Extension of the CL&Pol Polarizable Force Field to Electrolytes, Protic Ionic Liquids and Deep Eutectic Solvents

The polarizable CL&Pol force field presented in our previous study, Transferable, Polarizable Force Field for Ionic Liquids (J. Chem. Theory Comput. 2019, 15, 5858, DOI: 10.1021/acs.jctc.9b00689), is extended to electrolytes, protic ionic liquids, deep eutectic solvents, and glycols. These systems are problematic in polarizable simulations because they contain either small, highly charged ions or strong hydrogen bonds, which cause trajectory instabilities due to the pull exerted on the induced dipoles. We use a Tang-Toennies function to dampen, or smear, the interactions between charges and induced dipole at short range involving small, highly charged atoms (such as hydrogen or lithium), thus preventing the ``polarization catastrophe''. The new force field gives stable trajectories and is validated through comparison with experimental data on density, viscosity, and ion diffusion coefficients of liquid systems of the above-mentioned classes. The results also shed light on the hydrogen-bonding pattern in ethylammonium nitrate, a protic ionic liquid, for which the literature contains conflicting views. We describe the implementation of the Tang-Toennies damping function, of the temperature-grouped Nosé-Hoover thermostat for polarizable molecular dynamics and of the periodic perturbation method for viscosity evaluation from non-equilibrium trajectories in the LAMMPS molecular dynamics code. The main result of this work is the wider applicability of the CL&Pol polarizable force field to new, important classes of fluids, achieving robust trajectories and a good description of equilibrium and transport properties in challenging systems. The fragment-based approach of CL&Pol will allow ready extension to a wide variety of protic ionic liquids, deep eutectic solvents and electrolytes.