where ν i is the stoichiometric coefficient of species i (e.g., ν CO 2 = -2, ν O 2 = 3.5 in Eq. ( )). Fig. shows the thermodynamic cycle to determine ∆ f H C 2 H 6 . However, ∆ f H of many species have never been measured and probably will never be measured, especially for highly reactive species such as radicals and other short-lived intermediates. Consequently, QM methods are used instead to compute ∆ f H of species that cannot be measured. In all equations, we use the general QM instead of DFT since all methods can be used with any level of theory of electronic structure data. QM methods can provide only electronic energies of individual species that are internally referenced to the QM reference frame of the software, which can then be used to compute the reaction enthalpies and finally deduce ∆ f H. To determine ∆ f H of a target molecule P from an electronic structure method, we form a hypothetical reaction from a set of reference species A, B, and C with known ∆ f H.