Optional parts of the submission can be, in the sequence of decreasing importance for the goals of this blind challenge, and in free format to be included in the supplementary information: a) A compact table of possible resonance partners of the hydrogen-bonded OH stretching mode of water, where predicted. This should include the wavenumber of the perturber and its fractional share of the intensity (square of the zero order OH stretching wavefunction), and for control purposes also the fractional share of the OH stretching wavenumber of the perturbed OH stretching state. From this and the mandatory submission of the predicted OH stretching wavenumber, a coupling matrix element can be obtained in the 2-level dark perturber approximation, see . Such information will only be possible if anharmonic calculations or coupling models are included. For other submissions, the submitted OH stretching wavenumber will be taken as the deperturbed zero order OH stretch which is obtained from experiment in such a 2-level deperturbation, in the few cases where such a resonance is observed. b) A table of any other OH stretching wavenumbers of the monohydrate, in particular the free OH stretch of the water and any other OH stretching mode in the acceptor molecule. If strong mixing of the OH oscillators is predicted, the mode with the strongest hydrogen-bonded water character should be submitted in the mandatory part and all the others in this part. One way to identify the mode with dominant water vibration character is to calculate the spectrum for H 2 O. The OH stretching mode with the strongest isotope downshift is the one of interest. c) IR intensities and Raman scattering activities for all the OH stretching modes of the monohydrate, where available, preferably in km/mol and in Å4 /u, including the depolarisation ratio for polarised light.