Dual control volume grand canonical molecular dynamics was a boundary-driven non-equilibrium molecular-dynamics technique for simulating gradient-driven diffusion in multi-component systems. Two control volumes were established at opposite ends of the simulation box. Constant temperature and chemical potential of diffusing species were 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. results and detailed analysis were presented for a constant-pressure variant of the dual control volume grand canonical molecular dynamics method in which one of the diffusing species for which a steady-state diffusion flux existed did not have to be inserted or deleted. Constant temperature, pressure, and chemical potential of all diffusing species except one were imposed in the control volumes (i.e., constant-μ1⋯μn−1NnPT). The constant-pressure method could be applied to situations in which insertion and deletion of large molecules would be prohibitively difficult. As an example, the method was used 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, it was shown how the concentration, chemical potential and flux profiles could be used to calculate local binary and Maxwell–Stefan diffusion coefficients. In the case of the 2:1 size ratio mixture, it was found that the binary diffusion coefficients were asymmetrical and composition-dependent whereas the Maxwell–Stefan diffusion coefficients changed very little with composition and were symmetrical. This last result verified that the Gibbs–Duhem relation was satisfied locally, thus validating the assumption of local equilibrium.

Direct Molecular Simulation of Gradient-Driven Diffusion of Large Molecules using Constant Pressure. Thompson, A.P., Heffelfinger, G.S.: Journal of Chemical Physics, 1999, 110, 10693