DIRECT EVALUATION OF VAPOR-LIQUID-EQUILIBRIA OF MIXTURES BY MOLECULAR-DYNAMICS USING GIBBS-DUHEM INTEGRATION

We present an extension of the Gibbs-Duhem integration method that per mits direct evaluation of vapour-liquid equilibria of mixtures by mole cular dynamics. The Gibbs-Duhem integration combines the best elements of the Gibbs ensemble Monte Carlo technique and thermodynamic integra tion. Given conditions of coexistence of pure substances, simultaneous but independent molecular dynamics simulations of each phase at const ant number of particles, constant pressure, constant temperature and c onstant fugacity fraction of species 2 are carried out in succession a long coexistence lines. In each simulation, the coexistence pressure i s adjusted to satisfy the Clapeyron-type equation. The Clapeyron-type equation is a first-order nonlinear differential equation that prescri bes how the pressure must change with the fugacity fraction of species 2 to maintain coexistence at constant temperature. The Clapeyron-type equation is solved by the predictor-corrector method. Running average s of mole fraction and compressibility factor for the two phases are u sed to evaluate the right-hand side of the Clapeyron-type equation. Th e Gibbs-Duhem integration method is applied to three prototypes of bin ary mixtures of the two-centre Lennard-Jones fluid having various elon gations. The starting points on the coexistence curve were taken from published data.