The curious case of the water hexamer: Cage vs. Prism

Small water clusters, such as the hexamer, provide a unique opportunity to advance the molecular-level understanding of water in all its phases. In particular, the water hexamer is the smallest cluster that possesses several nearly iso-energetic non-planar isomeric forms whose relative stability at low temperatures can be probed experimentally and investigated theoretically. Here, we report on the equilibrium populations of the isomers in the temperature range from 30 K to 150 K for both H2O and D2O as predicted by four different water potentials. The simulations, performed using path-integral molecular dynamics combined with the replica exchange method, highlight some deficiencies of empirical water models while providing support for the accuracy of more recent ab initio-based potentials. The theoretical predictions for the cage/prism isomeric equilibrium upon isotopic substitution suggest that rotational spectra measured for the deuterated cluster could deliver further insights on the ground-state properties of the water hexamer.