  We show that harmonic vibrations in amorphous silicon can be decomposed to
transverse and longitudinal components in all frequency range even in the
absence of the well defined wave vector ${\bf q}$. For this purpose we define
the transverse component of the eigenvector with given $\omega$ as a component,
which does not change the volumes of Voronoi cells around atoms. The
longitudinal component is the remaining orthogonal component. We have found the
longitudinal and transverse components of the vibrational density of states for
numerical model of amorphous silicon. The vibrations are mostly transverse
below 7 THz and above 15 THz. In the frequency interval in between the
vibrations have a longitudinal nature. Just this sudden transformation of
vibrations at 7 THz from almost transverse to almost longitudinal ones explains
the prominent peak in the diffusivity of the amorphous silicon just above 7
THz.
