It stands to reason that the necessary context around the reaction center cannot be determined from a single reaction. Such information can be inferred by considering a set of reactions with the same reaction mechanism. A (mechanistically defined) reaction type or named reactions, such as the Diels-Alder reaction, in Figure above, however, is usually not annotated in most reaction data record. We, therefore, start from a (large) collection R of reactions and use a clustering approach to at least approximate a classification into mechanistically equivalent classes. Following ideas from Hendrickson's seminal publication, we first subdivide R into classes determined by isomorphic reaction centers. That is, we set