Accurate a b i n i t i o calculations which demonstrate a 3Πu ground state for Al2

The spectroscopic parameters and separations between the three low-lying X 3Πu, A 3Σ−g, and a 1Σ+g states of Al2 are studied as a function of both the one-particle and n-particle basis sets. Approximate correlation treatments are calibrated against full CI calculations correlating the six valence electrons in a double-zeta plus two d-function basis set. Since the CASSCF/MRCI 3Πu–3Σ−g separation is in excellent agreement with the FCI value, the MRCI calculations were carried out in an extended (20s13p6d4f)/[6s5p3d2f] Gaussian basis. Including a small correction for relativistic effects, our best estimate is that the 3Σ−g state lies 174 cm−1 above the 3Πu ground state. The 1Σ+g state lies at least 2000 cm−1 higher in energy. At the CPF level, inclusion of 2s and 2p correlation has little effect on De, reduces Te by only 26 cm−1, and shortens the bond lengths by about 0.02 a0. Further strong support for a 3Πu ground state comes from the experimental absorption spectra, since both observed transitions can be convincingly assigned as 3Πu→3Πg. The (2)3Πg state is observed to be sensitive to the level of correlation treatment, and to have its minimum shifted to shorter r values, such that the strongest experimental absorption peak probably corresponds to the 0→2 transition.