The mandatory part of the submissions for this blind challenge will be compact. For each of the training and test set members, a single predicted wavenumber for the hydrogen-bonded water OH stretching mode in the most stable monohydrate in cm -1 must be provided in a standardised online form. We strongly recommend to provide together with this number an error estimate. This can be based on an actual statistical analysis (e.g., from the errors observed in the training systems) or even from personal convictions. Each participant group has the freedom to select which systems to compute (and ultimately submit data). In other words, all submissions will be accepted even if incomplete. This compact table of up to 20 numbers (plus uncertainties) must be accompanied by a detailed description of the rational prediction method employed, to be included as a separate, autonomous file in the supplementary information of the planned publication. The description must contain all the details required for the reproduction of the results, such as the employed methods and program packages, with suitable references. Scaling parameters, employed atomic masses and any other parameters and computational keywords required for reproduction must be provided. This document will be in the full responsibility of the submitting authors and should include all the names and affiliations of the contributing co-authors as well as the role of each co-author in the submission. The use of ORCID ids is strongly recommended.