The new experimental data for the 10 members of the test set are detailed in the ESI †, as pre-published after the challenge submission deadline , together with the supersonic jet spectra and their assignments. They are listed in Fig. together with the experimental values for the training set, which were discussed before . In many cases, the assignments are straightforward and leave little room for interpretation, already based on infrared spectroscopy alone . In a few cases, complementary Raman spectroscopy was needed to safely identify the hydrogen-bonded O-H stretch of the water unit in the monohydrate. Here, we only discuss some key results for the most complicated case of MLA, where there is evidence for mode mixing between the hydrogenbonded OH groups of the two complexed molecules.