Harmonic Dynamics in Supercooled Liquids: The Case of Water

We present an analysis of the molecular motion in supercooled liquid states, based on the topology of the configurational phase space and using the instantaneous normal mode technique. We aim at characterizing the harmonicity of the dynamics and evaluating its role in the short-time dynamics. For supercooled water, we find that modes with frequency higher than 40cm^-1 have a very strong harmonic character and control, in the low- T regime, the fast process of exploration of phase space around the local minima. The fast decay of correlation functions for time shorter than 1 ps is attributed mainly to a dephasing (as opposed to a relaxation) process.