The importance of tetrahedrally coordinated molecules for the explanation of liquid water properties.

Ab initio calculations on molecular clusters and a quantum statistical model are used to probe the structure of liquid water and its anomalies. Characteristic temperature dependent mixtures of ring and three-dimensional, voluminous water clusters provide the famous density maximum. The mixture model also reproduces the shift of the density maximum as a function of pressure and isotopic substitution. This finding is consistent with femtosecond spectroscopy data suggesting that two distinct molecular species exist in liquid water. The given structures also reproduce the oxygen-oxygen pair correlation function and the vibrational IR spectrum of liquid water. The results underline the importance of three-dimensional, tetrahedrally coordinated structures for the understanding of water anomalies and the existence of two liquid phases in the supercooled region.