A theoretical model which successfully predicts such a single observable number like νOH b is likely to initially get the right answer for the wrong reason, as multidimensional anharmonicity is not easy to model rigorously. This is particularly true if the experimental answer is known beforehand, due to a multitude of available and conceivable model variants and parameters. Therefore, large training sets, blind test components and iterative refinements are essential ingredients on the path towards systematic success for the modeling of OH stretching wavenumbers of monohydrates. Even a largely empirical model with a high success rate for a selected observable can be useful for the experimental gas phase cluster community. This is still close to the current status, where each observable and sometimes even research group has its favourite recipes (functionals, basis sets, scaling factors) to assist experiment. There may be more or less systematic correlations between the observed wavenumber shifts and experimental or computed proton affinities and gas phase basicities or calculated hydrogen bond lengths , to name just a few possible links to be exploited. Ultimately, one must hope for a number of systematically successful models which connect theory and experiment on multiple observables, not only the vibrational wavenumber. Meaningful approximations such as elaborate scaling techniques , local mode models , parameterised harmonic DFT predictions , polarisable force fields or hierarchical models building on high level treatments of the smallest systems or subgroups may be identified or we may even witness the victory of machine-learning approaches 29 towards such challenges.