Using Car-Parrinello simulations and microscopic order descriptors to reveal two locally favored structures with distinct molecular dipole moments and dynamics in ambient liquid water

Abstract

Water is essential for life and technological applications, mainly for its unique thermodynamic and dynamic properties, often anomalous or counterintuitive. These anomalies result from the hydrogen-bonds fluctuations, as evidenced by studies for supercooled water. However, it is difficult to characterize these fluctuations under ambient conditions. Here, we fill this knowledge gap thanks to the Car-Parrinello ab initio molecular dynamics (MD) simulation technique. We calculate the local structural order parameter ζ, quantifying the coordination shells separation, and find two locally-favored structures or states: High-ζ and Low-ζ. On average, High-ζ molecules have a tetrahedral arrangement, with four hydrogen bonds, and the first and the second coordination shell well separated. The Low-ζ molecules are less connected, partially merging the first and the second shells. The appearance of isosbestic points in the radial distribution functions and the collective density fluctuations at different length scales and timescales reveal that the two-state model, consistent with available experimental data for supercooled water, also holds under ambient conditions, as we confirm by analyzing the vibrational spectrum of both types of water molecules. Significant consequences of the structural differences between the two states are that High-ζ molecules have a dipole moment 6 % higher than Low-ζ. At the same time, Low-ζ structures are more disordered and with more significant angular fluctuations. These differences are also reflected in the dynamics under ambient conditions. The Low-ζ molecules decorrelate their reorientation faster than High-ζ and merge their coordination shells within 0.2 ps, while the High-ζ preserve the shell separation for longer times. Our analysis shows that first-principle calculations make predictions under ambient conditions calling for new and definitive experiments confirming the two-state model.