The overall layer-normal vector was determined using a set of N local layer-normal vectors determined using a suitable reference atom in each molecule, as identified in the main text. The local layer-normal vector of each molecule was determined by combining the reference atom position with those of the equivalent reference atoms in surrounding molecules that were within a cutoff distance of 1 nm, fitting a plane through the set of reference atoms, and then the local layer-normal vector is a normal vector to that plane. The overall layer-normal vector was then determined for each trajectory frame as the eigenvector associated with the largest eigenvalue of an ordering tensor equivalent to Equation 1 but assembled from the set of local layer-normal vectors determined for that frame.