Fig. 3. (a) The definition of a hydrogen-bond between two water molecules and the free energy landscape of the hydrogen-bonds in the (b) CCD/aug-cc-pVDZ and (c) MP2/aug-cc-pVDZ simulated liquid water. The definition for a hydrogen-bond is that the distance between two oxygen atoms r_oo < 3.5 Å and θ_{∠OA···OD–HD} < 30°.