Dynamical properties across different coarse-grained models for ionic liquids

TL;DR

This paper compares different coarse-grained models of ionic liquids to understand their dynamical properties and transferability, revealing that structure-based models better capture relative diffusion and that dynamical behaviors can be unified onto a single curve.

Contribution

It systematically analyzes five CG models of RTILs, linking their parametrization strategies to dynamical behaviors and proposing a route for constructing dynamically consistent models.

Findings

Limited structural transferability over temperature.

Structure-based models show more consistent cation-anion diffusion.

Dynamical properties can be collapsed onto a single curve.

Abstract

Room-temperature ionic liquids (RTILs) stand out among molecular liquids for their rich physicochemical characteristics, including structural and dynamic heterogeneity. The significance of electrostatic interactions in RTILs results in long characteristic length- and timescales, and has motivated the development of a number of coarse-grained (CG) simulation models. In this study, we aim to better understand the connection between certain CG parametrization strategies and the dynamical properties and transferability of the resulting models. We systematically compare five CG models: a model largely parametrized from experimental thermodynamic observables; a refinement of this model to increase its structural accuracy; and three models that reproduce a given set of structural distribution functions by construction, with varying intramolecular parametrizations and reference temperatures. All five CG models display limited structural transferability over temperature, and also result in various effective dynamical speedup factors, relative to a reference atomistic model. On the other hand, the structure-based CG models tend to result in more consistent cation-anion relative diffusion than the thermodynamic-based models, for a single thermodynamic state point. By linking short- and long-timescale dynamical behaviors, we demonstrate that the varying dynamical properties of the different coarse-grained models can be largely collapsed onto a single curve, which provides evidence for a route to constructing dynamically-consistent CG models of RTILs.