Large Curvature Ground-state model

Unsatisfactory features of these particular calculations are due to the limitations of the HF-RPA procedure to produce quantitatively correct spectroscopic patterns, and to the very small basis set that we adopted in order to carry out evaluations of several thousands of HF ground-electronic-state wave functions and their associated RPA spectra. Such a quantitative inadequacy of the computational model, however, does not alter the physical meaning of the conclusion that can be drawn the appearance of a single broad band in the optical spectrum of Na9 is simply a thermal broadening effect, which, in general, is particularly effective in the presence of several low-lying isomers or in the case of cluster structures characterized by low-frequency vibrations, accompanied by large nuclear displacements due to the low curvature of the BO surface. 

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