A CASSCF study of the potential curves for the X 1Σ+, B 1Σ+, and A 1Π states of the BH molecule

Abstract

The complete active space (CASSCF) method was applied to compute the potential curves for three singlet states of the BH molecule. The vibrational and rotational levels were calculated by solving numerically the radial Schrödinger equation and the spectroscopic constants were obtained by a least square fit of the term values to the Dunham expansion. For each of the states X ¹Σ⁺, A ¹Π, and B ¹Σ⁺, only four vibrational levels are known from experiment. The results for the ground state are in excellent agreement with experiment: the maximum deviation between calculated and experimental G_v+1/2 (v = 0,2) is 3.5 cm⁻¹. Spectroscopic constants obtained by fitting 160 term values (v = 0,9, J = 0,15) are reported. The existence of a double minimum in the potential curve for the B ¹Σ⁺ state is confirmed. The calculation shows that all four experimentally observed levels correspond to the inner minimum. The differences between calculated, for this minimum, and experimental ΔG_v+1/2 (v = 0,2) values for the B ¹Σ⁺ state do not exceed 22 cm⁻¹. A fifth vibrational level and a number of levels corresponding to the outer minimum or lying above the maximum are also obtained in the calculation. For the A ¹Π state a dissociation energy of 0.50 eV (exptl. = 0.70 eV) was obtained, and the potential curve was found to have a maximum lying 0.20 eV above the dissociation limit.