• The number of edges is at most proportional to the number of vertices, because of the limited valence of atoms. Such graphs are generally called sparse. This remains true when vertices represent clusters instead of single atoms, although in that case the number of neighbours can be higher than the maximum valence of an atom -for instance, SBUs with up to 66 points of extension have been built. . • The set of edges is small to medium-sized because the number of vertices per unit cell is, in practice, between 1 and 10,000 in most real-world crystalline structures. This is small compared to the trillion of edges of some graphs in social media modelling for instance .