The relative temperature range (delta T (g) /T (g) ) characterizing the liquid-to-glass transition is a single-valued function of the fluctuation volume fraction f (g) frozen at the glass transition temperature T (g) . The fluctuation volume Delta V (e) of the amorphous material is determined by delocalization of active (excited)) atoms, i.e., their thermal displacements from equilibrium positions. The fluctuation volume fraction f = Delta V (e) /V controls the molecular mobility that is characteristic of delocalized atoms in the liquid-glass transition region. It is shown that delta T (g) for the majority of amorphous materials is only 0.7% of the glass transition temperature T (g) . The narrow temperature range delta T (g) is consistent with the classical Simon concept on the insignificant temperature range in which the structure is frozen.
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