Let Ψ and Ψ ′ be extended reaction centers, i.e. subgraphs of Υ(G, H, α) and Υ(G ′ , H ′ , β), that contain the reaction centers Γ (G, H, α) and Γ (G ′ , H ′ , β) as well as all vertices in both reactant and product graphs within a distance at most r from a vertex in the reaction centers.