MOLECULAR-DYNAMICS OF SUPERCRITICAL WATER - A COMPUTER-SIMULATION OF VIBRATIONAL-SPECTRA WITH THE FLEXIBLE BJH POTENTIAL

Molecular dynamics (MD) computer simulations have been performed for a system of 200 water molecules interacting by means of the Bopp-Jancso-Heinzinger (BJH) intermolecular interaction potential under supercritical conditions (630 < T < 770 K, or similar to 350-500 degrees C) over a very wide range of densities (0.17 < rho < 1.28 g/cm(3)) and pressures (0.25 < P < 30 kbar). The results are compared with available experimental data and simulations using other water models. The flexibility of the BJH water model made it possible to analyze the temperature and density dependencies of the intramolecular geometry and vibrational frequencies of water molecules along with the information on thermodynamic, structural, and kinetic properties of water, usually calculated from MD simulations. With temperature and density (pressure) increase, the average intramolecular O-H distance also increases, while the average intramolecular H-O-H angle decreases. Both effects increase the average dipole moment of a water molecule, which changes from 1.99 to 2.05 Debye at 400 degrees C and 0.1666 g/cm(3) and 0.9718 g/cm(3), respectively. The spectra of intramolecular vibrations are calculated as Fourier transforms of the velocity autocorrelation functions of hydrogen atoms. The frequencies of both symmetric and asymmetric stretching vibrations increase with temperature and decrease with density (pressure), while the frequency of the H-O-H bending vibrations remains almost constant over the wide range of thermodynamic conditions studied. These findings are in good agreement with available IR and Raman spectroscopic measurements and allow us to expect the BJH potential to be able to predict changes in the vibrational behavior of water molecules in response to changes of thermodynamic parameters covering the entire range of temperatures, densities, and compositions characteristic of hydrothermal systems.