Based on more than a dozen submissions from half a dozen countries over half a year, a small committee consisting of a PhD student, an undergraduate research student and two experimental spectroscopy group leaders made a selection of 10 systems to be recommended to the theory community for the training of their models. Among others, methanol was not included into the core training set because of its very large amplitude internal motion and a capped phenylalanine The 10 systems selected for the HyDRA training set are shown in Figure together with the three-letter acronym used in this work and with their CAS registry number. Each of them fulfills a number of desirable properties for this initial training process (Table ). All are believed to have water acting as a hydrogen bond donor in the global minimum structure, all contain less than 100 electrons to facilitate accurate quantum-chemical modeling. All except APH (where IR spectroscopy was required to detect the second isomer ) have only one experimentally observed monohydrate conformation. For all except IMZ, the free OH stretching wavenumber of the solvating water is also known experimentally. All except for the methyl ketones ACE and APH with their low barrier methyl torsions are fairly stiff, with no monomer fundamental wavenumber significantly below 30 cm -1 . For most monohydrates of the training set, there is more than one spectroscopic study in the literature, and in a few cases (ACE, IMZ) there are also isotope substitution experiments. In particular, many monohydrates (APH, ANL, CBU, DBF, IMZ) have been characterised structurally by rotational spectroscopy . In two cases (ACE, APH), there is a well-established vibrational b2lib resonance (a resonance of the water OH stretch with the bending overtone (b2) and a hydrogen bond librational (lib) mode) which makes these systems particularly interesting for advanced anharmonic models, but models not including such a resonance can also be applied, because we provide the deperturbed OH stretching wavenumber as well. In two other cases (ANL, DBN), there may be an analogous resonance, but this remains experimentally open and therefore the deperturbed value for OH b spans the experimentally observed main transition. To analyse such resonances, it is essential to avoid embedding of the monohydrates in matrices, because matrices may also induce site splittings which are difficult to distinguish from a resonance .