Consequently, the generalization of accurate reaction enthalpies from DFT for all surface elementary reactions due to error cancellation is not possible. It is possible to maximize the degree of error cancellation by constructing reactions that conserve the bonding environment and hybridization on both sides of the reaction. This approach moves away from atomic reference sets, instead selecting reference reactions by maximizing the similarity of molecular fragments. The gas-phase community has used these errorcancellation reactions to derive ∆ f H of species with chemical accuracy at the DFT level of theory . Instead of conserving the reaction enthalpies of surface reactions that are relevant to a given chemical reaction network, the goal is to conserve the reaction enthalpies for reactions with the highest degree of error cancellation. The key is to construct reactions that conserve the molecular structure and bonding environment of the target molecule to maximize error cancellation . Raghavachari and co-workers 199,200 developed the connectivitybased hierarchy (CBH) that automatically constructs error cancellation reactions for a gas-phase target. The order of the CBH approach is conservation of spin pairs (isogyric, CBH-0), bond types (isodesmic reaction, CBH-1), immediate connectivity of the heavy atoms (hypohomodesmotic, CBH-2), or immediate connectivity of the bonding environment (hyperhomodesmotic, CBH-3). Climbing this ladder leads to increasing error cancellation and more accurate ∆ f H. Fig. shows the order of the error cancellation reactions with a gas-phase example for 1-propyl.