Finally, in step 8, reaction centres of the remaining analogue reactions are obtained to ensure that they are within designated carrier fragments. The reaction centre constitutes all atoms that change in connectivity from reactants to products. For each mapping index number, corresponding atoms in reactant and product are compared with respect to eight criteria: atom SMARTS, atomic number, degree, formal charge, aromaticity, number of bonded hydrogens, number of radical electrons, and nearest neighbour identities (bond order and atomic number).