Same or different-that is the question: Identification of crystal forms from crystal structure data

The comparison of new structural data with crystal structures stored in the Cambridge Structural Database (CSD) is becoming ordinary when dealing with recognition and identification of original solid forms. Indeed, this is a consequence of the ever-increasing number of deposited crystal structures and of the development of new structural comparison tools. We performed a CSD search for single-component crystal structures of organic compounds, resulting in 8102 refcode families with more than one entry. The crystal structure similarity (CSS) and the powder pattern similarity (PXS) tools implemented in the CSD Materials module of Mercury were used to quantitatively assess the degree of similarity for pairs of structures in each family, in order to identify potential polymorphs or structure redeterminations. A total of 47422 pairwise comparisons shows that in most cases (84%) both comparison methods agree in discriminating between the two categories. An analysis of selected examples for which the comparison methods disagree (16%) reveals that ambiguity is due to several reasons including experimental conditions employed for diffraction data collections (i.e. temperature and/or pressure) or crystallographic errors during structure solution. For those borderline cases, the decision to designate a pair of entries as redeterminations of the same structure or polymorphs still relies on the combination of several comparison techniques performed by an expert practitioner.