Reference compounds and bonds used in the isodesmic reac-tion method are depicted in Figure . Experimental BDEs were taken from Luo, 14 except for alanine, for which no experimental BDE is available. Instead, the BDE was computed directly using G4(MP2)-6X. Including alanine as a reference compound seemed preferable to the use of another compound because C α radicals are captodatively stabilized, and a reference compound should include this special electronic stabilization if compensation of systematic errors is desired. BDEs of reference bonds computed at the G4(MP2)-6X and BMK/6-31+G(2df,p) levels of theory agree well with experimental values 14 (??). The mean absolute errors (MAEs) are 6.2 kJ/mol and 8.6 kJ/mol, respectively. In addition to BDEs, for the sake of completeness, bond dissociation free energies (BDFEs) are reported as well. BDFEs were calculated directly, i.e. analogously to Equation .