Based on an event-oriented classification for the different stages of the fusion pore and in resemblance with the biological process of a vesicle about to fuse with the plasma membrane, as schematically shown in figs. 1a and 1b, we have used independent collective variables for membrane fusion (ξ f ), fusion pore nucleation (ξ n ) and expansion (ξ e ), see eqs. 1, 2 and 3. From initially flat and parallel opposed POPC:POPS:PIP 2 bilayers, using restrained molecular dynamics simulations we have sequentially induced the fusion stalk (figure ), a traversing water channel through it (figure ) and its final expansion (figure ).