Non-linear effects in rotational dynamics in the liquid state

Through general theoretical considerations, based only on the Markoffian assumption, it is shown that equilibrium non-gaussian properties can be used to predict how the decay of excited states deviates from a linear response theory behaviour. To check this prediction, a computer experiment on a ‘two-dimensional liquid’ of discs interacting via a Lennard-Jones plus electric dipole potential is carried out. The results obtained for both equilibrium and excited correlation functions confirm completely the intimate relation between equilibrium non-gaussian properties and excitation behaviour.Via approximate analytical expressions and exact solutions, it is also shown that a non-linear version of the popular itinerant oscillator provides the same equilibrium non-gaussian properties and, in consequence of that, the same kind of deviation from linear response as the two-dimensional system.