Ab initio absorption spectra and optical gaps for CdnSen (n=17,26,38) quantum dots are calculated using time-dependent density functional theory within the local density approximation. The spectra computed using the time-dependent local density approximation technique differ from the ones calculated using conventional approaches in both absorption threshold and spectral features. The time-dependent density functional spectra suppress surface contributions in the lower-energy region of the spectra. These contributions come from the Se atoms at the surface with two dangling bonds. The optical gaps calculated using the time-dependent approach are in good agreement with experimental results. (C) 2003 American Institute of Physics.