Coarse-Grained Force Field for Imidazolium-Based Ionic Liquids

We develop coarse-grained force fields (CGFFs) for computationally efficient and accurate molecular simulation of imidazolium-based ionic liquids. To obtain CGFF parameters, we employ a systematic coarse-graining approach based on the relative entropy (RE) method to reproduce not only the structure but also the thermodynamic properties of the reference all-atom molecular model. Our systematic coarse-graining approach adds a constraint to the RE minimization using the Lagrange multiplier method in order to reproduce thermodynamic properties such as pressure. The Boltzmann inversion technique is used to obtain the bonded interactions, and the non-bonded and long-range electrostatic interactions are obtained using the constrained relative entropy method. The structure and pressure obtained from the coarse-grained (CG) models for different alkyl chain lengths are in agreement with the all-atom molecular dynamics simulations at different thermodynamic states. We also find that the dynamical properties, such as diffusion, of the CG model preserve the faster dynamics of bulky cation compared to the anion. The methodology developed here for reproduction of thermodynamic properties and treatment of long-range Coulombic interactions is applicable to other soft-matter.