A structural indicator for water built upon potential energy considerations.

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Understanding the local structure of water is crucial for explaining its unique properties.
• Liquid water exhibits complex structural dynamics, especially in supercooled states.

Purpose of the Study:

• To develop a parameter-free indicator for classifying local water molecule environments.
• To elucidate the structural basis for water's anomalous thermodynamic behavior.

Main Methods:

• Introduction of a novel structural indicator for water's local environments.
• Analysis of molecular coordination and hydrogen bonding in liquid water states.

Main Results:

• A clear two-peak distribution of local structures was identified: tetrahedrally coordinated (T molecules) and three-coordinated (D molecules).
• The concentration of D molecules decreases with temperature, and their inter-conversion with T molecules explains increasing specific heat.
• T molecules resemble low-density amorphous ice, while D molecules are linked to high-density amorphous ice structures.

Conclusions:

• The T and D molecular environments provide a fundamental framework for understanding liquid water's structure and properties.
• This classification offers insights into the transitions observed in water's phase behavior, including amorphous ice forms.