The lowest energy states of the group-IIIA-group-VA heteronuclear diatomics: BN, BP, AlN, and AlP from full configuration interaction calculations

Full configuration interaction (CI) calculations on the group-IIIA-group-VA diatomic molecules BN, BP, AlN, and AlP have been performed with the cc-pVTZ correlation-consistent basis set and compared to CCSD(T) calculations with the same basis set. The CCSD(T) calculations are good to better than 1 kcal/ mol in comparison with the full CI results if the T ₁ diagnostic is small and to within about 2 kcal/mol if the T ₁ diagnostic is large. Inspection of the T ₂ amplitudes shows that the simple T ₁ diagnostic is providing useful insight into the quality of the starting wave function. The ground state of BN, BP, and AlN is predicted to be the ³Π and, for AlP, the ground state is predicted to be ³Σ ^- For all molecules except BP, there is an excited state within 1.1 kcal/mol of the ground state. The ordering of the state energies can be explained in terms of simple orbital and bonding models. The results provide little evidence for placing the π orbital below the σ orbital for the ground state of these heteronuclear diatomic molecules.