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.