For R19 the HF-3c Dev-2 error is -33.5 kcal/mol, much larger in magnitude than other Dev-2 error for HF-3c. For comparison the corresponding value for HF/STO-3G is -12.0 kcal/mol. In R19 an oxirane ring is formed, which may pose a special challenge to minimal basis sets that may not offer sufficient flexibility to describe the ring strain. Indeed, the Dev-2 error for the first reaction shown in Figure is -14.6 and -12.6 kcal/mol for HF-3c and HF/STO-3G, respectively. While these errors are similar to the HF/STO-3G Dev-2 error for R19, they are still significantly smaller than the HF-3c Dev-2 error for R19, so we performed similar calculations for the second reaction shown in 2a, which is more similar to R19. The HF-3c and HF/STO-3G errors are -16.4 and -11.7 kcal/mol, respectively. The source of this increased difference in error between HF-3c and HF/STO-3G can be understood by decomposing the Dev-2 error into contributions from the two "half-reactions" (2c). The HF-3c and HF/STO-3G Dev-2 errors for the "product reaction" are very similar (-12.5 and -11.5 kcal/mol), while the HF-3c error is 4.0 kcal/mol higher than that for HF/STO-3G for the reactant reaction. Thus, the higher Dev-2 error observed HF-3c is primarily due to a higher error for the reactant. So, one possible explanation for -33.5 kcal/mol error for HF-3c is that roughly 13 kcal/mol comes from the oxirane ring in the product while the remaining error comes from the four rings in the reactant (4(4.0) = 16 kcal/mol).