Effect of model flexibility on the behavior of supercritical water in molecular dynamics simulation

ZH Jiao and MM Song and Y Liu and LJ Guo, JOURNAL OF MOLECULAR LIQUIDS, 382, 121997 (2023).

DOI: 10.1016/j.molliq.2023.121997

Supercritical water plays an irreplaceable role in the technological application and geological process. Building accurate water potential models from the atomic scale is crucial to reveal the nature of water. However, the application of the rigid model is limited due to the more pronounced vibration of water molecules in the supercritical region. Conflicts exist on whether the model's performance could be improved by introducing flexibility. Here we employ rigid SPC/E, TIP4P, and flexible SPC/Fw models to perform molecular dynamics simulations under extensive thermodynamics states (447 K to 847 K and 15 MPa to 31 MPa). By analyzing the water's structure and dynamics along isobars and isotherms, we find that introducing flexibility in SPC/Fw is a double- edged sword. The increase in the molecular degree of freedom makes the system's density and selfdiffusion coefficient closer to the experimental value while negatively contributing to a more structured solvation shell. Furthermore, diffusion coefficient and hydrogen bond show opposite correlation to flexibility change. A robust linear relationship between water geometry, density, averaged H-bond number and self-diffusion coefficient is established. This work provides a deeper insight into water's structure and dynamics and sheds light on more accurate atomic model construction for water-containing systems.

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