shows that the most densely packed regions are at the center and occupied by molecules with both a high and a low frequency. Additional PCA plots were generated with some key descriptors that help pinpointing the different regions of the chemical space. Figure shows that pixels at the right have mostly cyclic bonds, implying more rings and fewer sidechains and linkers. This area is mostly covered by molecules that have low frequency. Moreover, Figure shows that pixels at the top have more heteroatoms. This closely matches the top lighter area in Figure , which features molecules with low frequency. From the previous plots, molecules with many heteroatoms or complex topologies have a lower probability of being sampled than molecules with less rings and more carbon atoms. However, Figure also shows that most of these structures are in lower density zones of the database, which implies that are only a small part of the database. In Table , 24 fragment-like molecules with frequency 0, 5, 10, 15, 20 and 25 were selected from GDB-13 and shows that molecules with lower frequency have a tendency to have a more complex structure, especially more cyclic bonds, although it is not possible to separate them clearly.