where xj is the j th token in the sequence. They compared their model to an existing expert rule-based system, matching its performance and attracting attention to the field. However, Liu et al. highlighted challenges with the generation of valid SMILES. SMILES are inherently fragile, which means that a single permutation of a token in the sequence can invalidate the SMILES. Additionally, SMILES are non-unique such that a molecule can have several valid SMILES representations. Thus, a model might encounter difficulty learning the different representations while proposing diverse reactant predictions. These shortcomings have been addressed by the community over the years; we highlight powerful approaches below.