COMPUTATION OF OSCILLATOR-STRENGTHS FROM KOHN-SHAM WAVE-FUNCTIONS - THE EXAMPLE OF SMALL LITHIUM CLUSTERS

Density functional calculations of ground and excited states of Li-n ( n less than or equal to 8) clusters have been performed, within two di fferent approaches. Using a set of Kohn-Sham orbitals to construct wav e functions, the calculation of the oscillator strengths of the electr ic dipole transitions is performed. Our results have been tested at tw o levels: first the necessary comparison with the experimental data, s econd the comparison with high level CI (MRD-CI) calculations. This la st point is not a trivial challenge, because such an ab initio method leads for small clusters to a highly accurate description of the elect ronic structure and optical absorption spectra. Finally, this is also a new test for the capability of using Kohn-Sham orbitals to construct physically meaningful wave functions. Transition energies, oscillator strengths and finally optical absorption spectra presented here are i n general in reasonable agreement with the experimental data and the M RD-CI calculations. That is very promising for bigger systems, with re gard to the modest computational effort (CPU time and memory size) of density functional calculations.