The Shape of Water – how cluster formation provides a unifying explanation of water’s anomalous properties

Water, essential to life on Earth, has many anomalous properties, that are not yet fully understood. We used computational chemistry to calculate the dynamic equilibrium of formation of water clusters in water. Clusters of certain shape are predicted to exist at ambient down to supercooled conditions and their presence almost quantitatively explains water’s anomalous properties as a function of temperature and pressure. The dodecahedron is the dominant cluster below ~240 K and below ~150 MPa and forms a miscibility gap with water below a critical point at ~225 K. The hexagonal torus is the dominant cluster above ~240 K and below ~75 MPa. Their presence explains the density maximum of pure water at 4 °C. Both the dodecahedron and the torus are low-density clusters and their tetrahedral water structure are consistent with spectroscopic data. At pressures above ~100 MPa, many of water’s anomalous properties diminish or vanish, which is also the pressure above which high-density water clusters become more stable and prevalent. Our simple unifying theory reproduces water’s anomalous properties from supercooled to ambient temperatures and from ambient pressure up to at least 400 MPa, including density anomalies, two-liquid behavior, compressibility and heat capacity.