We have studied structural and electronic properties of models of abrupt interfaces between Si(001) and Si0₂ within density functional theory in the local density approximation. We have adopted a model interface between Si and tridymite, a crystalline form of Si0₂, and have allowed for full relaxation. We find structures in which all dangling bonds are saturated. Comparison between calculated and experimental x-ray scattering intensities seems to rule out the tridymite hypothesis. On the other hand, the absence of electronic states in the Si gap indicates that our model interface is a valuable starting point for further investigations.