Inherent structures of water using fully ab initio simulations.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Understanding the complex structure of liquid water is crucial for various scientific disciplines.
• Previous models have struggled to fully explain water's anomalous properties, such as its density maximum.

Purpose of the Study:

• To investigate the inherent local structures of liquid water using *ab initio* molecular dynamics.
• To elucidate the temperature dependence of these structures and their role in water's properties.

Main Methods:

• Employing fully *ab initio* molecular dynamics simulations.
• Utilizing the SCAN functional for accurate electronic structure calculations.
• Classifying local water structures based on the translational ordering of the second oxygen coordination shell.

Main Results:

• Identified three distinct local structures in water: Structure I (disordered), Structure II (similar to ice II/V), and Structure III (similar to ice III).
• Observed differences in tetrahedral orientational and bond-angle distributions between liquid water structures and crystalline ices, despite similar translational ordering.
• Demonstrated that the competition between these structures explains the density maximum of water.

Conclusions:

• Liquid water and crystalline ices, while sharing some translational ordering, possess fundamentally different local atomic structures.
• The proposed mixture model of water, incorporating Structures I, II, and III, is supported by *ab initio* simulation evidence.
• These findings offer a deeper understanding of water's unique behavior and its underlying molecular mechanisms.