Crystalline diversity. Figure shows a comparison of the distribution of crystallographic space groups for most small molecules in the CSD to the rigid-molecule subset selected for our CSP survey. There is no significant difference in the relative occurrence of different crystalline symmetries between the two sets, though the relative ordering of space groups by frequency varies slightly. For example, space group 15 (C2/c) is observed slightly more frequently in our rigid set and space group 2 (P 1) slightly less so. Regardless, our rigid molecule set is reasonably representative of the range of crystal packing symmetries observed in the CSD. In contrast, our rigid molecule set displays a diminished frequency of hydrogen bonding (H-bonding) in the observed crystal structures compared to the CSD more generally, as might be expected by excluding rotatable bonds. 26.7% (277) of crystal structures of molecules in our subset contain at least one intermolecular H-bond. This compares to 62.3% of structures of molecules of similar size without rotatable bond restrictions, demonstrating the bias introduced by the omission of common H-bonding groups due to their flexibility, including alcohols ( -OH), carboxylic acids ( -COOH), and primary amines ( -NH 2 ). However, the proportion of H-bonded systems is still significant enough that meaningful H-bond chemistry is incorporated in our survey, albeit underrepresented. Consequently, our set in turn overrrepresents chemistry such as π-π stacking and "weak" (i.e. more isotropic, less localised) interactions, and assessments of our CSP energy model's performance must be made with these biases in mind.