Lattice Density Functional Theory (LDFT) has been used to develop a general property whose gradient is linear with the flux of a species. The path-dependent property, Γ_n, appears in LDFT equations as the impingement rate of n-molecules onto the malleable constituents in the surroundings and it is proposed that the gradient of Γ_n is the driving force for diffusion. Malleable constituents include vacancies and molecules of a species whose density gradients can be influenced by diffusion. In this work, Molecular Dynamics simulations are used to study the effect molecule mobility has on the flux of a diffusing species. Simulation results for “color” counter-diffusion (half of the molecules of species n are labeled A and half are labeled B) are compared to the simulation results of A molecules diffusing through static B molecules. These conditions are analogous to comparing the diffusion of species A through an amorphous polymer membrane and through a crystalline polymer membrane. Comparisons between simulation and theory will be discussed.