Permeation of metabolic substrates across biological membranes is a fundamental process in cellular life. This process is largely driven by the concentration gradient of various molecules between the outside and inside of a cell. To meet the need for creating such concentration gradients in MD simulation, and to calculate permeation under natural conditions, we developed a technique in NAMD to continually drive permeant molecules near the periphery of the simulation box across the periodic boundary, which results in a sustained gradient in the center of the simulation system where the membrane is located. This allows for purely diffusive motion of particles across a membrane, enabling one to directly calculate permeability the same way as in experiment. Read more in a recent paper.