where the adsorption enthalpy is derived from the DFT energies and ∆ f H * = 0 kJ mol -1 by assertion as discussed in Section 4.1. Thus, the only unknown to solve is ∆ f H of the gas-phase precursor of the adsorbate. Many gas-phase precursors are unstable and, therefore, not tabulated in the ATcT. An isogyric reaction to form the species from CH 4 , H 2 O, and H 2 is used to estimate ∆ f H of the precursor, which is similar to the ANL-0 approach . ∆ f H of the gas-phase precursor also provides the reference to an existing thermochemical network as illustrated in Fig. . We derive the approach in detail in the SI and show how it is mathematically equivalent to Eq. ( ).