where ∆G 0 j (298.15; P 0 ) avg is the average value of the reaction free energy using G4, G3, CBS-QB3 and CBS-APNO calculations and [∆G 0 j (T ; P 0 ) -∆G 0 j (298.15; P 0 )] G4 is that of G4 calculations. We have previously shown that the second term, which accounts for the effect of temperature on the ideal-gas free energy, is insensitive to the QM method/theory level; however, the absolute ideal-gas free energy of the species (the first term) can vary substantially across different QM methods. As noted previously, improved predictions can arise from the use of the combination of several high-level QM methods for the calculation of this term. We therefore used the average value of ∆G 0 j (298.15; P 0 ) obtained from the G4, G3, CBS-QB3 and CBS-APNO calculations and the standard deviation was taken as a surrogate measure of the uncertainty of the ∆G 0 j (T ; P 0 ) values.