On paper, purely harmonic estimates should be the class of calculations with the least chance of success. They stand as the most accessible calculations, but will carry the full error of anharmonic effects. However, we have already observed in previous blind challenges that harmonic approaches can be surprisingly effective. In the furan-methanol challenge, the delicate energy balance between different conformers was critically dependent on the quality of the zero-point vibrational energy (ZPVE) estimates. However, the most successful entries in general only included harmonic estimates, while some anharmonic ZPVE corrections provided the wrong energetic ordering. It all boils down to the degree of error compensation in the model chemistry chosen. In the present case, most of the anharmonicity of an OH group is diagonal in nature and does not change much upon hydrogen bonding. By a quite significant margin, the best submission Fig. Deviations of the submitted O-H stretch shifts relative to the measured experimental values (raw values) in cm -1 for the 10 test systems. A positive error means that the predicted wavenumber downshift from the monomer is too large. Each value is represented as a white dot (only one value is outside of the visualisation window, AC1 submission for CON, with an error of 460.5 cm -1 ). The color coding of the graph is provided by the probability density function of the error for each system. The bandwidth selection has been made with Scott's rule. The dotted line delimits the RMSD range of the null hypothesis, obtained as described in the text. in this class was PH5, making use of MP2/6-31++G(2d,p) counterpoise-corrected harmonic values. The RMSD for all submissions in this class are depicted in Fig. . We provide values incrementally removing the worst prediction of each set, in order to compare in a more balanced way the different submissions. The largest deviations for PH1 and PH3 are due to wrong structures for the PCD minimum upon which the frequencies are based. In the case of PH1 the wrong organic molecule was used, 1phenylcyclohexane-cis-2,5-diol instead of 1-phenylcyclohexanecis-1,2-diol. In the case of PH3, a different minimum was found, leading to an error of 98.7 cm -1 for PCD. The water is correctly pointing to the ring system, but also is hydrogen bonding to one of the alcohols (in the accepted global minimum it should be accepting a bond from the alcohol). We provide the overlap of a few selected PCD complex structures in Fig. . Both sets are of relatively similar quality, with B3LYP-D3(BJ)/def2-TZVPP (PH1) and ωB97xD/def2-TZVP 40 (PH3) delivering the overall same level of accuracy. Once the outliers are removed, the RMSD is found to be below 40 cm -1 . The PH2 submission worst value (for MLA, error of 155 cm -1 ) does not appear to be due to the minimum chosen, but solely the approach used. The PBE0-D3/aug-cc-pVTZ level of theory reveals large errors for the whole set, at least as submitted by the participating group. The only geometry which was not correctly predicted was for TPH. The water binds to the side of the trifluoro group, when it should be docking to the carbonyl group from the opposite end. Despite the wrong geometry, and through error compensation, the deviation is only 23 cm -1 , the second best prediction in PH2. Curiously enough, despite the similarity in method between PH1 and PH4, the smaller basis set chosen in the latter submission seems to have a sizeable impact (an almost constant 15 cm -1 increase in RMSD). All structures of PH4 were in line with the best results. Attempting to build a summary for Fig. Comparison of PCD hydrate complex structures submitted by LS1 (red) and PH3 (blue). In the latter case, the water molecule functions both as acceptor and donor to the alcohol moieties. In the structures featured in submissions with lower RMSD, the water is actually pointing towards the π-system, interacting with only one of the alcohols. This gives a much lower shift, in line with the experimental observations. this class of submissions, and removing the impact of wrong structure submissions, there was a decisive advantage in the use of the MP2 method and a small basis set. This could be due to the well documented fact that MP2 will overcorrect Hartree-Fock in predicting OH hydrogen bond shifts (too large downshifts). By reducing the basis set, the amount of electron correlation described decreases, compensating the harmonic and method errors. This is partly confirmed in the harmonic estimates underlying the AC4 submission, where the basis was expanded to 6-311++G(d,p) and the results were overall worst (if no anharmonic corrections were included).