Force Field And The Transport Properties Computation Of Ionic Liquids

As the rapidly increase of computer and molecular simulation techniques,the molecular simulation has been applicated in more and more research field.However,some problems limitated the applications of molecular simulation,including accuracy of the force field and the time-and length scale.The goal of this paper is on the development of classical force field for ionic liquids?ILs?and investigating how to predict transport properties of ILs.We extend TEAM force fields to cover ILs system by using 30 kinds of ILs as training set.Two different equilibrium molecular dynamical?MD?methods including Green-Kubo relation and Einstein-Helfand relation have been investigated on calculating the transport properties?self-diffusion,shear viscosity,conductivity?,using[C₄PY][BF₄]as sample.For shear viscosity,a non-equilibrium method?periodic perturbation method?PPM??also has been studied.Firstly,we developed an accurate and reliable all-atom force field for ILs based on TEAM force field.The philosophy of TEAM development is transferability,extensibility,accuracy and modularization.The definition of the atom type is a key factor that determines the completeness and transferability.The nonbond parameters are critical for predicting condense phase properties.With the valence and charge parameters determined using quantum chemistry data,the Van der Waals parameters are usually optimized using liquid data?density and enthalpy of vaporization?.Our results show the force field parameters can accurately predict molecular structures,energies,and liquid properties.The deviation between experimental and calculated are 2%and 5%for density and enthalpy of vaporization,respectively.Using[C₄PY][BF₄]as example,we evaluated two equilibrium MD method,namely Green-Kubo relation,Einstein-Helfand function,in calculating transport properties including self-diffusion coefficient,electronic conductivity,and shear viscosity.By using 30 independent samle and moderate data period and save frequency,two method give consistent results.For diffusion coefficient and electronic conductivity,calculated values are in agreement with experimental data.However,for shear viscosity the calculated values are less than experimental data by 50%.A non-equilibrium method?Periodic Perturbation Method?PPM??also was used to calculated shear viscosity.However,different model give different value with large deviation.