Figure shows a two-dimensional embedding of our atom database. Each point correspond to one of the 227,520 atoms, each of which belong to one of 4000 molecules. The color of each point corresponds to the (magnitude) of the atom's ATC value, calculated as described above. The plot was obtained by applying t-distributed stochastic neighbor embedding (tSNE) to the SOAP descriptors of each atom. As can be seen in the plot, the ATC value varies smoothly across the embedding. While some degree of clustering can be observed, suggesting the presence of distinct classes of atom families within the data set, the ATC values vary smoothly between them. This smooth variations strongly suggests that ATC values vary smoothly with changes in SOAP descriptor values, and hence that SOAP descriptors would be good descriptors for predicting ATC values with machine learning.