TS at about 2/3 of the entire reaction course. The reaction profile is similar to other [4+2] cycloadditions previously studied using the same XP-PCM method. In the current endo-cycloaddition, the first 1/4 of the PES is rather flat, where two isolated 1,3cyclohexadiene molecules reorient themselves in a proper geometry for the cycloaddition and begin to approach one another; the latter motion would lead to a decrease of the separation between the molecules. At high pressure, a minimum of a vdW complex develops prior to the TS, as noted previously by us and by Loco et. al. The minimum is very shallow at low pressures (may be difficult to see in the figure, but it is there) but becomes apparent at 5.7 GPa. This minimum also shifts towards the TS as the pressure increases. The emergence of a pre-TS minimum and the shift of it towards the TS could be explained by the pressure-enhanced vdW complex formation. The higher the pressure, the shorter the vdW separation between the molecules, and the smaller the volume of such complex. The favorable pV term of the enthalpy leads to the appearance of a minimum for the complex. This phenomenon of a pre-TS minimum seems to be common in bimolecular reactions; the other three reactions in Figure The vdW cavity of TS-threo-4 is 142 cm 3 /mol, compared with 145 cm 3 /mol for TS-endo-2.