LIBRATIONAL DYNAMICS OF WATER IN TERMS OF ANGULAR VELOCITY CORRELATION-FUNCTIONS AND ORIENTATIONAL STRUCTURE

The high frequency (librational) dynamics of water is studied by molecular dynamics simulation on the TIP4P model at 245 K. The single-molecule part of the hydrogen current is described by a combination of autocorrelation functions of components of the angular velocity along the principal axes of inertia of the molecules. The distinct part of the hydrogen current is represented by a sum of cross correlation functions of the same components. The spectrum of the self current is almost quantitatively reproduced by the sum of the spectra of two of these contributions. The first is that relative to the component along the axis normal to the molecular plane and the second along the axis normal to the dipole vector, in the molecular plane, both multiplied by a geometrical parameter of the molecule. In the case of the distinct part, the interparticle correlation function of the projection of the angular velocity over the same two axes is weighted by the dot product of the dipole moments of each pair of molecules. The agreement between the results obtained this way and according to the usual definition is qualitatively satisfactory. The axis normal to the dipole in the plane of the molecule turns out to be a favourite channel for the propagation of rotational correlations in the liquid. The analysis of hydrogen current in terms of rotational correlations allows to show that the transition from single-molecule to collective dynamics is essentially complete when clusters of two or three shells of neighbors are considered. A simple model of the distinct hydrogen current as a time-propagated self function is able to qualitatively account for all features of the spectra of both longitudinal and transverse current. The ratio of the transverse to longitudinal delay time is shown to be equal to the square root of the amplitude of the librational band of the dielectric spectrum.