  We investigate a prominent vibrational feature in amorphous silica, the
so-called boson peak, by means of molecular dynamics computer simulations. The
dynamic structure factor S(q,nu) in the liquid, as well as in the glass state,
scales roughly with temperature, in agreement with the harmonic approximation.
By varying the size of the system and the masses of silicon and oxygen we show
that the excitations giving rise to the boson peak are due to the coupling to
transverse acoustic modes.
