1024 lipid bilayers are relatively large and naturally exhibit undulations that render the Z center between both membranes inadequate for calculations. Instead, we have used here the local center Z l of the cylindrical volume of height D and radius R max where the water channel locally forms in the XY plane (see figure ). The position of this cylindrical volume is dynamically calculated at each molecular dynamics step, always centered around the water channel. In this way, large membrane undulations have negligible effects on the Z l position now taken as reference by ξ e for the dynamic location of the horizontal layer (parallel to the membranes) of radius R max and height D.