Excited-state relaxations in molecules are responsible for a red shift of the absorption peak with respect to the emission peak (Franck-Condon shift). The magnitude of this shift in semiconductor quantum dots is still unknown. Here we report first-principle calculations of excited-state relaxations in small (diameter < 2.2 nm) Si nanocrystals, showing that the Franck-Condon shift is surprisingly large (~60 meV for a 2.2 nm-diameter nanocrystal). The physical mechanism of the excited-state relaxations changes abruptly around 1 nanoeter in size, providing a clear demarcation between ``molecules and ``nanocrystals.
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