Development of United-Atom Force Fields for Monomeric and Oligomeric Ionic Liquids through Regression-Guided...

Ionic liquids are salts that exist in the liquid state at room temperature and exhibit high viscosity because of their strong electrostatic interactions. It was difficult to reproduce their viscosity by molecular dynamics simulations with conventional nonpolarizable force fields; however, recent development of force fields implementing electronic continuum correction (ECC), which accounts for polarizability, has enabled accurate predictions. Here, we present a regression-guided strategy to optimize scaling factors for ECC charges and Lennard-Jones parameters for monomeric (BMIM⁺, MOEMIM⁺) and oligomeric (IL2²⁺, IL4⁴⁺) imidazolium-based cations paired with TFSI^- with united-atom models. The scaling factors were optimized to simultaneously reproduce experimental density and viscosity. To validate the strategy, we calculated the temperature dependence of density, diffusion coefficient, conductivity, and viscosity of BMIM-TFSI, achieving good agreements with experiments. Moreover, scaling factors optimized for MOEMIM⁺, which shares similar chemical structure and elemental compositions with IL2²⁺ and IL4⁴⁺, were found to be transferable to these compounds. Thus, this work not only provides a practical regression-guided workflow for selecting ECC-based united-atom force-field parameters but also suggests their transferability across chemically related ionic liquids.