Molecular scale precursor of the liquid–liquid phase transition of water

Abstract

Spatiotemporal fluctuations of the local structure in liquid water are studied with the molecular-dynamics simulation. At temperatures around and above the melting point, each molecule alternately goes through the structured period and the destructured period. Lifetime of each period spreads from several hundred fs to 10 ps at 0 °C at 1 atm. The order parameter to describe this structural switching fluctuations is derived by carefully filtering out the fast oscillating components from the simulation data. By analyzing the neutron-weighted pair correlation function, we find that the clusters of the structured molecules and the clusters of the destructured molecules are similar to the clusters of low-density amorphous (LDA) ice and the clusters of high-density amorphous (HDA) ice, respectively. Simulated liquid water is, therefore, a composite of the LDA-like clusters and the HDA-like clusters even at temperatures well above the melting point. The large amplitude structural fluctuation of water at around and above the melting temperature should be regarded as the molecular-scale precursor of the possible liquid–liquid phase transition in the supercooled region.