  Recently a new approach to the determination of dynamic correlation lengths,
{\xi}, for supercooled liquids, based on the properties of the slow
(picosecond) vibrational dynamics, was carried out [L. Hong, V.N. Novikov, and
A.P. Sokolov, Phys. Rev. E 83, 061508 (2011)]. Although these vibrational
measurements are typically conducted well below the glass transition
temperature, Tg, the assumption is that the structure of the liquid is frozen
at Tg, so that the {\xi} characterize dynamic heterogeneity in the supercooled
liquid state. We compare {\xi} from this method to values calculated using an
approximation to the four-point dynamic susceptibility. For 26 different
materials we find good correlation between the two measures; moreover, the
pressure dependences are consistent within the large experimental error.
However, {\xi} from Boson peak measurements above Tg have a different, and
unrealistic, temperature dependence.
