These orbital inconsistencies represent a second test of the robustness of MC-PDFT. Figure shows the absolute deviation in the reaction energy, ∆E, and the activation energy, E a (both forward and backward), for all examined reactions from the single reference limit (SRL) for CASSCF (SRL: HF), tPBE (SRL: HF-PBE), and NEVPT2 (SRL: MP2). This deviation is stratified by three degrees of multirference (MR) character (low (M < 0.05), moderate (0.05 < M < 0.1), and high (0.1 < M ). As shown clearly, both the mean absolute deviation (MAD) from the SRL and overall spread of the data increases from the low M to the high M categories. In the cases with low multiconfigurational character, M < 0.05, tPBE successfully reproduces the single-reference limit with a mean deviation of ± 1.8 kcal/mol for ∆E and ± 2.8 kcal/mol for E a , with an average between these two of ± 2.2 kcal/mol. In contrast, CASSCF and NEVPT2 reproduce these limits with a mean deviation of ± 7.9 kcal/mol and 4.4 kcal/mol repectively, with much larger maximum deviations (as high as 20 kcal/mol). These results show that MC-PDFT is significantly more robust in the single-reference limit towards active space inconsistency error (ASIE) than competing multiconfigurational approaches, making it ideal for high-throughput application. Surprisingly, we find that this robustness carries over to the performance of hybrid PDFT as well, despite it being an admixture of CASSCF and tPBE; this point bears technical discussion and is discussed in the Supporting Information. A similar analysis, using the square of the coefficient of the leading configuration, C 2 0 , as the multireference diagnostic can also be found in the Supporting Information. Two examples where tPBE shows improved reliability for a single-reference reaction are shown in Figure . The first is a trimethylamine rearrangement reaction, where the APC(12,12)-CASSCF wave functions for the reactant and product are mostly well-described by a single determinant, with M diagnostics below 0.03. Thus, the overall reaction energy is expected to be similar between each MR approach and its single reference parallel. As is seen, APC-tPBE successfully reproduces HF-PBE to within 3 kcal/mol, a result that is similarly in-line with B3LYP-D3 and CCSD(T). Despite this, APC (12,12)-NEVPT2 shows a clear deviation from all other methods, overestimating the energy of the reactant by roughly tPBE. This drastic difference from the single-reference result is emblematic of ASIE, where orbital rotation between the product and reactant results in drastically unphysical results.