  We report on the possibility of describing the absorption and emission
characteristics of an ensemble of silicon nanocrystals (NCs) with realistic
distributions in the NC size, by the sum of the reponses of the single NCs. The
individual NC responses are evaluated by means of ab-initio theoretical
calculations and the summation is performed by taking into account the trend of
the optical properties as a function of NC size and oxidation degree. The
comparison with experimental results shows a nice matching of the spectra, also
without any tuning of the parameters. Finally, the possibility of adapting the
model in order to reproduce the experimental data is explored and discussed.
