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: http://doi.org/10.1021/acs.jctc.9b0068910.1021/acs.jctc.9b00689),
is extended to electrolytes, protic ionic liquids (PIL), deep eutectic
solvents (DES), 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
(TT) function to dampen, or smear, the interactions between charges
and induced dipole at a 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 PIL, for which the literature
contains conflicting views. We describe the implementation of the
TT damping function, of the temperature-grouped Nosé–Hoover
thermostat for polarizable molecular dynamics (MD) and of the periodic
perturbation method for viscosity evaluation from non-equilibrium
trajectories in the LAMMPS MD 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 PILs, DES, and electrolytes.