crystal. A seminumerical frequency analysis using X-HCFF allowed the characterization of the transition state of a [2,3] sigmatropic (Mislow-Evans) rearrangement and its disappearance at high pressure. XP-PCM was used to calculate intramolecular vibrations and their pressure-induced shifts in sulfur hexafluoride, P 4 S 3 , C 60 and C 70 , and diborane, reaching overall good agreement with experiments. The computational approach subsequently allowed to elucidate the geometrical and energetic basis for the observations.