Unravelling Water Anomalies Using a Percolation Approach

Computational studies suggest that the maximum density of water at 277 K and the divergence from experimentally measured compressibility at 228 K at 1 bar arise from the competition between low and high-density molecular populations. However, the relationships among these anomalies, including the minimum density at 203 K, are still debated. Using molecular dynamics with the TIP4P/2005 model, we investigate the pressure-temperature behavior of metastable water along isothermal and isobaric lines to show that in both cases the density and compressibility anomalies result from the emergence, coexistence and extinction of low and high-density percolating clusters. We connect these behaviors with distinct stages of metastable water phase separation, which occur in regions of steep increase in sample density. Our results are consistent with liquid-liquid critical point scenarios while establishing a phenomenological percolation-based framework that systematically organizes water anomalies across diverse temperatures and pressures. This framework may explain percolation transitions observed in oxide and metallic glasses, providing a perspective to describe amorphous transformations in pure substances.