The approaches in Sections 4.2.1 to 4.2.3 have shown that the reaction enthalpies calculated from the total DFT energies are conserved, regardless of the choice of atomic reference species. We can use this fact and calculate ∆ A H if only reaction enthalpies are reported for a mechanism. This problem cannot be formulated for a single species, and it directly becomes a linear algebra problem. For a chemical kinetics network involving n chemical species, a total of n a + n s reference species must be defined, where n a is the number of unique atomic elements of which all species consist and n s is the number of surface sites (typically one). Let E be the zero-point corrected energies of chemical species, H A be the enthalpies of formation of the same species given defined references A and H r be the reaction energies according to the stoichiometry matrix M. The relationship between H r and E is a simple linear combination, shown in Eq. (72).