A Unified Approach to Derive Atomic Partial Charges and Polarizabilities of Ionic Liquids.

We propose a unified approach to fit simultaneously a set of atomic partial charges and polarizabilities of the polarizable model against the electrostatic potential (ESP) and polarizability. The polarizable model is represented with interactive atomic dipoles with distance-dependent attenuation. For the polarizable model employed in this study, the internal electric field on the polarization sites is fully turned on, and thus allows self-induced dipoles, which persist even for an isolated molecule/ion. By such treatment, the contribution of ESP stems not only from the partial charges but also from the self-induced dipoles, and the atomic partial charges and polarizabilities can be fitted simultaneously against ESP in a unified manner. The fitting with 1-ethyl-3-methylimidazolium (EMIM) and nitrate (NO), a prototypical organic cation and inorganic anion, respectively, that can form ionic liquid, demonstrates that allowance of the self-induced dipoles gives much better fitness. Moreover, test on the total dipole of an EMIM/NO ion pair shows that the agreement with the dipole is also much improved for the polarizable model, which highlights the importance of the polarization effects of ionic liquids.