Molecular dynamics simulation of vibrational energy relaxation of highly excited molecules in fluids. III. Equilibrium simulations of vibrational energy relaxation of azulene in carbon dioxide

The expressions for vibrational energy relaxation (VER) rates of polyatomic molecules in terms of equilibrium capacity time correlation functions (TCF s) derived in the first paper of this series [J. Chem. Phys. 110, 5273 (199 9)] are used for the investigation of VER of azulene in carbon dioxide at l ow (3.2 MPa) and high (270 MPa) pressure. It is shown that for both cases t he VER times evaluated on the basis of the same potential model via solute- solvent interaction capacity TCFs by means of equilibrium molecular dynamic s (EMD) simulations satisfactorily agree with the nonequilibrium (NEMD) mol ecular dynamics [J. Chem. Phys. 110, 5286 (1999)] and experimental [J. Chem . Phys. 105, 3121 (1996)] results as well. Thus it follows that these metho ds can complement each other in characterizing VER from different points of view. Although more computational power and refined methods of dealing wit h simulated data are required for EMD simulations, they allow the use of po werful tools of equilibrium statistical mechanics for investigating the rel axation process. To this end, an analysis of VER mechanisms on the basis of normal mode and atomic representations is carried out. The influence of te mperature and CO2 pressure on azulene normal mode spectra and solvent assis ted intermode coupling in connection with the eigenvector structure is inve stigated in great detail. The normal mode capacity cross-correlation matrix reveals the significance of intermode coupling, which significantly contri butes to intramolecular vibrational energy redistribution (IVR). As a new c oncept, partial normal mode relaxation rates are introduced. It is shown th at these rates demonstrate similar properties as the energy exchange rates through particular normal modes in nonequilibrium simulations. Atomic spect ra and friction coefficients are characterized by a complicated frequency d ependence due to contributions from many normal modes. Atomic capacity TCFs and partial relaxation rates are analyzed and reveal a similar picture to that obtained from NEMD simulations. These results show that VER and IVR ca nnot be separated from each other and have to be considered as mutually con nected processes. (C) 1999 American Institute of Physics. [S0021-9606(99)51 641-3].