All experimental data of this blind challenge refer to translationally and rotationally cold molecules (T < 20 K) without any significant environment, but with some residual vibrational excitation in soft modes from the initial thermal population. There is the possibility of conformational trapping behind barriers, which may partially avoid relaxation to the global minimum structure. This is the typical situation in adiabatic carrier gas expansion experiments into vacuum, either free or skimmed to form a molecular beam , with a starting temperature of 300 K or somewhat higher for low-volatility compounds. The specific monohydrates are chosen to minimise the risk of significant conformational trapping in most cases. The trapping can be intermolecular, if two or more hydrogen bond formation sites are available in the solute molecule. In such a case, cold monohydrates with water docking on the less favourable site cannot quantitatively interconvert into the most favourable site on the timescale of the experiment in all cases. This is particularly relevant for the three alcohols in the test set, where water can attach to any lone pair or insert into a pre-existing hydrogen bond of the solute 87 . The insertion is typically an activated process and if the activation energy significantly exceeds 5 kJ/mol, it may not happen sufficiently fast under the low collision rates of the supersonic expansion. The trapping can also be of intramolecular nature, if the solute exists in different conformations with higher barriers. For ring puckering modes, the barriers are not always sufficiently low for efficient funneling into the vibrational ground state. For intramolecular hydrogen bonds, the solutes are chosen such that relaxation into the global minimum structure was typically proven experimentally, or is at least likely. Constitutional relaxation is evidently not considered. Thus, the decomposition of formaldehyde into CO and H 2 is disregarded for the global minimum criterion, although it is exothermic by about 9 kJ mol -1 at low temperature . The solutes always remain in the constitution given in Figure .