A tight-binding model is used to investigate confinement and shape effects on the optical absorption spectrum of CdSe nanocrystals up to 5 nm diam. The effects of size dispersion are studied and we show that to simulate the spectrum of a collection of nanocrystals it is not always sufficient to broaden inhomogeneously the mean diameter cluster spectrum. Our results are compared with calculations based on other methods and show good agreement with experimental data. We have also calculated the absorption spectra of CdSe crystallites with different morphologies: spherical clusters and oblate and prolate elliptic nanocrystals. We have performed a comparison of the energies and absorption bands as a function of the morphological changes and we present a detailed study of the modifications in the features of the optical spectra. ͓S0163-1829͑98͒03932-0͔

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