Transferable, Polarizable Force Field for Ionic Liquids.

A general, transferable, polarizable force field for molecular simulation of ionic liquids (ILs) and their mixtures with molecular compounds is developed. This polarizable model is derived from the widely used CL&P fixed-charge force field that describes most families of ILs, in a form compatible with OPLS-AA, one of the major force fields for organic compounds. Models for ILs with fixed, integer-ionic charges lead to pathologically slow dynamics, a problem that is corrected when polarization effects are included explicitly. In the model proposed here, Drude-induced dipoles are used with parameters determined from atomic polarizabilities. The CL&P force field is modified upon inclusion of the Drude dipoles to avoid double-counting of polarization effects. This modification is based on first-principles calculations of the dispersion and induction contributions to the van der Waals interactions using symmetry-adapted perturbation theory (SAPT) for a set of dimers composed of positive, negative, and neutral fragments representative of a wide variety of ILs. The fragment approach provides transferability, allowing the representation of a multitude of cation and anion families, including different functional groups, without the need to reparametrize. Because SAPT calculations are expensive, an alternative predictive scheme was devised, requiring only molecular properties with a clear physical meaning, namely, dipole moments and atomic polarizabilities. The new polarizable force field, CL&Pol, describes a broad set of ILs and their mixtures with molecular compounds and is validated by comparisons with experimental data on density, ion diffusion coefficients, and viscosity. The approaches proposed here can also be applied to the conversion of other fixed-charge force fields into polarizable versions.