The representation of the frequency dependent friction coefficient ia a four-particle two-time correlation function is used to analyze the applicability of collisional and hydrodynamical models of vibrational energy relaxation (VER). The solute–solvent binary dynamics is separated from collective equilibrium correlations by means of Green's functions. The collective contributions manifest themselves mainly ia the solute–solvent radial distribution function (RDF), which reflects peculiarities of the particular solvent thermodynamical (e.g.,
supercritical) state. The binary dynamics is also closely related to many-body equilibrium correlations, as initial conditions sample microscopic system states in the vicinity of the solute which are the most important for
VER. VER rates along a close to critical isotherm are calculated on the basis of the breathing sphere model
and the Douglas approximation for force–time correlation functions, while Monte Carlo simulations are used
for calculating RDFs. The results are compared with molecular dynamics simulations at low, intermediate and
high densities. It is shown that at near-critical conditions as well as far from the critical point the key contribution to VER comes from the short and intermediate time behavior of the force–time correlation function. In configuration space only short range binary solute–solvent correlations are important. Analytical
estimations, Monte Carlo and molecular dynamics simulations clearly show that the dynamics of VER can only be understood on the basis of a detailed description of local solute–solvent interactions and correlations.
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