  We introduce a general method which allows reconstruction of electronic band
structure of nanocrystals from ordinary real-space electronic structure
calculations. A comprehensive study of band structure of a realistic
nanocrystal is given including full geometric and electronic relaxation with
the surface passivating groups. In particular, we combine this method with
large scale density functional theory calculations to obtain insight into the
luminescence properties of silicon nanocrystals of up to 3 nm in size depending
on the surface passivation and geometric distortion. We conclude that the band
structure concept is applicable to silicon nanocrystals with diameter larger
than $\approx$ 2 nm with certain limitations. We also show how perturbations
due to polarized surface groups or geometric distortion can lead to
considerable moderation of momentum space selection rules.
