We have used the amount of water molecules inside the cylinder that expands the fusion pore to directly estimate its radius (see figure ). Martini W beads randomly accommodate in the available space in the most compact way, which results in a maximum density of ∼63. 33 per cent (see supplementary information). Theoretically, the most compact way to pack spheres (random close packing) results in a maximum density of ∼64 per cent. Following a statistical description of jammed states, Makse and collaborators showed that random packing of hard spheres in three dimensions cannot exceed a density limit of ∼63.4 per cent. This last result is in excellent agreement with the behavior of Martini beads under unbiased conditions in an NPT equilibrated water box (see supporting information). Therefore, in the Martini space, any volume V occupied by N water beads is the sum of all the individual volumes � N 1 v i divided by the 0.634 random packing correction, V = 1 0.634