For theoreticians, ionic liquids represent a major challenge. This is due to the fact that the intermolecular interactions are particularly strong and long ranged. This, in turn, causes a subtle interplay between different scales which is encoded in the measured macro- and mesoscopic properties and also in the molecular electrostatic characteristics. Therefore, force fields have to describe the microscopic processes correctly in order to reproduce macroscopic properties accurately over a large range of state variables. We will present our multiscaling approach to refine  classical force fields, taking as example imidazolium-based ionic liquids. Our approach extracts the Coulomb interactions from the detailed quantum electronic scale. In particular, we adress the issue of deriving suitable partial charges for usage in classical force fields. To that end we exployed the Bl�chl method to generate partial charges that reproduce the multipole distribution accurately for bulk systems. This led naturally to values of the absolute ionic charges of less than |1 e| , i.e. charge scaling. The monopole structure of the thereby introduced force field mimicks the quantum chemical behaviour observed in liquid phase. This led to a substantial improvement in the description of dynamical properties of immediate experimental interest as e.g. electric conductivity. However due to the drastic change in the partial charge distribution, also the short range interactions have to be reparametrized. As the dynamical properties of the liquids are dominated by the electrostatic interactions, short range parametrization is only performed for static properties, as the density of mass and the radial distribution functions of the liquid-phase ab-inito MD, allowing to avoid computationally expensive simulations. Hence, our scale-combined study offers a gateway to rational design of models, based on the relevant underlying physics rather than on mere numerical parametrisation, and thereby to (possibly) more direct physical interpretation of experimental results.

Ionic liquids studied across different scales: A computational perspective

243rd National Meeting (2012)

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