Theoretical Description of Molecular Rydberg States: B Σ+1 and Lowest Σ+3 States of BH

Ab initio calculations, including electron correlation, have been carried out on the X 1Σ+, B 1Σ+, and lowest Σ+3 states of BH. The last two states can be thought of as arising from the 1σ22σ23σ 4σ electron configuration. A double-zeta-plus-polarization and Rydberg function basis of Slater functions is used and electron correlation is considered through the use of approximate ``first-order'' wavefunctions. Optimum wavefunctions for the B 1Σ+ state are obtained by repeated diagonalization of the density matrix arising from the second eigenvalue of the Σ+1 secular equation. For the ground state the ab initio dissociation energy is 3.27 eV, compared to Gaydon's experimental value 3.54 ± 0.04 eV. Other spectroscopic constants for the X 1Σ+ and B 1Σ+ states are also in good agreement with experiment. The X—B separation is calculated to be 51 770 cm−1, compared to the experimental value 52 347 cm−1. The B 1Σ+ calculations confirm the double minimum predicted by Browne and Greenawalt. The lowest Σ+3 state is predicted by the present treatment to be Rydberg-like (with a minimum at 1.173 Å) for short internuclear separations and valencelike (repulsive) for large separations. A maximum occurs in the Σ+3 potential curve at 1.45 Å. Natural-orbital analyses and electron density plots are used to describe the valence to Rydberg ``transition.''