Here, all three states exhibit an M of less than 0.05, indicating both the reaction and activation energies should be well described by a single-determinant wave function. Despite this, both the forward and reverse barriers are predicted to be 20 kcal/mol lower with APC(12,12)-NEVPT2 than MP2. By comparison, APC-tPBE agrees to within chemical accuracy (1 kcal/mol) with the single-reference limit of HF-PBE and CCSD(T). Once again, the smaller APC(4,4) active space largely remedies this unphysical error with NEVPT2, demonstrating the error to be due to ASIE. An in-depth evaluation of the active space dependence of tPBE and NEVPT2 for these two reactions, as well as CASSCF is included in the Supporting Information. Figure presents a second case, in which the ring-opening of a 3-membered heterocycle forms an oxygen diradical with significant multiconfigurational character. As is seen, the HF determinant is completely incapable of describing this state, overestimating the energy of this product relative to the reactant by 60 kcal/mol -higher in energy than the transition state. Due to this terrible description given by HF, CCSD(T) inherits this error and also drastically overpredicts the energy of the reactant relative to the transition state. The unrestricted nature of B3LYP-D3 is able to account for the multiconfigurational character of the biradical somewhat, predicting a shallow barrier of 8.5 kcal/mol relative to the transition state. In contrast, APC-tPBE predicts a significantly more stable product, with a barrier of 31.3 kcal/mol relative to the transition state, and in much better agreement with the CCSD(T) reference values for the single-reference reactant and product. Here, we believe the APC-tPBE results give a much more accurate description than either DFT or CCSD(T), and are reproduced by NEVPT2 in a smaller active space containing only the correlating orbitals (Supporting Information). These results serve to highlight the necessity of multiconfigurational approaches for some reactions containing significant multiconfigurational character.