Relations between thermodynamics, structures, and dynamics for modified water models in their supercooled regimes

Area of Science:

• Computational chemistry
• Physical chemistry
• Materials science

Background:

• Water exhibits complex phase behavior, including a proposed liquid-liquid critical point (LLCP).
• Understanding the relationship between structure, thermodynamics, and dynamics is crucial for explaining water's anomalous properties.

Purpose of the Study:

• To investigate the thermodynamic, structural, and dynamical properties of water models with systematically reduced hydrogen bond strength.
• To explore the existence and characteristics of a liquid-liquid critical point (LLCP) and liquid polyamorphism in these models.
• To elucidate the origin of the dynamical crossover observed in water.

Main Methods:

• Molecular dynamics simulations of TIP4P/2005 water models with reduced partial charges.
• Analysis of P-T diagrams to identify isochore crossings and critical points.
• Fitting to a two-structure equation of state to determine critical parameters (Tc, Pc, ρc).
• Examination of structural and dynamical properties in elongated systems at varying pressures and temperatures.

Main Results:

• Water-like models with reduced charges display an accessible liquid-liquid critical point (LLCP), indicating liquid polyamorphism between high-density liquid (HDL) and low-density liquid (LDL) phases.
• The critical temperature, pressure, and density (Tc, Pc, ρc) were determined as a function of the charge-scaling factor.
• Elongated systems showed distinct HDL-like and LDL-like regions, with differing local order and dynamics; fragility remained largely unchanged.

Conclusions:

• The observed dynamical crossover in water is attributed to the rapid conversion between HDL-like and LDL-like environments near the LLCP, not a fragile-to-strong transition.
• Water models with weaker hydrogen bonds exhibit liquid polyamorphism over a broad range of interaction parameters.
• The study provides insights into the fundamental mechanisms driving water's unique dynamical behavior.