  We use a novel non-equilibrium algorithm to simulate steady-state fluid
transport through a two-dimensional (2D) membrane due to a concentration
gradient by molecular dynamics (MD) for the first time. We confirm that, as
required by the Onsager reciprocal relations in the linear-response regime, the
solution flux obtained using this algorithm agrees with the excess solute flux
obtained from an established non-equilibrium MD algorithm for pressure-driven
flow. In addition, we show that the concentration-gradient solution flux in
this regime is quantified far more efficiently by explicitly applying a
transmembrane concentration difference using our algorithm than by applying
Onsager reciprocity to pressure-driven flow. The simulated fluid fluxes are
captured with reasonable quantitative accuracy by our previously derived
continuum theory of concentration-gradient-driven fluid transport through a 2D
membrane [J. Chem. Phys. 151, 044705 (2019)] for a wide range of solution and
membrane parameters even though the simulated pore sizes are only several times
the size of the fluid particles. The simulations deviate from the theory
especially for strong solute--membrane interactions relative to the thermal
energy, for which the theoretical approximations break down. Our findings will
be beneficial for molecular-level understanding of fluid transport driven by
concentration gradients through membranes made from 2D materials, which have
diverse applications in energy harvesting, molecular separations, and
biosensing.
