STSM to Vienna: MD simulations of ionic liquid mixtures with polarizable force fi elds

Generic force fields are widely used in molecular level computations. However, in real physical systems, the magnitude and localization of the charges are variable due to the interatomic polarization and the molecular dynamics. There are two ways to take these into account: (i) to reduce the atomic charges to non-integer values or (ii) to include polarizableforces.

In this STSM the latter method was used adding Drude oscillators to the system to add the polarizability to the particles. The polarizable force fields (FFs) developed by the group of Prof. Schroder where used to simulate the mixtures of ionic liquids with classical molecular dynamics. Namely mixture of [EMIm][BF4] and [EMIm]I was used. For comparison, the same system with non-polarizable FFs and the system of pure [EMIm][BF4] using polarizable FFs were simulated.

The main problem with classical MD simulations of ions is that the viscosity is overestimated which comes from the fact of imprecise non-bonding interactions. The polarizable FFs estimate it better and taken from the simulations the diffusion constant for the same system with polarizable FFs was much higher than for non-polarizable (1.03 A2/ns and 0.48 A2/ns respectively for cations). The system of pure [EMIm][BF4] showed that iodine affects the movement of ions as the diffusion constant differs by 10% (1.15 A2/ns for cations in pure system). All in all, the polarizable FFs improves the system and adding the small amount of iodine does not change the diffusion nor the structure that much.