AbstractQuasi‐spherical Si nanocrystals with diameters from 0.8 to 1.6 nm are embedded into amorphous SiO2 networks and further optimized using density functional methods. After relaxation the nanocrystals exhibit a smooth interface without defects to the glass matrix. Tensile strain is observed within the nanocrystalline core, with a maximum strain for sub‐surface Si atoms close to the suboxide interface. We find the highest occupied states of the valence band being located at Si–Si bonds close to the interface forming a shell‐like structure around the central core of the nanocrystal. The lowest unoccupied states are centered within the nanocrystal. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)