficiency. However, the neglect of explicit polarization can jeopardize their accuracy and predictive capabilities, particularly for properties that involve a change in electrostatic environment (e.g., phase changes). In order to mitigate this intrinsic shortcoming, highly simplified analytical polarization corrections describing the distortion of the molecular dipole are commonly applied in force field development and validation. In this paper, we perform molecular dynamics simulations and thermodynamic integration to show that applying the current state-of-the-art polarization corrections leads to a systematic inability of current nonpolarizable water models to simultaneously predict the experimental enthalpy of vaporization and hydration free energy. We go on to extend existing theories of polarization and combine them with data from recent ab initio molecular dynamics simulations to obtain a better estimate of the real contribution of polarization to phase-change energies and free energies. Our results show that for strongly polar molecules like water, the overall polarization correction is close to zero, resulting from a cancellation of multipole distortion and purely electronic polarization effects. In light of these findings, we suggest that parametrization of classical nonpolarizable models of water should be revisited in an attempt to simultaneously describe phase-change energetics and other thermodynamic and structural properties of the liquid.