MOLECULAR-ORIGIN OF FRICTION IN LIQUIDS

Relaxation in liquids can often be formally attributed to ''friction,' ' a generally time-dependent quantity that can be quantitatively speci fied, but is difficult to break into molecular components. We show her e how the instantaneous normal modes of a fluid can be used to derive this friction, thereby identifying its molecular origins. This approac h is illustrated by dissecting the vibrational relaxation of a dissolv ed diatomic molecule into contributions from specific kinds of solvent dynamics.