  We study the structure of the energy spectra along with the character of the
states participating in optical transitions in colloidal CdS quantum dots (QDs)
using the {\sl ab initio} accuracy charge patching method combined with the
%pseudopotential based folded spectrum calculations of electronic structure of
thousand-atom nanostructures. In particular, attention is paid to the nature of
the large resonant Stokes shift observed in CdS quantum dots. We find that the
top of the valence band state is bright, in contrast with the results of
numerous {\bf k$\cdot$p} calculations, and determine the limits of
applicability of the {\bf k$\cdot$p} approach. The calculated electron-hole
exchange splitting suggests the spin-forbidden valence state may explain the
nature of the ``dark exciton'' in CdS quantum dots.
