To further improve our XRPD model, we created an enlarged unit cell (a supercell) to simulate faulted xanthine structures . Our protocol (described in S6) allows for the random introduction of stacking faults to a Form II supercell made of 20 layers (5 unit cells), creating a model of planar disorder. This inclusion of stacking disorder improved the model, in particular in fitting to the asymmetric peak profiles. This model refined to an Rwp of 1.55% and GooF of 2.46% (Figure ).