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

An application of the Gibbs-Duhem integration [D. A. Kofke, J. Chem. P hys., 98, 4149 (1993)] for the direct evaluation of solid-liquid equil ibria by molecular dynamics is presented. The Gibbs-Duhem integration combines the best elements of the Gibbs ensemble Monte Carlo technique and thermodynamic integration. Given conditions of coexistence at one coexistence point, simultaneous bur independent constant pressure-con stant temperature molecular dynamics simulations of each phase are car ried out in succession along saturation lines. In each simulation, the saturated pressure is adjusted to satisfy the Clapeyron equation, a f irst-order nonlinear differential equation that prescribes how the pre ssure must change with the temperature to maintain coexistence. The Cl apeyron equation is solved by the predictor-corrector method. Running averages of enthalpy and density of each phase are used to evaluate th e night-hand side of the Clapeyron equation. The Gibbs-Duhem integrati on method is applied to a two-centre Lennard-Jones system with elongat ion 0.67. The starting coexistence point is determined as the point of intersection of solid and liquid isotherm branches in the pressure vs chemical potential plane.