The membrane must be large enough to capture the correct transport phenomena without introducing system size effects, while avoiding excessive computational costs. To investigate this, membranes of three different sizes were constructed: 6×6×6 nm 3 , 8×8×8 nm 3 , and Figure : Effect of system size on CO 2 /N 2 perm-selectivity and CO 2 permeability in PIM-PI-8. Triangles, squares, and circles correspond to membranes of 6×6×6 nm 3 , 8×8×8 nm 3 , and 10×10×10 nm 3 , respectively. Simulations are performed for three independent membrane configurations for each system size. The error bars represent the standard deviation. The figure also shows additional results for the 6 nm membrane (black sphere). The results correspond to the same configuration of the membrane and initial conditions, and hence represent the discrepancies that arise solely from two independent trajectories for the same structure.