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

Dual control volume grand canonical molecular dynamics (DCV-GCMD) is a boundary-driven nonequilibrium molecular-dynamics technique for simulating gradient-driven diffusion in multicomponent systems. Two control volumes are established at opposite ends of the simulation box. Constant temperature and chemical potential of diffusing species are imposed in the control volumes (i.e., constant-μ1⋯μn−1μnVT). 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 constant-pressure variant of the DCV-GCMD method in which one of the diffusing species for which a steady-state diffusion flux exists does not have to be inserted or deleted. Constant temperature, pressure, and chemical potential of all diffusing species except one are imposed in the control volumes (i.e., constant-μ1⋯μn−1NnPT). The constant-pressure method can be applied to situations in which insertion and deletion of large molecules would be prohibitively difficult. As an example, we used the method to simulate diffusion in a binary mixture of spherical particles with a 2:1 size ratio. Steady-state diffusion fluxes of both diffusing species were established. The constant-pressure diffusion coefficients agreed closely with the results of the standard constant-volume calculations. In addition, we show how the concentration, chemical potential, and flux profiles can be used to calculate local binary and Maxwell–Stefan diffusion coefficients. In the case of the 2:1 size ratio mixture, we found that the binary diffusion coefficients were asymmetric and composition dependent, whereas the Maxwell–Stefan diffusion coefficients changed very little with composition and were symmetric. This last result verified that the Gibbs–Duhem relation was satisfied locally, thus validating the assumption of local equilibrium.