The fully anharmonic submissions were perhaps the biggest upset overall. Some submissions were able to break below an RMSD of 40 cm -1 , but they were far from reaching the top 4. One should say that this is partly due to the way the challenge was conceived. The test set consisted of both large and small systems, significantly limiting the level of theory that could be applied across all (or at the least the majority of) systems. It would have been perhaps of interest to have a few higher-level calculations for the smallest hydrates. This can be a task for future studies, although one will miss the blind character of the challenge. In future editions, one could also attempt to include more groups with a focus on ab initio vibrational spectroscopy which could potentially restrict themselves to the smallest systems featured. Another deciding factor was to estimate the shift in the OH frequency, i.e. to estimate a difference of frequencies instead of the absolute value. This decreases the importance of anharmonic effects and strongly favours methods which most heavily rely on error compensation (e.g., the PH submissions). The fully anharmonic (FA) methods have a much better comparative performance when looking at absolute fundamental band positions.