For future reference, we label the 95k molecular compounds that were contained in QM9 and successfully processed by the DFTB calculations to generate their optimized molecular geometries and associated HOMO-LUMO gaps as the "original data set". As described in Section 4, this data set was split into training and test data sets. Figure shows the comparison of the HOMO-LUMO gap as predicted by DFTB for the molecules contained in the original data set (blue) as well as for the newly generated ones (orange). At first glance it becomes apparent that the generated molecules possess much lower HOMO-LUMO gap values, following the user-defined constraint. It further appears that, while the molecules in the original data set show a multi-modal distribution of their HOMO-LUMO gaps corresponding to the different molecular classes (aliphatic molecules > olefinic molecules > conjugated molecules > molecules with double bonds and strained rings), the new predicted molecules have a more concentrated distribution centered near the 3-3.5 eV (see Figs. and). The latter figure likewise indicates that most of the newly generated molecules contain more than 9 atoms, and that such structures were not at all included in the original training set of the GCNN surrogate. We do note that the generated molecules show a tendency towards including small, strained rings with double bonds included, such as three-and four-membered rings that are often fused to larger rings. Such molecules derive their small HOMO-LUMO gaps from their ring strains which typically pushes the HOMO levels up and the LUMO levels down, reducing the HOMO-LUMO gap , but at the same time increasing their chemical reactivity and the difficulty of synthesis.