Revisiting OPLS Force Field Parameters for Ionic Liquid Simulations

Our OPLS-2009IL force field parameters (J. Chem. Theory Comput. 2009, 5, 1038-1050) were originally developed and tested on 68 unique ionic liquids featuring the 1-alkyl-3-methylimidazolium [RMIM], N-alkylpyridinium [RPyr], and choline cations. Experimental validation was limited to densities and a few, largely conflicting, heat of vaporization (ΔH_vap) values reported in the literature at the time. Owing to the use of Monte Carlo as our sampling technique, it was also not possible to investigate the reproduction of dynamics. The [RMIM] OPLS-2009IL parameters have been revisited in this work and adapted for use in molecular dynamics (MD) simulations. In addition, new OPLS-AA parameters have been developed for multiple anions, i.e., AlCl₄^-, BF₄^-, Br^-, Cl^-, NO₃^-, PF₆^-, acetate, benzoate bis(pentafluoroethylsulfonyl)amide, bis(trifluoroethylsulfonyl)amide, dicyanamide, formate, methylsulfate, perchlorate, propanoate, thiocyanate, tricyanomethanide, and trifluoromethanesulfonate. The computed solvent densities, heats of vaporization, viscosities, diffusion coefficients, heat capacities, surface tensions, and other relevant solvent data compared favorably with experiment. A charge scaling of ±0.8 e was also investigated as a means to mimic polarization and charge transfer effects. The 0.8-scaling led to significant improvements for ΔH_vap, surface tension, and self-diffusivity; however, a concern when scaling charges is the potential degradation of local intermolecular interactions at short ranges. Radial distribution functions (RDFs) were used to examine cation-anion interactions when employing 0.8∗OPLS-2009IL and the scaled force field accurately reproduced RDFs from ab initio MD simulations.