To further understand how molecules are generated, the composition of the SMILES was analyzed. As shown in Figure (dashed orange line), the 1-gram (token) count distribution is exponential and mostly features C (40 %). In order, less featured tokens are 1, N, =, (, ), O and 2. The rest of the tokens sum up to less than 7% of the total. SMILES representing simple topologies use mostly the tokens enumerated before and molecules that have complex shapes tend to have more rings, so they have less common tokens, such as 3,4, …, 7. The frequency of the molecules containing each token is also plotted in Figure , showing that the frequency correlates with the counts. Note especially, marked in red in Figure , the numeric tokens starting from 4 tend to have a significantly lower average frequency than the neighboring tokens. This means that molecules in GDB-13 with 4 or more rings are significantly less likely to be sampled than others.