The other side of the coin are the purely harmonic (PH) calculations, which heavily rely on error compensation. PH5 is built on a very conscious decision of using the Pauling point. Albeit calculations with larger basis sets were available (and provide the basis for AC4), cutting the number of functions leads to an improved error compensation. The robustness is impressive given the amount of approximations involved. We find two questions of interest for the future. The first one is whether this robustness will still hold when increasing the number or diversity of systems under study. This could be verified in future editions of the challenge or simply calculating some of the other systems which were suggested for the training set. The other question is whether this type of error cancellation can be found in other (relatively cheap) electronic structure methods, in particular DFT functionals. The three models used in this category (B3LYP, PBE0 and ωB97xD) seem to not hold to the task. However, the PH data sets are mostly subproducts of other computational protocols, and the 'Pauling point' was not being explicitly sought for. There could be alternatives.