If there are differences, that atom is part of the reaction centre. Additionally, the reaction centre must lie within carrier fragments of analogue species; this implies that relevant carrier fragments are reacting with each other and could lead to sensible impurities when applied to query species. For instance, in Figure , atoms 8, 4 and 6 (highlighted in red) are identified as the reaction centre since they change with respect to the eight criteria. Atom 8 lies within the aromatic hydroxyl carrier fragment (highlighted in green), whilst atoms 4 and 6 lie within the anhydride carrier fragment (highlighted in green), so this is a reaction of interest. On the other hand, Figure is invalid as the reaction centre, notably atoms 1 and 9 (aldehyde, circled in red) are outside the two valid aromatic hydroxyl fragments. The aldehyde involved is not a valid carrier fragment as it is not present in any query species. Other examples include reactions that don't contain a reaction centre or feature any changes in connectivity for example in the case of isomerism. These types of reactions are removed, leaving a set of 17,504 analogue reactions that are guaranteed to feature interactions between carrier fragments.