Electronic structure of the ground and low-lying states of the dimer and trimer of aluminum

Abstract

All electron MCSCF/MRCI calculations using basis sets of double-zeta plus polarization quality are presented for the low-lying states of Al 2 and Al 3. In the dimer, the 3Π u state is found to be marginally more stable than the 3Σ g − state at the (MC) SCF and CI level. However, the relative stability is reversed after correcting for the contribution from unlinked clusters using Davidson's formula. Similar results are obtained with the Möller-Plesset perturbation theory based on UHF wavefunctions. In the latter case, the 3Σ g − state is found to be most stable only if the triple substitutions are included. This finding is at variance with a previous ab initio pseudopotential MCSCF/CI calculation, but agrees with the experimental assignment and a recent GVB/CI calculation. The theoretical spectroscopic constants are also in excellent agreement with experiment. For Al 3 the 2A 1 state is found to have the lowest energy. The geometry of the ground-state molecule is an isoceles triangle with an apex angle of 63°C and AlAl bond lengths of 2.55 Å. Several excited states are found to lie within only a few kcal mol −1 above the ground state.