Probing the triplet correlation function in liquid water by experiments and molecular simulations.

Area of Science:

• Physical Chemistry
• Materials Science
• Liquid State Physics

Background:

• Elucidating tetrahedral molecular configurations in liquid water using scattering experiments remains challenging.
• Pair correlation functions from scattering may not fully describe water's tetrahedral ordering.
• Previous work indicated triplet correlation functions are crucial for understanding water's tetrahedrality.

Purpose of the Study:

• To link the isothermal pressure derivative of the structure factor with triplet correlation functions for water.
• To investigate the effect of pressure on the structure of liquid water.
• To determine if structure factor derivatives can reveal three-body correlations in liquid water.

Main Methods:

• Combined X-ray scattering experiments and molecular dynamics (MD) simulations.
• Determined triplet correlation functions for water up to 3.3 kbar at 298 K.
• Validated findings using a rigid-body TIP4P/2005 water model and direct simulation trajectory analysis.

Main Results:

• Triplet functions derived from simulations using the TIP4P/2005 model align with experimental results.
• Higher pressure enhances agreement with the Kirkwood superposition approximation (KSA) for tetrahedral liquids.
• Both q-space (H[combining tilde](q)) and real-space (g⁽³⁾(r,s,t)) triplet correlation functions contain similar information on liquid tetrahedrality.

Conclusions:

• The isothermal pressure derivative of the structure factor provides valuable information on three-body correlations in liquid water.
• Structure factor, despite containing only pair correlation information, can illuminate higher-order correlations.
• This approach offers a novel way to probe the tetrahedral nature of liquid water beyond traditional pair correlation analysis.