Computational studies of ionic liquids: size does matter and time too.

Taking the molecular ionic liquid 1-ethyl-3-methylimidazolium triflate as a reference system, the size and time dependence of molecular dynamics simulation studies is analyzed in a systematic way. Based on an all atom force field, trajectories of 70 ns length, covering samples of 8-2000 ion pairs, were generated and analyzed in terms of structure as well as single particle and collective dynamics. Although 50 ion pairs seemed sufficient for structure, at least 500 ion pairs were needed for the correct handling of dynamics. For larger systems a linear regime is found, i.e., the respective dynamical properties are a linear function of the inverse box length. In case of translational diffusion coefficients, this linear relation can be rationalised in hydrodynamic terms. The respective formula is essentially determined by viscosity and the inverse box length. Concerning the time dependence, consistent dynamical properties required a time period of 20-30 ns. Nevertheless, size dependence dominates time dependence and has to be primarily addressed.