The target benchmark observables are summarised in Table . Either the absolute OH b -wavenumbers νexp (OH b ) of the hydrogen-bonding water or equivalently their downshifts ∆ OH from the isolated water symmetric stretching fundamental can be targeted. For models including full anharmonicity and thus the postulated Darling-Dennison 1-3 resonance for four systems, the most intense OH b transition can be addressed directly. For models which do not include such resonances, it may be better to address the values in square brackets, which attempt to correct for this anharmonic perturbation with a coupling matrix element on the order of 10 cm -1 and involve a larger error bar due to the correction uncertainty. For imidazole it is worth mentioning that most techniques detect both the NH-and OH-bonded isomers , whereas theory and a slit jet expansion study 17 indicate a clear energetical preference for the OH-bonded isomer which is of interest in the context of this challenge.