The ab initio calculations have systematic errors. These systematic errors can be made to cancel in a well-chosen reaction. Isodesmic (same number of same type of bonds on each side of the reaction) reactions are a good chioce. The example above is not an isodesmic reaction. There is a C-C bond in glyoxal on the left side and no C-C bonds on the right side of the reaction. An example of an isodesmic reaction involving glyoxal is:
CHOCHO + CH₄ = CH₃CHO + CH₂O
There are 2 C=O bonds, 1 C-C bond, anf 6 C-H bonds on both sides of the reaction.

Of course, we still have the constraint that there must be experimental values for the other species in the reaction. In this case where we want the enthalpy of formation of glyoxal (CHOCHO) we would need the experimental values for CH₄ , CH₃CHO, and CH₂O.
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