Quantitative investigation of the two-state picture for water in the normal liquid and the supercooled regime

Several evidences have helped to establish the two-state nature of liquid water. Thus, within the normal liquid and supercooled regimes water has been shown to consist of a mixture of well-structured, low-density molecules and unstructured, high-density ones. However, quantitative analyses have faced the burden of unambiguously determining both the presence and the fraction of each kind of water “species”. A recent approach by combining a local structure index with potential-energy minimisations allows us to overcome this difficulty. Thus, in this work we extend such study and employ it to quantitatively determine the fraction of structured molecules as a function of temperature for different densities. This enables us to validate predictions of two-state models.