On the prediction of transport properties of ionic liquid using 1-n-butylmethylpyridinium tetrafluoroborate as an example

Abstract

The Green–Kubo and Einstein–Helfand approaches are examined for calculating diffusion coefficient, electronic conductivity and shear viscosity of ionic liquid using 1-n-butylmethylpyridinium tetrafluoroborate [C4PY][BF4] as an example. Both methods suffer numerical errors accumulated at long simulation time, resulting divergences in the integrated autocorrelation time (IAT) and nonlinearity in the mean square displacement (MSD). The numerical errors can be reduced using smaller time step in the simulation. By identifying a converged plateau in IAT and a linear segment in MSD both approaches yield consistent predictions. Using a validated force field, the predicted diffusion coefficient and electrical conductivity agree reasonably well with the experimental data. However, the shear viscosity is significantly underestimated. Analysis of the simulation data indicates that a much slow relaxation in the pressure tensor must be considered, which is unfortunately infeasible due to the accumulated numerical error. Alternatively, the non-equilibrium periodic perturbation method shows promising improvement in the prediction.