Stepwise addition 2.00 Å late TS. The cavity volume of the system decreases from the beginning of the reaction to about the point at which the H-transfer completes (i.e., a bit pass the TS). Then the cavity increases, as the ene-adduct opens up to adapts to its optimal conformation at 0 GPa. At high pressures, a pre-TS vdW complex minimum develops, similar to the case of the [4+2] cycloaddition. But this minimum in the [6+4]-ene reaction is much deeper compared with that in the [4+2] cycloaddition. A most likely reason is that the separated reactant molecules occupy a larger volume (164 cm 3 /mol) than in the [4+2] cycloaddition (158 cm 3 /mol), so that a greater volume reduction is possible in forming the vdW complex.