A workflow has been implemented to identify chemical functional groups from the molecular structure in two steps: firstly, we save the Cartesian coordinates of molecules in a MDL Molfiles format using the standard implementation in Open Babel 88 . Secondly, Checkmol 89 is employed to detect the functional groups (204 tags) based on the connectivity tree. In total, 61 unique functional groups were detected for the subset of the ≈ 13 k QM7-X-molecules , demonstrating that the dataset covers a considerable sector of CCS (cf. Fig. S 2 of the ESI † ). Since Open Babel predicts valencies only based on the distance between pairs of atoms, the functional group detection scheme is prone to errors for molecules with rare functional groups. Moreover, the functional group definitions of Checkmol have significant overlaps, e.g., the molecules detected as alkylamines are also detected as primary amines. To ensure that these shortcomings do not influence our conclusions, we base our analyses only on the subset of the 14 k molecules that have certain functional groups. These groups are chosen to be chemically important, non-overlapping, and each of these categories contain at least 500 entries. The number of structures containing one of these functional groups is 9604. For the analysis in section 3.3, only molecules containing a single functional group are considered, i.e., 1626 entries of our dataset (see Fig. 3). Based on these constraints, the following eleven classes of molecules are identified: aldehydes, carbonitriles, dialkyl ether, enol ether, hydrazones, ketones, oximes, primary alcohols and amines, as well as secondary alcohols and amines (see labels in Fig. (a)).