PCD as the most complex member of the test set is of stereochemical and hydrogen bond topological interest. Note that the two OH groups are cis-configured, whereas the absolute chirality is not relevant for the monohydrate. The diol forms two weak, but cooperative hydrogen bonds (OH-OH and OH-π) which give rise to separate OH stretching transitions in CCl 4 solution . The water in the monohydrate has several options to attach or insert into this hydrogen bond chain and the most stable conformation is sought. In this particular case, we recommend an extensive search among the possible conformations to identify the global minimum conformation. It will be characterised by IR/UV spectroscopy and isotope labelling. We are not aware of previous gas phase work on this system. Acceptors which were considered as suitable candidates for the test set, but are postponed for later investigation to maximise its diversity and to reduce its conformational complexity, include acetaldehyde (AAH: 75-07-0), 4'-fluoroacetophenone (FPH: 403-42-9), hexafluoroisopropanol (HFP: 920-66-1) , N,N-dimethylformamide (DMF: 68-12-2) and 2,2,4,4-tetramethylpentan-3one (TPO: 815-24-7). Other candidates which require further testing or turned out to be less suitable at this stage include benzaldehyde, 4-fluorobenzaldehyde, furan, thiophene, propylene oxide, and 4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy. Further interesting candidates which were suggested by microwave groups and will have to be further analysed in terms of suitability include propargyl alcohol , furfuryl alcohol, and furfuryl mercaptane 90 as well as thenyl alcohol and thenyl mercaptane 83 .