Diabatic research of resonant states in MO-CI calculations of negative ions

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Abstract
The information supported by the lowest adiabatic eigenstates resulting from ab initio MO-CI calculations using finite basis sets with diffuse AO many be transformed into a small effective Hamiltonian through a unitary transformation recently proposed. This diabatisation procedure of ab initio MO-CI potential curves is applied to the lowest 2 Sigma +u and 2 Sigma +g states of H2-. In both cases one of the vectors supporting the effective Hamiltonian has large components on the (H-H+or-HH-) description taken from the bound asymptotic limit. It is rather spatially concentrated, of predominant valence character and may be considered as a description of the resonant state. Its mean (diabatic) energy varies smoothly with the interatomic distance, becoming unbound for r(1.6 AA for 2 Sigma +u and r(2.5 AA for 2 Sigma +g. The diabatic potential curve of the resonant state is almost independent on the basis set. In contrast, the other diabatic potential curves (a sampling of diffuse states) are parallel to the ground-state potential curve of the neutral molecule, and their position is very sensitive to the basis set. An estimate of the width of the resonant state is proposed. The results are in good agreement with the experimental evidence for both symmetries and with most refined calculations by Domcke and co-workers.
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