Vibrational relaxation in simple fluids: Comparison of theory and simulation

General theoretical expressions for the dephasing and energy relaxation times of a stiff oscillator in simple fluids are derived from the GLE and a critical discussion of the dynamic processes in these systems is given. In addition new methodological aspects of stochastic and full molecular dynamics simulations are discussed. The new reversible integrator based on the Trotter factorization of the classical propagator is used to directly simulate the vibrational energy and phase relaxation of a stiff classical oscillator dissolved in a Lennard-Jones bath. We compare the "real" relaxation from full MD simulations with that predicted by Kubo theory and by the generalized Langevin equation (GLE) with memory friction determined from the full molecular dynamics. It is found that the GLE gives very good agreement with MD for the vibrational energy relaxation, even for nonlinear oscillators far from equilibrium. The dephasing relaxation is also well approximated by the GLE.