If this results in atom surplus on the LHS, one compatible small help specie is added to the RHS. A library of manually curated help species was tailored to include 32 molecules and ions, ranging from water to chlorobenzene. These molecules are the most frequently appearing side products in organic synthesis. The full list of help compounds is shown in Table . These help species are added sequentially to the RHS of a reaction and used to balance the reaction. The reaction is defined as complete with help species if it is balanced with the addition of these species to the RHS. Otherwise, it is classified as a 'RHS species insufficient' reaction. quantified by the atom mapping confidence score. The confidence scores for the candidate reactions are ranked to select the most optimal candidate molecule to update the LHS of the reaction. ChemBalancer treats the updated reaction as a new reaction record and tries to balance the updated reaction again with the same workflow. This corresponds to the arrow from the bottom-right box to the top box in Figure .