  The vibrational properties of a high-quality realistic model of amorphous
silicon are examined. The longitudinal and transverse dynamical structure
factors are calculated, and fitted to a damped harmonic oscillator (DHO)
function. The width $\Gamma$ of the best-fit DHO to the longitudinal dynamical
structure factor scales approximately as $k^{2}$ for wavevectors
$k\lesssim0.55\textrm{\AA}^{-1}$, which is above the Ioffe-Regel crossover
frequency separating the propagating and diffusing regimes, occurring at
$k=0.38\pm0.03\textrm{\AA}^{-1}$. Using the DHO function as a fitting function
for the transverse dynamical structure factor (without theoretical
justification), gives a dependence of $\Gamma\propto k^{\alpha}$ with
$\alpha\sim2.5$ for wavevectors $k\lesssim0.7\textrm{\AA}^{-1}$. There was no
evidence for $\Gamma\propto k^{4}$ behaviour for either polarization.
