Eq. ( ) is identical to Eq. ( ); the only difference is the source of the reaction enthalpy. Since the reaction is hypothetical, it is possible to use any stoichiometrically balanced reaction. QM calculations are performed at a temperature of 0 K, so ∆ f H derived from the QM reaction enthalpy are at 0 K, too. They can be converted to ∆ f H at 298 K by applying temperature corrections . In principle, all the approaches described in this work are mathematically identical to Eq. (13); they only differ in the choice of reference species and the value of ∆ f H of the reference species.