The flat TS region of the PES at 0 GPa becomes tilted with the post-TS region lowering in enthalpy at high pressures, due to the monotonical volume decrease in this region. A clear shift of the TS towards the reactant is seen at high-pressures. The shift of TS at high pressure is consistent with the Hammond postulate 74 that the TS shifts towards the reactant when the reaction becomes more exothermic. However, note that the TS shift is apparent only in the [6+4]-ene reaction, not so in the other three reactions in Figure , though all these reactions become more exothermic (or less endothermic) at high pressures. One distinct feature of the [6+4]-ene reaction is that the cavity volume decreases in the TS region, while in the other three reactions, the cavity volume stays almost constant in the TS region. Therefore, the effect of pressure is larger on the PES in the TS region in the [6+4]-ene reaction, leading to an apparent shift of the TS towards the reactant. The shift of TS along reaction coordinate under pressure has been noted previousely. Concerted [4+2]-ene reaction. In contrast to the flat TS region in the [6+4]-ene reaction, the [4+2]-ene reaction has a sharp maximum in the TS region (Figure ). We inspected the TS structures in the two reactions to understand the reason. In the [6+4]-ene TS-threo-4, the C-C bond is almost fully formed at a distance of 1.68 Å while the H is barely transferred with distances of 1.20 and 1.90 Å for the breaking and forming C-H bonds (Figure ). On the contrary, in the [4+2]-ene TS-6, the H-transfer is substantial with 1.50 and 1.28 Å for the two C-H bonds but the C-C bond to be formed is at a large distance of Stepwise addition. Moving on to the stepwise mechanism, Figure shows the reaction profiles of the meso addition. Compared with the above-discussed concerted reactions, this meso addition has a rather short course, especially in the post-TS region. The C-C distance of the formed bond changes from 2.00 Å in TS-meso to 1.57 Å in the adduct meso-8. The cavity volume of the system decreases in the meso-addition, but with a much less volume reduction compared with the above concerted reactions where two bonds are formed. Consequently, pressure provides much less reduction of the reaction barrier for the meso addition. The meso-adduct meso-8 is calculated to be higher in enthalpy than the reactants at pressures lower than 6.2 GPa.