Each molecule was then labelled with the constraints used to enumerate GDB-13 that it breaks (see methods). Figure includes a plot with the percent of molecules that break each constraint (Figure ) and another histogram with the number of constraints broken per molecule (Figure ). The most common broken constraint, not allowed functional groups (26.2 %), is the most complex one to learn, as any given functional group can have multiple SMILES strings, depending on where it is positioned in the molecule, thus making it more difficult to learn the string patterns to avoid. Also, 19.8 % of the molecules have a graph that was filtered during the GDB-13 enumeration process, which correlates with the problems encountered when generating molecules with complex graph topologies: the model is not able to correctly learn the underlying graph topologies of the molecules. Additionally, due to the probabilistic nature of the model, 17.5 % of the molecules generated outside of GDB-13 have more than 13 heavy atoms. Heteroatom / Carbon ratios used to create GDB-13 are generally followed (10.9 %) and there are a similar number (10.1 %) of molecules with disallowed neighboring heteroatom configurations. These constrains can easily be learnt by the model, as they have very little topological complexity compared to the previous two. For the same reason, 9.4 % of the database are tautomers of molecules existing in GDB-13 and less than 7 % of the molecules have problems with double or triple bonds. Interestingly, the miscellaneous category (22.7 %) includes all molecules that are not in GDB-13 and that have broken none of the previous filters. This occurs partially due to problems with the chemical library used (GDB-13 was created with JChem from 2008 and this research uses RDKit from 2018) and because GDB-13 is not completely exhaustive: many molecules would never have been obtained with the enumerative approach used to create the database. Lastly, Figure shows that 72 % of the molecules only break one constraint, hence the chemical space generated outside of GDB-13 is very similar to the space represented by GDB-13.