Direct molecular simulation of gradient-driven diffusion of large molecules using constant pressure

Dual control volume grand canonical molecular dynamics (DCV-GCMD) is a boun dary-driven nonequilibrium molecular-dynamics technique for simulating grad ient-driven diffusion in multicomponent systems. Two control volumes are es tablished at opposite ends of the simulation box. Constant temperature and chemical potential of diffusing species are imposed in the control volumes (i.e., constant-mu(1)... mu(n-1)mu(n) VT). This results in stable chemical potential gradients and steady-state diffusion fluxes in the region between the control volumes. We present results and detailed analysis for a new co nstant-pressure variant of the DCV-GCMD method in which one of the diffusin g species for which a steady-state diffusion flux exists does not have to b e inserted or deleted. Constant temperature, pressure, and chemical potenti al of all diffusing species except one are imposed in the control volumes ( i.e., constant-mu(1) ... mu(n-1)N(n)PT). The constant-pressure method can b e applied to situations in which insertion and deletion of large molecules would be prohibitively difficult. As an example, we used the method to simu late diffusion in a binary mixture of spherical particles with a 2:1 size r atio. Steady-state diffusion fluxes of both diffusing species were establis hed. The constant-pressure diffusion coefficients agreed closely with the r esults of the standard constant-volume calculations. In addition, we show h ow the concentration, chemical potential, and flux profiles can be used to calculate local binary and Maxwell-Stefan diffusion coefficients. In the ca se of the 2:1 size ratio mixture, we found that the binary diffusion coeffi cients were asymmetric and composition dependent, whereas the Maxwell-Stefa n diffusion coefficients changed very little with composition and were symm etric. This last result verified that the Gibbs-Duhem relation was satisfie d locally, thus validating the assumption of local equilibrium. (C) 1999 Am erican Institute of Physics. [S0021-9606(99)50418-2].