The spectroscopic parameters and separations between the three lowhyphen;lyingXthinsp;3Pgr;u,Athinsp;3Sgr;minus;g, andathinsp;1Sgr;+gstates of Al2are studied as a function of both the onehyphen;particle andnhyphen;particle basis sets. Approximate correlation treatments are calibrated against full CI calculations correlating the six valence electrons in a doublehyphen;zeta plus twodhyphen;function basis set. Since the CASSCF/MRCI3Pgr;undash;3Sgr;minus;gseparation is in excellent agreement with the FCI value, the MRCI calculations were carried out in an extended (20s13p6d4f)/lsqb;6s5p3d2frsqb; Gaussian basis. Including a small correction for relativistic effects, our best estimate is that the3Sgr;minus;gstate lies 174 cmminus;1above the3Pgr;uground state. The1Sgr;+gstate lies at least 2000 cmminus;1higher in energy. At the thinsp;CPF level, inclusion of 2sand 2pcorrelation has little effect onDe, reducesTeby only 26 cmminus;1, and shortens the bond lengths by about 0.02a0. Further strong support for a3Pgr;uground state comes from the experimental absorption spectra, since both observed transitions can be convincingly assigned as3Pgr;urarr;3Pgr;g. The (2)3Pgr;gstate is observed to be sensitive to the level of correlation treatment, and to have its minimum shifted to shorterrvalues, such that the strongest experimental absorption peak probably corresponds to the 0rarr;2 transition.
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