The structure of the primitive phase, and therefore the x1, x2 and x3 axis frame, is rotated into three different orientations within the x,y,z frame of the unit cell: with the [111], [100], and [110] directions perpendicular to the film surface (i.e. in the z direction). These three orientations were also used in our previous work on the diamond and gyroid phases At the interface with the external "water", the structure of the primitive phase is curved in on itself to maintain a continuous bilayer without exposed edges; this process closes off the water channels on one side of the bilayer . We achieve this by setting 𝜙 = ±1 in the external phase, at one end of cuboid unit cell (in the z direction). We apply a fixed boundary condition in the z direction and periodic boundary conditions in the x and y directions, and calculate and minimise the functional