Extended Data Figure 7: MD simulations for a 9-Å-high graphene capillary.

a, b, Top (a) and side (b) views of a bilayer of water formed at P = 0.5 GPa. Different colours in a correspond to different vertical positions of water molecules in different layers: dark blue and red mark the bottom and top positions, respectively and lighter colours correspond to intermediate positions. Although the water molecules are clearly arranged in two layers, no intralayer ordering is present and hydrogen bonds preserve their tetrahedral coordination as in bulk water. c, d, As in a and b but at 1.0 GPa. Ordered bilayer ice is formed, with identical square lattices in the two layers, as illustrated by snapshots of the top and bottom layers in e. The ordering is accompanied by switching hydrogen bonds to in-plane coordination. f, g, Schematic illustration of AB and AA stacking. The stacking found in c and d is AB.