  We present a new method for introducing stable non-equilibrium concentration
gradients in molecular dynamics simulations of mixtures. This method extends
earlier Reverse Non-Equilibrium Molecular Dynamics (RNEMD) methods which use
kinetic energy scaling moves to create temperature or velocity gradients. In
the new scaled particle flux (SPF-RNEMD) algorithm, energies and forces are
computed simultaneously for a molecule existing in two non-adjacent regions of
a simulation box, and the system evolves under a linear combination of these
interactions. A continuously increasing particle scaling variable is
responsible for migration of the molecule between the regions as the simulation
progresses, allowing for simulations under an applied particle flux. To test
the method, we investigate diffusivity in mixtures of identical, but
distinguishable particles, and in a simple mixture of multiple Lennard-Jones
particles. The resulting concentration gradients provide Fick diffusion
constants for mixtures. We also discuss using the new method to obtain coupled
transport properties using simultaneous particle and thermal fluxes to compute
the temperature dependence of the diffusion coefficient and activation energies
for diffusion from a single simulation. Lastly, we demonstrate the use of this
new method in interfacial systems by computing the diffusive permeability for a
molecular fluid moving through a nanoporous graphene membrane.
