A simple, polarizable, rigid, 3-point water model using the direct polarization approximation

We present dPol, a 3-point, rigid, polarizable water model that uses the direct approximation of polarization. We show that, with a moderate computational cost (~3x slower than TIP3P), dPol achieves additional accuracy over widely used non-polarizable 3-point rigid water models. Unlike most polarizable force fields, dPol allows use of a 2fs time-step with a conventional molecular dynamics integrator. The partial charges and polarizabilities used in dPol are derived from quantum chemistry calculations, while the Lennard-Jones parameters and geometry are adjusted to reproduce liquid properties under ambient conditions. The final dPol water model reproduces key room temperature physical properties used in training, with heat of vaporization 10.43 kcal/mol, dielectric constant 80.7, high-frequency dielectric constant 1.60, molecular polarizability 1.41 A$^3$, gas-phase dipole moment 1.89 D, and liquid-phase dipole moment 2.2-3.1 D. Importantly, dPol also closely reproduces properties outside the training set, including the oxygen-oxygen radial distribution function of liquid water, as well as the self-diffusion coefficient (2.3 x $10^{-5} cm^2 s^{-1}$) and shear viscosity (0.87 mPa s). Predicted temperature-dependent properties are also largely reproduced; although dPol does not correctly place the density maximum, this is not expected to impede successful application of the model to biomolecular systems near room temperature. The dPol water model is, by design, compatible with our AM1-BCC-dPol polarizable electrostatic model for small organic molecules [DOI: 10.1021/acs.jctc.3c01171]. These models in combination establish a foundation for the integration of electronic polarizability into efficient force fields for heterogeneous systems of biological and pharmaceutical interest.