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

Abstract

Molecular dynamics simulation, and theoretical chemistry in general, have been central tools in the study of ionic liquids 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 ionic liquid 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 molecular dynamics codes, so the main challenge to develop a useful polarizable model for ionic liquids and eutectic solvents is how to handle their diversity in chemical structures and interactions, in order 1 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 usually, force field development is not only science: it's 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 ionic liquids, 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, etc.

Computer Simulations of Deep Eutectic Solvents. Challenges, Solutions, and Perspectives

“…So far, almost all MD simulations of DES have been done with nonpolarizable models only. However, efforts are emerging towards simulations with polarizable FFs [121][122][123][124]. They are all based on the classical Drude oscillator model, which will be briefly described next.…”

“…Besides the high computational cost, another main problem with polarizable FFs is their poor transferability. Recently, using the CL&P fixed-charge FF [125] as the basis, Goloviznina et al developed a transferable polarizable FF for ionic liquids [126] and extended it to DESs [122,125]. In order to increase transferability, they applied a fragment approach, which was validated by calculating density, ion diffusion coefficients and viscosity for a range of ionic liquids and their mixtures [126].…”

“…In order to increase transferability, they applied a fragment approach, which was validated by calculating density, ion diffusion coefficients and viscosity for a range of ionic liquids and their mixtures [126]. In DESs, the strong combination of H-bonds and Drude-induced dipoles causes stability problems, which were also addressed by Goloviznina et al [122].…”

“…Atomic polarizabilities are determined by first principle calculations. In CL&Pol [121,122], the force constants of all of the harmonic bonds between the DCs and the DPs are assigned to be 𝑘 = 4184 kJ • mol and the masses of all DPs to 𝑚 =DP=0.4 u. It is also necessary to scale down the Lennard-Jones energy parameter 𝜀 to avoid double counting the polarization effects, since induction effects are implicitly included in the CL&P parameter set [125].…”

“…However, SAPT calculations are computationally intensive, especially if applied to a broad range of compounds. In this context, an alternative predictive scheme was devised by Goloviznina et al [121,122,126] to obtain the scaling factor (kij) for the interaction between fragments i and j,…”