In the first step, two reactants, S 11 and S 12 , produce the intermediate product P 1 , followed by the second step, which produces the second intermediate product P 2 by reacting P 1 and a newly selected reactant S 21 . In the third step, the intermediate product P 2 and reactant S 31 react to produce final product P := P 3 . This reaction cascade can be expressed using the single-step reaction model, as follows: