The vibrational densities of states for theSiO2Bethe lattices of different Si–O–Si angles and various local bonding structures ina-SiO2:Hare calculated. It is found that the Si–O–Si bond-stretching mode shifts to a higher frequency and the bond-bending mode shifts to a lower frequency when the Si–O–Si bond angle increases. The SiH bond-bending mode lies at876 cm−1and there is an additional mode, which comes from the coupled motions of SiH and Si–O–Si bonds, at a lower frequency. This mode is not revealed by experiments. The Si-dangling bond has minor effects to the vibrational spectrum of thea-SiO2.The Si–O bond-stretching mode, of the O atom with dangling bond, shifts to a much lower950 cm−1compared to the1200 cm−1of the Si–O–Si bond-stretching mode of bulkSiO2.The Si–OH bond-stretching mode has a peak at932 cm−1which is slightly less than that of the Si–O bond of the dangling O atom. Under thermal annealing, some H atoms will evolve and the O atom will become a dangling atom or form a Si–O–Si bond with the other Si atom. The Si–Si bond gives an additional Si–Si vibrational mode at464 cm−1in the spectrum. ©1997 American Institute of Physics.