Polarisation corrections and the hydration free energy of water

Milne, Andrew W. and Jorge, Miguel (2018) Polarisation corrections and the hydration free energy of water. Journal of Chemical Theory and Computation. ISSN 1549-9618 (https://doi.org/10.1021/acs.jctc.8b01115)

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Classical non-polarisable water models play a crucial role in computer simulations due to their simplicity and computational efficiency. However, the neglect of explicit polarisation can jeopardise 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 polarisation 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 polarisation corrections leads to a systematic inability of current non-polarisable water models to simultaneously predict the experimental enthalpy of vaporisation and hydration free energy. We go on to extend existing theories of polarisation and combine them with data from recent ab initio molecular dynamics simulations to obtain a better estimate of the real contribution of polarisation to phase-change energies and free energies. Our results show that for strongly polar molecules like water, the overall polarisation correction is close to zero, resulting from a cancellation of multipole distortion and purely electronic polarisation effects. In light of these findings, we suggest that parametrisation of classical non-polarisable models of water should be revisited in an attempt to simultaneously describe phase-change energetics and other thermodynamic and structural properties of the liquid.