Vapour liquid equilibrium of a pure fluid from test particle method in combination with NpT molecular dynamics simulations

Abstract

In order to determine the vapour liquid equilibrium of a pure fluid, the liquid and the vapour branch of the isotherms in the chemical potential μ vs pressure p-diagram, are constructed explicitly. The liquid branch is obtained by molecular dynamics simulations in an NpT-ensemble into which test particles are inserted to calculate the chemical potential. The vapour branch is obtained at lower temperatures by using the second virial coefficient, at higher temperatures it is determined again by simulations. As an example the two-centre Lennard-Jones fluid with elongation L = 0·505 is considered at temperatures ranging from 0·69 to 0·92 of the estimated critical temperature. As expected, the inaccuracies of the liquid chemical potential increase with decreasing temperature as a consequence of the increasing saturated density. The uncertainties in μ/RT range from 0·02 at the highest to 0·10 at the lowest temperature which creates an uncertainty in the reduced vapour pressure Pσ³/ϵ of the order of 0·002. Within that uncertainty, the vapour pressures agree with those obtained previously from perturbation theory. The saturated liquid densities agree within 2 per cent which is consistent with a previous comparison between perturbation theory and experimental results for fluorine. Finally, we note that all simulations were performed with vectorized codes on a CYBER 205.