A systematic theoretical investigation of the valence excited states of the diatomic molecules B2, C2, N2 and O2

A quantitative survey on the performance of multireference (MR), configuration interaction with all singles and doubles (CISD), MRCISD with the Davidson correction and MR-average quadratic coupled cluster (AQCC) methods for a wide range of excited states of the diatomic molecules B-2, C-2, N-2 and O-2 is presented. The spectroscopic constants r(e), omega (e), T-e and D-e for a total of 60 states have been evaluated and critically compared with available experimental data. Basis set extrapolations and size-extensivity corrections are essential for highly accurate results: MR-AQCC mean-errors of 0.001 Angstrom, 10 cm(-1), 300 cm(-1) and 300 cm(-)1 have been obtained for r(e), omega (e), T-e and D-e, respectively. Owing to the very systematic behavior of the results depending on the basis sst and the choice of method, shortcomings of the calculations. such as Rydberg state coupling or insufficient configuration spaces. can be identified independently of experimental data. On the other hand, significant discrepancies with experiment for states which indicate no shortcomings whatsoever in the theoretical treatment suggest the re-evaluation of experimental result. The broad variety of states included in our survey and the uniform quality of the results indicate that the observed systematics is a general feature of the methods and, hence, is molecule-independent.