The gas-phase thermodynamic community has defined the enthalpies of formation ∆ f H (see Eq. ( )), which use the elements in their most stable configuration as the reference thermochemical species . By referencing all species to the standard set of anchor species (i.e. the elements in their IUPAC standard states ), it is possible to integrate data from multiple sources, e.g., experimental and theoretical determinations with various degrees of accuracy, into a global thermochemical network. There are many highly accurate QM methods available for gas-phase molecules (e.g., W4 , HEAT , focal point , and ANL0 ) that are capable of computing reaction enthalpies with an accuracy of ±1 kJ mol -1 . With these accurate reaction enthalpies, the gasphase chemistry community has established standard methods to convert the electronic energies from QM into highly accurate ∆ f H in thermochemical networks, which can be created in two ways: sequential or using the Active Thermochemical Tables approach . We briefly describe these two different approaches in this review, but the reader is referred to refs 86,91,92 for a complete description.