Instantaneous normal mode analysis of short-time dynamics in hydrogen-bonded liquids

We discuss to which extent the presence of strong directional bonds influences the short-time dynamics of hydrogen-bonded liquids. We use, as a main tool in this analysis, the instantaneous normal mode approach, which is based on the quadratic approximation of the true interaction potential. We exploit the instantaneous eigenmodes and eigenvalues to analyse the correlated motions of clusters of molecules in various H-bonded liquids, projecting the actual motions on some a priori modes, which can be expected on the basis of symmetry considerations. In the case of methanol and hydrogen fluoride, linear chain forming liquids, the emphasis is about the motion of triplets of bonded molecules. In the case of water we analyse the motion of pentamers and triplets with accepted or donated bonds.