The Excessive Nature Of The Molecular Simulation Of Model Fluids And Activity Coefficient Model

The calculation of the excess Gibbs free energy G~E is a challenging topic in molecular simulation. A formula for calculating the excess Gibbs free energy with the test particle removal method has been derived in this paper. Simulated results show that this formula is suitable for the mixtures with equal molecular energy parameters but different molecular sizes, in particular. It can also be used to describe mixtures with different molecular energy parameters qualitively. Moreover, this method substantially reduce the effort in program design and computing time.Long—range corrections for theenergy, pressure and chemical potential for linear and sphere molecular system have been derived and used for molecular simulation.The effect of composition on the excess Gibbs free energy, internal energy and density for Square-well fluid has been calculated with the Monte Carlo method.The Monte Carlo NPT ensemble method has been applied to the calculation of the internal energies, densities and excess properties of the Lennard-Jones fluid mixtures under different temperatures, pressures, compositions, molecular size parameters and interaction energy parameters.The Wilson, NRTL, Wang-Chao and UNIQUAC local composition activity coefficient models have been used to calculate G~Efor the LJ simple fluid mixtures. Their values have been compared with those from the Monte Carlo simulation. The results show that the method combining the UNIQUAC model and the Wang—Liang coordination number expression are more accurate for theLJ fluid mixtures with equal molecular energy parameters than the original model with Z = 10.For systems with equal molecular sizes, the method combining the Wang-Chao model, Wang—Liang coordination number expression and its local composition defination is the best one to calculate G~Ecompared with the methods used here.