The first Ioffe-Regel crossover for vibrational plane waves (when wavelengt
h and mean free path are comparable) has been investigated theoretically fo
r models of Vitreous silica (upsilon-SiO2) constructed by molecular dynamic
s. The crossover is found to be from a state of weak scattering to one of s
trong scattering, not vibrational localization. Three methods have been use
d to investigate the crossover: an analysis of the time evolution of a vibr
ational plane wave, a spectral-density analysis in frequency space, and an
analysis of the final scattered state in momentum space. The first Ioffe-Re
gel crossover frequency is found by all three methods to be similar to 1 TH
z for upsilon-SiO2, for both longitudinal and transverse polarizations. A s
econd Ioffe-Regel crossover occurs at similar to 6 THz for upsilon-SiO2, co
rresponding to the frequency at which the mean free path is minimal (compar
able to the interatomic spacing), and the spectral-density width is maximal
(comparable to the frequency range of the vibrational density of states).