Multiscale coarse-grained simulations of ionic liquids: comparison of three approaches to derive effective potentials.

A coarse-grained model, with three sets of effective pair potentials for 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) ionic liquid, is introduced and used to study the structural and dynamical properties over extended length and time scales. Three sets of effective pair potentials between coarse-grained beads are obtained using the Newton Inversion and the Iterative Boltzmann Inversion methods, respectively, with different treatment of electrostatic interactions. The coarse-grained simulation results are compared systematically with corresponding atomistic simulation results on several thermodynamical and structural quantities together with charge density distributions. In addition, the scattering and dynamical properties are also calculated and compared to both atomistic simulation results and experimental measurements. While all three sets of the effective potentials provide perfect agreement with the atomistic simulation for radial distribution functions, our analysis shows that in coarse-grained simulations, the long-range electrostatic interactions between ionic groups are important and should be treated explicitly to improve the reliability of other simulation results. With explicit incorporation of electrostatic interactions derived from the Newton Inversion, the coarse-grained potentials provide the most consistent description of thermodynamical, scattering and dynamical properties including their temperature dependence as compared to atomistic simulations. We conclude also that the current atomistic force field should be further improved to meet specific requirements for studying the dynamical properties of the [Bmim][PF6] system over a large temperature range.