This work is part of our continuing efforts to apply a new quantum chemical method, the extreme pressure polarizable continuum model (XP-PCM), to the study of reaction energy profiles under pressure, and to calculate activation volumes. The method has been tutorially reviewed and applied by the present authors and Hoffmann 17 in 2017 to a set of pericyclic reactions where interesting phenomena on reaction energy profiles under pressure were revealed by calculations, such as the shift of the transition state, changes in the rate-determining step, etc. In addition, a reasonable agreement of the computed activation volumes with the experimental values was discovered. The evolution of the van der Waals volume of the reactive system upon proceeding from the reactants to products, as a chemical relevant diagnostic to analyze the effect of the pressure on the reaction energy profile, was emphasized.