The disparity in the number of molecules with many vs few heavy atoms is attributed to the combinatorial explosion of enumerable isomers as the number of heavy atoms increases. In fig. , we plot the distribution of QM9 atoms by their number of heavy 15.8% 67.6% 0.41% The number of reported and unreported molecules from the QM9 dataset, ordered by the number of heavy atoms (i.e. C, O, N, F) in each molecule. QM9 molecules reported in the PubChem or eMolecules 33 databases are considered synthesizable while those not reported in these databases are assumed to be unsynthesizable. Note that the ordinate is on a logarithmic scale; most molecules found in QM9 have higher numbers of heavy atoms. Among all molecules in QM9, 68% have 9 heavy atoms and are unreported, while 16% have 9 heavy atoms and are reported. Only 0.41% of QM9 molecules are reported with 6 heavy atoms, and there are even fewer molecules with formulas involving < 6 heavy atoms.