The intramolecular and intermolecular contributions to the longitudinal current fluctuations of the hydrogens of liquid water have been computed by molecular dynamics simulation on the TIP4P model at 245 K. This indicates that the recently discovered high frequency (approximately 165 THz) 'optical-like' collective mode of the hydrogens is due to the intermolecular contributions and that these and the intramolecular part cancel each other in the frequency range 20-120 THz. Conversely, they mutually reinforce outside this region to produce the acoustical (approximately 8 THz) and 'optical like' collective bands, in the low-k region. The high frequency mode is rooted in the librational motion of the molecules. This is shown by a comparison of the intermolecular part spectrum with that of the interparticle correlation function of the projection of the angular velocity over the principal axes of inertia of the molecule. In particular, we are able to show that the high frequency band is almost entirely due to correlations of the rotation of the tagged molecule and that of the cluster of neighbours around the axis normal to the dipole, in the molecular plane. Large cage effects are also observed in the translational dynamics, and the interparticle centre of mass velocity correlation function is found to be almost equal to the time propagated auto-correlation function.
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