Low-temperature isomers of the water hexamer as predicted by q-TIP4P/F and TTM3-F potentials

The hexamer is the smallest non-planar water cluster exhibiting several isomeric forms whose equilibria are governed by the subtle interplay between energetic and entropic contributions affected significantly by nuclear quantum effects. The hexamer thus can serve as a sensitive test for model potential energy surfaces (PESs), probing both the relative depth and shape of the minima corresponding to the different isomers. Here we report on the equilibrium populations of the hexamer isomers in the temperature range from 30 K to 150 K as predicted by two popular flexible force fields (q-TIP4P/F and TTM3-F) and compare them against the results obtained recently for an ab initio-based potential (WHBB). The calculations were performed using path-integral molecular dynamics combined with the replica exchange method.