for which the thermochemistry has been determined in the gas phase. This calibration suggests that the relative energy of C⩸C itself is too high by ~28 kcal/mol when computed using the ωB97XD functional and the Def2-SVP basis set. If this correction is applied to the ωB97XD results, then the computed free energy G298 of the reaction 1a → "C2" is reduced from +68.2 to ~+40 kcal/mol. This is in broad agreement with the simple argument advanced above from experimentally based BDEs (43-53 kcal/mol).