There are indeed many interesting monohydrates which were not considered for the present challenge for various reasons. This includes systems where the global minimum structure is hidden behind a large monomer isomerisation barrier , and some systems which are closely related to members of the training or test set . It also includes the numerous systems where the global minimum structure involves water as a pure acceptor. In terms of acceptor elements beyond O and N, there are many extension possibilities towards S 83,106,107 which remain to be explored. For the structurally well characterised acid hydrates , one may expect complex vibrational dynamics whenever the OH stretching excitation comes close to degenerate proton transfer barriers.