To avoid waters from permeating the leaflets and changing the hydration of the inner layer, we applied flat-bottomed position restraints to all water beads. The reference positions of all flat-bottomed position restraints were taken as the center of the box along z. The force constant for the quadratic potential was set to 100 kJ mol -1 nm -2 . Together, these flat-bottomed position restraints potentials allowed normal diffusion and fluctuation of water in both compartments and applied only if water beads deeply penetrated the hydrophobic membrane.