Application of Static Charge Transfer within an Ionic‐Liquid Force Field and Its Effect on Structure and Dynamics

Abstract

The effects of linear scaling of the atomic charges of a reference potential on the structure, dynamics, and energetics of the ionic liquid 1,3-dimethylimidazolium chloride are investigated. Diffusion coefficients that span over four orders of magnitude are observed between the original model and a scaled model in which the ionic charges are +/-0.5 e. While the three-dimensional structure of the liquid is less affected, the partial radial distribution functions change markedly--with the positive result that for ionic charges of +/-0.7 e, an excellent agreement is observed with ab initio molecular dynamics data. Cohesive energy densities calculated from these partial-charge models are also in better agreement with those calculated from the ab initio data. We postulate that ionic-liquid models in which the ionic charges are assumed to be +/-1 e overestimate the intermolecular attractions between ions, which results in overstructuring, slow dynamics, and increased cohesive energy densities. The use of scaled-charge sets may be of benefit in the simulation of these systems--especially when looking at properties beyond liquid structure--thus providing an alternative to computationally expensive polarisable force fields.

Intrinsic Structure of the Interface of Partially Miscible Fluids: An Application to Ionic Liquids

“…Later on, many studies have suggested that using integer charges leads to a system much too cohesive probably due to the exaggerated ionic character that fixed point charge models (lacking sufficient level of sophistication) can provide. [53][54][55][56] In reality, the bulky ions of ionic liquids have more molecular character due to their high polarizability and the high degree of delocalization of their charge density. It has also been shown that the reduced ionic character can be accounted for in an effective way by setting ionic net charges, in absolute terms, to less than unity leading to the emergence of a series of new "reduced charge models".…”

“…It has also been shown that the reduced ionic character can be accounted for in an effective way by setting ionic net charges, in absolute terms, to less than unity leading to the emergence of a series of new "reduced charge models". 17,[53][54][55][56] Note that these models treat the effect of polarization in a simplified manner by fixed reduced ionic charges which makes the model unable to adapt to local 8 changes of the electric field. As the degree of polarization varies significantly from one ionic liquid to another these modified force fields are designed to accurately describe one single ionic liquid species, thus precluding general transferability.…”

Improved United-Atom Force Field for 1-Alkyl-3-methylimidazolium Chloride

“…Youngs et al showed 41 that a reduction from 1.0 to 0.8 resulted in about a 5% decrease in the simulated density of [C 1 mim][Cl]. Thus, additional adjustment of LJ parameters is necessary.…”

Improved United-Atom Force Field for 1-Alkyl-3-methylimidazolium Chloride

“…For example, even the simple point charge model for water yields very good predictions of both structural and dynamic properties after optimization of a few parameters. 38 Many recent studies 35,[39][40][41][42][43][44][45][46][47][48] have shown that force fields of ILs that do not contain a polarization term, and charges fitted via ab initio calculations of the isolated ions, usually result in substantially lower predictions of transport properties and overestimation of the heat of vaporization. For example, the self-diffusion coefficient of [C 2 mim][BF 4 ] is underestimated by about an order of magnitude by two widely used force fields.…”