Accurate ab initio calculations which demonstrate a ^{3}Π_{u} ground state for Al_{2}
Abstract
The spectroscopic parameters and separations between the three lowlying X ^{3}Π_{u}, A ^{3}Σ^{}_{g}, and a ^{1}Σ^{+}_{g} states of Al_{2} are studied as a function of both the oneparticle and nparticle basis sets. Approximate correlation treatments are calibrated against full CI calculations correlating the six valence electrons in a doublezeta plus two dfunction basis set. Since the CASSCF/MRCI ^{3}Π_{u}^{3}Σ^{}_{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 ^{3}Σ^{}_{g} state lies 174 cm^{}^{1} above the ^{3}Π_{u} ground state. The ^{1}Σ^{+}_{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 D_{e}, reduces T_{e} by only 26 cm^{}^{1}, and shortens the bond lengths by about 0.02 a_{0}. Further strong support for a ^{3}Π_{u} ground state comes from the experimental absorption spectra, since both observed transitions can be convincingly assigned as ^{3}Π_{u}→^{3}Π_{g}. The (2)^{3}Π_{g} 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.
 Publication:

Journal of Chemical Physics
 Pub Date:
 June 1987
 DOI:
 10.1063/1.452349
 Bibcode:
 1987JChPh..86.7007B
 Keywords:

 Aluminum;
 Conduction Electrons;
 Ground State;
 Valence;
 Computational Chemistry;
 Dimers;
 FranckCondon Principle;
 Vibrational Spectra;
 Atomic and Molecular Physics