Molecular dynamics of supercritical water using a flexible SPC model

This study probes the structural and dynamic phenomena of flexible supercritical SPC water through molecular dynamics simulations, performed using the reversible reference system propagator algorithm (r-RESPA) multiple time step method. The intramolecular potential consisted of harmonic bond and bond angle terms. The reaction field method was used for treating long-range interactions. Simulations were conducted for liquid water at 300 K and 997 kg/m³ and for supercritical water at a temperature of 773 K and densities in the range 115-659 kg/m³. We present thermodynamic data, pair correlation functions, self-diffusivity, velocity autocorrelation functions, power spectra, and the finite system analog of the Kirkwood g factor for both liquid and supercritical water. The simulated pressures along the supercritical isotherm were in good agreement with experimental pressures. The structural features of liquid and supercritical water using the flexible potential exhibited trends similar to those obtained using rigid models. The librational peaks of supercritical water were at lower frequencies compared to liquid water. The dielectric constant for liquid water calculated from our simulations was within 8% of the experimental value. At supercritical conditions, the calculated dielectric constants were between 1 and 25% higher than the experimental values and the agreement improved with increasing density. The self-diffusivities obtained from the simulations were within the accuracy of the experimental values. This flexible water model exhibited the correct trends for a wide variety of properties and thus, is a promising candidate for use in simulating aqueous supercritical solutions using flexible potentials.