USING MOLECULAR-DYNAMICS TO OBTAIN MAXWELL-STEFAN DIFFUSION-COEFFICIENTS IN LIQUID-SYSTEMS

Two methods are compared for the calculation of Maxwell-Stefan diffusi on coefficients. The first method is a non-equilibrium molecular dynam ics (NEMD) algorithm, in which the system is driven away from equilibr ium and the system response is monitored. The second method is the equ ilibrium molecular dynamics (EMD) calculation of the appropriate Green -Kubo equation. Simulations were performed for systems of 250 and 300 Lennard-Jones particles at various densities and temperatures. The sys tems were divided into two or three components by attaching a colour l abel to the particles. Since a colour label plays no role in the dynam ics, the Maxwell-Stefan diffusion coefficients of the binary and terna ry systems are equal to the self-diffusion coefficient. In dense fluid s, the system response to an external perturbation is not a first-orde r process, and the diffusion coefficients cannot be determined from th e short term response in the NEMD method. Only the long term response can be used, after a steady state has been reached. In binary systems the Maxwell-Stefan diffusion coefficients, determined by the Green-Kub o (EMD) method, are more accurate than the NEMD coefficients. Since in the NEMD method only the long term response can be used, the Green-Ku bo method is also less time consuming and is therefore preferred for t he calculation of the Maxwell-Stefan diffusion coefficients. In ternar y systems the Green-Kubo method is tested for the 250 particle system. The Maxwell-Stefan diffusion coefficients agree well with the selfdif fusion coefficient. For low mole fractions of the coloured components the diffusion coefficients were less accurate.