Towards multiscale modeling of ionic liquids: From electronic structure to bulk properties

In this paper a multiscale approach for the description of ionic liquids and first results according to this scheme are presented. The basic idea is to combine post-Hartree-Fock calculations (pHF), density functional theory (DFT) and classical all-atomistic molecular dynamics simulations to go from individual molecules to bulk properties in a setup that is able to bridge the large time and length scale gaps involved. The idea is initially applied to a very simple, but well studied ionic liquid compound, namely dimethylimidazolium chloride. Results obtained on the post-Hartree-Fock level for one ion pair are compared to Car-Parrinello density functional results. Subsequently density functional results for several ion pairs up to 30 ion pairs are determined, that are intended to approach bulk features. The DFT results are finally compared to molecular dynamics simulations. A crucial step in this multiscale scheme is the analysis of the electrostatic potentials. The Blöchl approach employed herein on the DFT level allows us to obtain values for partial charges, that are considerably different from the ones used in the standard force fields. The results suggest that the total charge of the cation and the anion should be scaled down to a value of around ± 0.6e to ± 0.8e. The usage of these partial charges within a standard force field yields structural and dynamical results that approach the structural results of the density functional calculations. Although this appears promising, another refinement step of the force field parameters will be necessary, as the density of mass obtained deviates by 15% from the experimental results. Possible further refinements of the force fields are discussed.