On the brink of self-hydration: the water heptadecamer

For pure, neutral, isolated molecular clusters, (H₂O)₁₇ marks the transition from structures with all water molecules on the cluster surface to water self-hydration, i.e., cluster structures around one central water molecule. Getting this right with water model potentials turns out to be challenging. Even the best water potentials currently available, which reproduce collective properties very well, still deliver contradicting results for (H₂O)₁₇, when different low-energy isomers from global structure optimizations are examined. Interestingly, ab initio quantum chemistry also struggles with the only seemingly simple question if (H₂O)₁₇ is all-surface or water-centered. Hence, although the long history of water potential development may be entering its final phase, it is not quite finished yet.