Generation of thermodynamic data for organic liquid mixtures from molecular simulations

Fluctuation solution theory (FST) is employed to analyze results of molecular dynamics (MD) simulations of liquid mixtures. The objective is to generate parameters for macroscopic thermodynamic property models. Two benchmark systems, benzene-methyl acetate at 303.15 K and benzene-ethanol at 298.15 K, are used. MD simulations are performed in the isobaric-isothermal ensemble (NPT) at the respective temperatures and at a pressure of 1 atm. We use the CHARMM27 force field at different mixing ratios. We sample positions to determine the binary (between the centers-of-mass of molecules of a pair) radial distribution functions (RDFs). The RDFs are integrated to give the total correlation function integrals (TCFIs). Errors in TCFIs due to uncertainties in RDFs from simulation have been overcome by introducing a simple expression to reproduce the indirect interactions allowing reliable extrapolation to infinite distances. We compare the results of our computations with measured data on both systems studied. The results for activity coefficients agree to within experimental uncertainty.