Molecular similarity is calculated using the reactant and product molecular fingerprints. However, the value of each bit in fingerprint depends on the length of the bit string and the cut-off radius, resulting in different values of molecular similarity. It is thus necessary to choose a suitable bit string length and the cut-off radius to yield one-to-one molecular similarity for each branch reaction. The reactant alkanes with the number of involving carbon atoms up to 16 are interested in this work due to their importance in combustion reactions. Therefore, all the alkanes and their isomers (less than 17 carbon atoms) with their SMILESs available from PubChem are collected and used to test the length and the cut-off radius. As a result, the length of the bit string is taken as 433 and the radius is taken as 8. Table shows the calculated similarities for the reactions with the reactant alkanes involving less than 6 carbon atoms. Clearly, the different branches for the reactions can be well distinguished. A full list of generated similarities is given in Tables S7-S17.