Dynamics of hydrogen-bonded water networks under high pressure: Neutron scattering and computer simulation

Molecular dynamics simulation of proton-ordered high pressure ice modificat ions II and CI: was performed. Dynamics of both isotope varieties, H2O and D2O, was simulated. Rectangular simulation box of ice II contained 576 and that of ice IX 768 molecules. The average kinetic energy corresponded to 82 and 201 K for ice II and to 87 and 203 K for ice IX. One-phonon densities of states were calculated via Fourier transformation of velocity autocorrel ation functions and compared with those found experimentally from inelastic incoherent neutron scattering. This characteristic was calculated for all the molecules, as well as for the molecules of a particular crystallographi c type. Both simulated ice modifications contain molecules of two different structural types. Dynamic characteristics of molecules of different types are slightly different. Splitting of the librational peak at about 60-70 me V observed in the ice II experimental spectrum is mainly due to such differ ence. In the case of ice II simulated spectra reproduce experimental ones q uite reasonably in the whole range of energies, while in the case of ice IX agreement with the experiment is worse.