Electron correlation effects on the excitation energies of the lowest triplet states of glyoxal

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Abstract
In an attempt to understand the ordering of excited states of glyoxal the 1Ag ground state and low‐lying triplet states 3Au, 3Bu, and 3Bg have been studied with ab initio correlated wavefunctions. A double zeta basis set was used in the configuration interaction calculations, and ∼6000 Slater determinants were selected for each electronic state. The predicted CI adiabatic excitation energies Te are 21 800 cm−1 (3Au), 25 100 cm−1 (3Bu), and 29 300 cm−1 (3Bg). The effects of d functions and higher (than double) excitations (unlinked clusters) have also been evaluated. For the 3Bu state, both d functions and higher excitations raise the predicted Te value by perhaps as much as 5 000 cm−1. For the 3Bg state, d functions raise the predicted excitation energy but higher excitations have the opposite effect. It is concluded that the excitation energy of the 3Bu state is dramatically increased by correlation effects, while Te for the 3Au and 3Bg states is markedly decreased by correlation.