The MAD of other leading range-separated hybrid functionals, such as ωB97X-D3BJ and ωB97X-V is about 1.80-2.40 kcal/mol. The double-hybrid functionals give similar average errors ranging from 1.50-2.70 kcal/mol, with B2GP-PLYP(-D3) having the lowest MAD of less than 2.00 kcal/mol. Based on all of these results we can identify dispersion-corrected M06-2X (meta-hybrid GGA), CAM-B3LYP and ωB97X (range-separated hybrids), and B2GPLYP (double hybrid) as among the best performing functionals. In the SI (SI-1.1), we provide errors for each step of the decomposition pathway from all methods, which provides a more detailed look into their performance. Here, we focus on their performance for the critical step 3, i.e., the transition state barrier for ring opening, which determines the kinetics of the decomposition.Figure shows the absolute deviation of all methods with respect to CCSD(T) for this step. The deviation ranges from 0.02-9.84 kcal/mol, with B2GPLYP-D3 (0.02 kcal/mol), B2GPLYP (0.04 kcal/mol), CAM-B3LYP-D3 (0.44 kcal/mol), and M062X-D3 (0.64 kcal/mol) agreeing well with the reference value. While ωB97X-V has a mean error of 1.88 kcal/mol when averaged over all reaction steps, it exhibits a significant error of 3.74 kcal/mol for this critical step. Importantly, we find that some of the most common functionals used for materials simulations resulted in large deviations. For example, PBE-D3, SCAN, TPSS, and B3LYP-D3 underestimate the barrier height by more than 6 kcal/mol. Similar behavior is seen for step 4, which is to be expected because the magnitude of its energy difference is the barrier height for the reversed (ring closing) reaction. As shown in Figure , the error for this step closely tracks that of the ring-opening barrier height (step 3), but with opposite sign. The observed challenge in describing the energy differences of these steps is presumably due to the self-interaction error and associated stabilization of delocalized electronic structure of the transition state geometry 4. Although we observe an improvement by over 7 kcal/mol for PBE0-D3 compared to PBE-D3 for these steps, attributable to its reduced self-interaction error, such improvement is not consistent across all steps of the reaction. For example, in steps 5 and 6 (Figure and Figure ), the errors obtained for PBE0-D3 are larger than for PBE-D3. Thus, it is difficult to rank PBE0-D3 and for this reaction pathway. This completes our analysis of the reductive decomposition of EC, but before moving on, we note that exhaustive benchmarking against CCSD(T) is computationally expensive, especially for increasingly large molecules. DLPNO-CCSD(T) provides a cheaper alternative to serve as a benchmark, especially given its good agreement with canonical CCSD(T).To test whether our conclusions are changed, we repeated the same analysis using DLPNO-CCSD(T) as the reference, and the results are shown in Figure . Overall, we observe very similar trends, although the MAD of almost all functionals is increased by about 0.50-1.40 kcal/mol due to errors introduced by the DLPNO approximations. For example, the 2.69 kcal/mol overestimation of the barrier height (step 3) by DLPNO-CCSD(T) changes the apparent accuracy of one of the best performing functionals, M06-2X-D3, from 0.63 kcal/mol to 3.33 kcal/mol. This effect needs to be kept in mind when benchmarking DFT functionals against affordable DLPNO-CCSD(T) calculations.