Step 7 involves atom-to-atom mapping to establish a one-to-one correspondence between reactant and product atoms. Each product atom is mapped to a corresponding reactant atom by way of a common index number. This is important for later steps to extract reaction centres and generate templates. There are many atom-to-atom mapping tools that are available publicly such as NameRXN, 38 ChemAxon Standardizer, Indigo, Reaction Decoder Tool (RDT) and IBM RXNMapper. Specifically, RXNMapper is based on transformer models relying on NLP, which is template-free, and a recent benchmarking study identified it as the most accurate tool (83.4 % accuracy on a cleaned dataset published by Jaworski et al. ).