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Molecular orbitals for Siz and Gez have been optimized in hyper-w calculations and utilized in valence CI treatments to describe the low-lying states of the molecules. The calculational results reveal pronounced similarities between the electronic structures of Siz and Gez. Thus, for both molecules the two lowest-lying electronic states, 31;,(4.rr2) and 311,,(w~.rr~), have crossing potential energy curves, and the two lowestlying states of ' 2; symmetry exhibit crossing of configurations. The sequence of the low-lying electronic states can be rationalized on basis of a simple molecular-orbital picture in which the ug and the .rrn valence orbitals are almost degenerate. The spectroscopic constants derived from the present work compare favorably with the results of more elaborate calculations. It appears that transition energies derived in valence CI calculations between states of identical configurations are improved in large CI calculations, whereas this is not the case for transition energies between states of different configurations. The valence CI calculations based on the molecular orbitals optimized in hyper-w calculations appear to offer reliable descriptions of the chemical bonds as well as of the electronic structures of the molecules Siz and Ge2.

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