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 (3)Pi and, for AlP, the ground state is predicted to be (3)Sigma(-). 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 pi orbital below the sigma orbital for the ground state of these heteronuclear diatomic molecules.