Molecular dynamics simulation of liquid anisotropy. II. Rise and fall transients on a picosecond time scale

The behavior of a liquid sample subjected to an external torque of picosecond duration is investigated by molecular dynamics simulation, using 108 C2v triatomics interacting with a 3×3 site–site Lennard-Jones potential. The resulting rise and fall transients are computed and their behavior is such as to fall on Langevin’s functions from the linear response region to saturation, i.e., when the energetic perturbation (E) is less than kT to the point at which it is about 10 times kT. The breakdown of the fluctuation–dissipation theorem is investigated at the point E/kT = 12, where the normalized fall transients no longer decay as the equilibrium autocorrelation functions but considerably faster. The simulation does not support the conventional view of rise and fall transients, which is based on Debye’s theory of rotational diffusion.