The CL&Pol polarizable force field for the simulation of ionic liquids and eutectic solvents

Molecular dynamics (MD) simulation, and theoretical chemistry in general, have been central tools in the study of ionic liquids (ILs) and eutectic solvents, both of which are fluid phases dominated by ionic interactions and strong hydrogen bonds. These systems are still relatively recent, so the amount of experimental information given their enormous diversity is still scarce. Computational studies have produced several important discoveries, besides helping in the interpretation of experimental findings. Here we review the latest developments in the models describing the interactions and conformations of IL phases with atomic detail, in particular polarizable force fields. In these all-atom models, the response of electron clouds to their electrostatic environment is represented by induced dipoles, therefore moving beyond the pair-wise additivity of fixed-charge force fields. Most of the conceptual framework to develop polarizable force fields has been available and implemented in major MD codes, so the main challenge to develop a useful polarizable model for ILs and eutectic solvents is how to handle their diversity in chemical structures and interactions, in order to build a model that can represent solutions, mixtures and interfaces with materials, and that is not too difficult to extend to new systems. As is often the case, force field development is not only science but it is also an art! The ingredients of successful force fields are often the result of pragmatism and heuristics, rather than mathematical complexity and absolutely rigorous physics (although retaining a sound physical basis in the parameters definitely helps). We focus on the CL&Pol polarizable force field for ILs, protic ionic liquids, and eutectic solvents, improving on previous-generation fixed charge models. The CL&Pol force field is built as a detailed but extendable and transferable model offering more reliable predictions of thermodynamic, structural, and transport properties, which should contribute to the advancement of the field and towards the design of better solvents, electrolytes, lubricants, and so on. This article is categorized under: Molecular and Statistical Mechanics > Molecular Interactions Molecular and Statistical Mechanics > Molecular Dynamics and Monte-Carlo Methods