Comparison of rigid and flexible simple point charge water models at supercritical conditions

Abstract

This study investigates the differences between the predictions of various properties of rigid and flexible simple point charge water models at supercritical conditions. Molecular dynamics simulations were conducted for supercritical water in a temperature range of 773-1073 K and densities in the range 115-659 kg/m3. We present thermodynamic data, pair correlation functions, selfdiffusivity, power spectra, dielectric constants, and variaous measures of hydrogen bonding at different state conditions. The flexible water model performs better in predicting the pressures along the supercritical isotherms simulated. Agreement between experimental and calculated dielectric constants is superior for the flexible water model, particularly at high densities. The flexible model exhibits a greater degree of hydrogen bonding and more persistent hydrogen bonds than does the rigid model. The structural features of supercritical water at high densities are identical for the two water models. At low densities, however, the flexible potential exhibits pair correlation functions with enhanced peaks. Inclusion of flexibility in the potential model does not result in a significant shift in the position of the rotational/librational peak in the power spectrum. The self-diffusivities obtained from the simulations are within the accuracy of the experimental values for both the rigid and flexible models. On balance the inclusion of flexibility improves agreement with the properties of real supercritical water while incurring little or no additional computational burden.

Promoting the Adsorption of Metal Ions on Kaolinite by Defect Sites: A Molecular Dynamics Study

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“…This leads to much longer lifetimes than obtained in case ͑1͒. Since this first, yet elaborate attempt to understand H bond dynamics in water almost 20 years ago 16 other authors have typically focused on either of these methods using a particular potential and definition for a bond to estimate HB lifetimes in several systems: ambient water, 18 pure supercritical water, [19][20][21] solvation in supercritical water, 22 pure methanol, 14 pure ethanol, 23 water near hydrogels, 24 by assuming a ''quasiexponential'' decay of calculated correlations functions.…”

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