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The vibrational properties of model amorphous materials are studied bycombining complete analysis of the vibration modes, dynamical structure factorand energy diffusivity with exact diagonalization of the dynamical matrix andthe Kernel Polynomial Method which allows a study of very large system sizes.Different materials are studied that differ only by the bending rigidity of theinteractions in a Stillinger-Weber modelization used to describe amorphoussilicon. The local bending rigidity can thus be used as a control parameter, totune the sound velocity together with local bonds directionality. It is shownthat for all the systems studied, the upper limit of the Boson peak correspondsto the Ioffe-Regel criterion for transverse waves, as well as to a minimum ofthe diffusivity. The Boson peak is followed by a diffusivity's increasesupported by longitudinal phonons. The Ioffe-Regel criterion for transversewaves corresponds to a common characteristic mean-free path of 5-7 \AA{} (whichis slightly bigger for longitudinal phonons), while the fine structure of thevibrational density of states is shown to be sensitive to the local bendingrigidity.

Boson peak and Ioffe-Regel criterion in amorphous silicon-like materials: the effect of bond directionality