Atom types in RMG are a set of atom descriptors that describe the local environment around an atom. They can accelerate graph isomorphism (by using specific types) and improve flexibility when defining reactions (by using more generic types). The set of available atom types has been revised and expanded to improve representation of heteroatoms. Particular focus has been placed on expanding atom type descriptors for the various bonding configurations of nitrogen and sulfur, for which the full list of updated atom types has been recently reported. New carbon and oxygen atom types for representing formal charges and varying numbers of lone pairs have been added, along with additional halogen atom types. For surface chemistry, atom types representing generic surface sites have been added, along with quadruple bonds for carbon and silicon. A list of these new atom types is shown in Table .