Understanding how water models affect the anomalous pressure dependence of their diffusion coefficients

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Water exhibits anomalous self-diffusion behavior under hydrostatic pressure.
• This anomaly is linked to the tetrahedral hydrogen bond network.
• Accurate water models are crucial for understanding these pressure-dependent properties.

Purpose of the Study:

• To investigate the pressure dependence of water's self-diffusion coefficient using molecular simulations.
• To evaluate the performance of different water models in reproducing anomalous water properties.
• To explore the relationship between pressure, hydrogen bonding, and diffusion in water.

Main Methods:

• Molecular dynamics simulations of water were performed from 1 bar to 5 kbar.
• Three distinct water models were employed: two four-site and one single-site multipole model.
• Analysis included diffusion coefficients, radial distribution functions, hydrogen bond lifetimes, and tetrahedral order parameters.

Main Results:

• All simulated water models exhibited a pressure-induced maximum in the diffusion coefficient.
• The models showed good agreement with limited high-pressure experimental structural data.
• Simulated properties suggest pressure initially distorts hydrogen bonds, weakening them, before behaving like a normal liquid above the diffusion maximum pressure (P_mD).

Conclusions:

• The pressure at the diffusion maximum (P_mD) may indicate the angular strength of hydrogen bonds.
• Four-site models tended to over-predict P_mD, while the multipole model under-predicted it.
• Incorporating out-of-plane charge in four-site models could potentially improve their accuracy.