To compute the MCESs, an established approach is to search for the maximum cliques in the modular product of the reactant and product molecular graphs, R and P , as illustrated in Figure . This approach focuses on the correspondences between unchanged bonds in the reactants and products. Specifically, the MCESs correspond to the bond-to-bond correspondences that maximize the identification of unchanged bonds between the reactants and products. Each vertex in the modular product represents a mapping from one bond in the reactants to one bond in the products; thus, a set of vertices defines a bond-to-bond correspondence. As explained below, the MCESs are defined by the largest sets of vertices where every pair of vertices is connected by an edge, i.e., the maximum cliques of the modular product.