Anomalous shift of the 1075 cm^-1 oxygen-hydrogen defect in silicon

First principles calculations are carried out on i) hydrogen and ii) water molecules trapped near an interstitial oxygen atom in Si. We find that it is possible for these molecules to cause an upward shift in the antisymmetric stretch mode of O_i when H is replaced by D, which could explain the analogous shift in the 1075 cm^-1 O-H related local vibrational mode. Both these molecules lead to modes in the 3500-4000 cm^-1 region but those of the H₂ lie close to those recently detected using Fourier transform infra-red spectroscopy.