Molecular modelling of ionic liquids: General guidelines on fixed-charge force fields for balanced descriptions

It has been increasingly common to investigate dynamic and thermodynamic properties of green solvents at atomistic scales with molecular simulation. In this work, we present a detailed evaluation of the widely used fixed-charge regime, i.e., the combination of scaled RESP charges and GAFF derivatives. The benchmark set contains three ionic liquids formed by cations 1-butyl-3-methylimidazolium and 1-hexyl-3-methylimidazolium and anions bis(trifluoromethylsulfonyl)imide and PF6. For the charge scaling issue, two physical properties including the mass density and the solvation free energies of external agents in ionic liquids are selected as the criteria. Large-scale fast-growth solvation free energy simulations are performed to obtain the solvation thermodynamics, and the results are further combined with hydration data to form a water-ionic-liquids transfer dataset, which considers the partition of solutes between water and ionic liquids. It is observed that the density-derived charge scaling factor is always smaller than that derived from solvation/partition thermodynamics, which supports the use of a slightly larger scaling factor to obtain a balanced description of solute-solvent and solvent-solvent interactions. For the bonded-terms assessment, we refit the transferable GAFF2 derivatives with generalized force-matching in a molecule- and component-specific manner. The results suggest that the bond-stretching and angle-bending terms in GAFF derivatives are often problematic, while the torsional potential shows satisfactory reproduction of ab initio results. Finally, combining the extensive computational perspectives accumulated in our series works, general guidelines for molecular modelling of ionic liquids with fixed-charge force fields are summarized.