Electron Correlation in the Lowest Σ+1 State of Beryllium Oxide

Ab initio first-order wavefunctions, which include electron correlation, are reported for the lowest Σ+1 state of BeO. A contracted Slater function basis of better than double-zeta plus polarization accuracy was used, resulting in 157 configurations, constructed from 569 distinct Slater determinants. Practical convergence in form of the molecular orbitals was obtained using the iterative natural orbital procedure. Considerable emphasis is placed on obtaining the correct dissociation behavior, in the present case to a two-configuration (1s22s2and 1s22p2) wavefunction for S1 Be plus a slightly better than Hartree–Fock wavefunction for the D1 state of oxygen. The calculated dissociation energy is 6.58 eV, compared to the Hartree–Fock value, 4.13 eV, and the spectroscopic value recommended by Gaydon 6.69 ± 0.4 eV. The other spectroscopic constants represent a substantial improvement over the Hartree–Fock values and are all within 10% of experiment. The dominant configurations in the wavefunction are presented, and it is seen that, contrary to the suggestion of previous workers, the 1σ22σ23σ24σ25σ21π2 is not particularly important near the equilibrium internuclear separation. The natural orbital occupation numbers complete our picture of the electronic structure of Σ+1 BeO.