Many structures in the stepwise pathways are open-shell singlet states with significant multi-reference character. Single-reference DFT calculations using the brokensymmetry approach, which is what we used to compute the energies of, for example, TS_meso and TS_rac (Table ), introduce spin contamination from the triplet to the openshell singlet wave functions, as indicated by the <S 2 > values from broken-symmetry DFT calculations, for instance <S 2 > = 0.62 for TS-meso. The spin-contamination can be removed by the Yamaguchi and Houk spin-projection procedure. However, either the spin-contaminated or the spin-projected energies shows that the stepwise TS-meso is lower in energy than the concerted TS-endo-2, in contradiction to the experimental activation energies 𝐸 ! in Table . In order to compare the computed barriers of the stepwise pathways with the concerted pathways at a consistent level of theory, we resorted to NEVPT2 calculations. These NEVPT2 calculations are based on CASSCF (8,8) wavefunctions and, in addition to the fully-accounted dynamic electron correlation within the active space, the dynamic electron correlation outside of the active space is recovered through the 2 nd order perturbation theory. As shown in Table , TSmeso was computed to be 1 kcal/mol higher in enthalpy than TS-endo-2 at the NEVPT2(8,8)/def2-TZVP//(U)ωB97XD/def2-TZVP level, consistent with the experimental 𝐸 ! . However, the 3-5 kcal/mol difference between TS-meso and TS-rac at ωB97XD and NEVPT2 levels is larger than the experimental 𝐸 ! , though this large difference is consistent with previous calculations. The [4+2] cycloadducts endo-2 and exo-2 may be formed by either a concerted or stepwise mechanism as shown in Figure . The NEVPT2 calculations in Table suggest that these two [4+2]-cycloadducts are formed by different mechanisms. For endo-2, the concerted mechanism is computed to be enthalpically more favorable whereas for exo-2, the stepwise mechanism is more favorable.