To assess the performance of the final hierarchy, we calculate the accuracy which we define as the percentage of SMARTS patterns that map to exactly one reference label. Therefore an accuracy of 100% indicates that every SMARTS pattern maps to one reference label, i.e. all {L|S 0 } are singletons. An accuracy of 0% indicates that every SMARTS resolves to two or more labels, i.e. there are no singletons. In this case it is not possible to determine the correct reference label of any chemical environments in the training dataset. Accuracies in between 0 and 100% are also permissible, but the aim is to make the SMARTS hierarchy accuracy as close to 100% as possible.