  The optical absorption spectrum of the three most stable isomers of the Ag11
system was calculated using the time-dependent density functional theory, with
the generalized gradient approximation for the exchange and correlation
potential, and a relativistic pseudopotential parametrization for the modelling
of the ion-electron interaction. The computational scheme is based on a real
space code, where the photoabsorption spectrum is calculated by using the
formalism developed by Casida. The significantly different spectra of the three
isomers permit the identification of the ground-state configuration
predominantly present in the laboratory beams in base to a comparison between
the calculated photoabsorption spectrum of the most stable configuration of
Ag11 and the measured spectra of medium-size silver clusters trapped in noble
gas Ar and Ne matrices at different temperatures. This assignment is confirmed
by the fact that this isomer has the lowest calculated energy.
