Nonempirical LCAO MO SCF studies of the low lying states of BeF2

Nonempirical LCAO MO SCF studies of the low lying excited states of BeF₂ are reported. These results provide new insight into the differences between the valence molecular orbitals for covalent and ionic AB₂‐type molecules. The lowest energy states in BeF₂ are predicted to result from transitions involving the highest occupied ${\pi }_g{\mathrm{(b}}_2)$ and ${\pi }_g{\mathrm{(a}}_2)$ molecular orbitals of the ground state to the lowest unoccupied ${\mathrm{\sigma ̄}}_g{\mathrm{(a}}_1)$ molecular orbital. This is in contrast to the case of CO₂ where experiments have confirmed Walsh's prediction that the lowest energy transitions are from the ${\pi }_g{\mathrm{(b}}_2)$ and ${\pi }_g{\mathrm{(a}}_2)$ molecular orbitals to the ${\mathrm{\pi ̄}}_u{\mathrm{(a}}_1)$ molecular orbital. This predicted change in the ordering of the ${\mathrm{\pi ̄}}_u{\mathrm{(a}}_1)$ and ${\mathrm{\sigma ̄}}_g{\mathrm{(a}}_1)$ orbitals for BeF₂ and CO₂ suggests that the lowest energy transition in BeF₂ should be quite weak in absorption since a g to g transition is electric dipole forbidden for the linear molecule. A modification of the Walsh type diagram for highly ionic species of the form $X^{\mathrm{-\delta }}{\mathrm{-M}}^{\text{+2δ}}{\mathrm{-X}}^{\mathrm{-\delta }}$ is discussed.