Percolation in supercritical water: Do the Widom and percolation lines coincide?

Hydrogen bonding distinguishes water from simpler fluids. Here, we use classical molecular dynamics to study the percolation transition in the hydrogen bond network of supercritical water. We find that, contrary to some previous work, the percolation line in both the pressure-temperature and density-temperature planes does not coincide with the Widom line. This difference stems from a fundamental distinction between the Widom line, which is thermodynamic in nature, and the percolation transition, which depends only on connectivity. For example, we show that percolation-related quantities collapse onto master curves when plotted with respect to a measure of connectivity rather than thermodynamic variables. We then use the Galam-Mauger formula to understand the properties of the hydrogen bonding network. We find that the percolation transition in supercritical water can shed light on the hydrogen bonding network in room temperature liquid water.