Improvement in hydrogen bond description using van der Waals-corrected DFT: The case of small water clusters

The method recently developed to include van der Waals interactions in DFT using Wannier functions is applied to small water clusters, characterized by Hydrogen-bonding interactions. In particular, water hexamer represents a critical test case since it is relevant for the condensed phases of water and has four low-energy isomers with similar binding energies. We are able to achieve results close to those obtained by high-level quantum chemistry methods and we find that inclusion of van der Waals effects is crucial for an appropriate description of the Hydrogen bonds and to get accurate dissociation-energy estimates. The relevance of these results for DFT simulations of liquid water is finally discussed.