Improved classical united-atom force field for imidazolium-based ionic liquids: tetrafluoroborate, hexafluorophosphate, methylsulfate, trifluoromethylsulfonate, acetate, trifluoroacetate, and bis(trifluoromethylsulfonyl)amide.

A cost-effective, classical united-atom (UA) force field for ionic liquids (ILs) was proposed, which can be used in simulations of ILs composed by 1-alkyl-3-methyl-imidazolium cations (C(n)mim) and seven kinds of anions, including tetrafluoroborate (BF(4)), hexafluorophosphate (PF(6)), methylsulfate (CH(3)SO(4)), trifluoromethylsulfonate (CF(3)SO(3)), acetate (CH(3)CO(2)), trifluoroacetate (CF(3)CO(2)), and bis(trifluoromethylsulfonyl)amide (NTf(2)). The same strategy in our previous work (J. Phys. Chem. B 2010, 114, 4572) was used to parametrize the force field, in which the effective atom partial charges are fitted by the electrostatic potential surface (ESP) of ion pair dimers to account for the overall effects of polarization in ILs. The total charges (absolute values) on the cation/anion are in the range of 0.64-0.75, which are rescaled to 0.8 for all kinds of ions by a compromise between transferability and accuracy. Extensive molecular dynamics (MD) simulations were performed over a wide range of temperatures to validate the force field, especially on the enthalpies of vaporization (ΔH(vap)) and transport properties, including the self-diffusion coefficient and shear viscosity. The liquid densities were predicted very well for all of the ILs studied in this work with typical deviations of less than 1%. The simulated ΔH(vap) at 298 and 500 K are also in good agreement with the measured values by different experimental methods, with a slight overestimation of about 5 kJ/mol. The influence of ΔC(p) (the difference between the molar heat capacity at constant pressure of the gas and that of liquid) on the calculation of ΔH(vap) is also discussed. The transport coefficients were estimated by the equilibrium MD method using 20-60 ns trajectories to improve the sampling. The proposed force field gives a good description of the self-diffusion coefficients and shear viscosities, which is comparable to the recently developed polarizable force field. Although slightly lower dynamics is found in simulations by our force field, the order of magnitude of the self-diffusion coefficient and viscosity are reproduced for all the ILs very well over a wide temperature range. The largest underestimation of the self-diffusion coefficient is about one-third of the experimental values, while the largest overestimation of the viscosity is about two times the experimental values.