Ab initio multireference study of the BN molecule.

The lowest 1Σ+ and 3Π states of the BN molecule have been studied using multireference configuration interaction (MRCI) and averaged coupled-pair functional (ACPF) methods and large atomic natural orbital (ANO) basis sets, as well as several coupled cluster methods. Our best calculations strongly support a 3Π ground state, but the a 1Σ+ state lies only 381±100 cm-1 higher. The a 1Σ+ state wave function exhibits strong multireference character and, consequently, the predictions of the perturbationally-based single-reference CCSD(T) coupled cluster method are not as reliable in this case as the multireference results. The best theoretical predictions for the spectroscopic constants of BN are in good agreement with experiment for the X 3Π state, but strongly suggest a misassignment of the fundamental vibrational frequency for the a 1Σ+ state. [ABSTRACT FROM AUTHOR]