The luminescence energies of a variety of crystalline silicon nanostructures are calculated using a time‐dependent tight‐binding technique. The calculated energies are compared with measured values for porous silicon samples to determine the geometries of the regions in which electron‐hole recombination occurs. We observe a correlation between luminescence wavelength and nanostructure which is consistent with the porosity changes during the etching process. We conclude that porous silicon samples which emit visible light are composed of small crystallites, and that the active regions in longer wavelength emitters are wirelike structures. ©1995 American Institute of Physics.