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 supercrit ical conditions (630 < T < 770 K, or similar to 350-500 degrees C) ove r a very wide range of densities (0.17 < rho < 1.28 g/cm(3)) and press ures (0.25 < P < 30 kbar). The results are compared with available exp erimental data and simulations using other water models. The flexibili ty of the BJH water model made it possible to analyze the temperature and density dependencies of the intramolecular geometry and vibrationa l frequencies of water molecules along with the information on thermod ynamic, structural, and kinetic properties of water, usually calculate d from MD simulations. With temperature and density (pressure) increas e, the average intramolecular O-H distance also increases, while the a verage 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), re spectively. The spectra of intramolecular vibrations are calculated as Fourier transforms of the velocity autocorrelation functions of hydro gen atoms. The frequencies of both symmetric and asymmetric stretching vibrations increase with temperature and decrease with density (press ure), while the frequency of the H-O-H bending vibrations remains almo st constant over the wide range of thermodynamic conditions studied. T hese findings are in good agreement with available IR and Raman spectr oscopic measurements and allow us to expect the BJH potential to be ab le to predict changes in the vibrational behavior of water molecules i n response to changes of thermodynamic parameters covering the entire range of temperatures, densities, and compositions characteristic of h ydrothermal systems.