Cis-trans isomerization of glyoxal

The CNDO/2 method with the original parameter set predicts the conformation with the mutually perpendicular CH=O groups to be the most stable isomer of glyoxal; the cis form is favoured over the trans form. The order of stabilities is not changed upon the full optimization of coordinates of the isomers. Calculations fail to reproduce the observed order of isomer stabilities unless allowance is made for limited configuration interaction with the lowest ππ* doubly excited state and for geometry optimization. Doubly excited configurations of the ππ* and σσ* types have negligible effect on the energies of isomers.