Self-intermediate scattering function of strongly interacting three-dimensional lattice gases: time- and wave-vector-dependent tracer diffusion coefficient.

We investigate the self-intermediate scattering function (SISF) in a three-dimensional (3D) cubic lattice fluid (interacting lattice gas) with attractive nearest-neighbor interparticle interactions at a temperature slightly above the critical one by means of Monte Carlo simulations. A special representation of SISF as an exponent of the mean tracer diffusion coefficient multiplied by the geometrical factor and time is considered to highlight memory effects that are included in time and wave-vector dependence of the diffusion coefficient. An analytical expression for the diffusion coefficient is suggested to reproduce the simulation data. It is shown that the particles' mean-square displacement is equal to the time integral of the diffusion coefficient. We make a comparison with the previously considered 2D system on a square lattice. The main difference with the two-dimensional case is that the time dependence of particular characteristics of the tracer diffusion coefficient in the 3D case cannot be described by exponentially decreasing functions, but requires using stretched exponentials with rather small values of exponents, of the order of 0.2. The hydrodynamic values of the tracer diffusion coefficient (in the limit of large times and small wave vectors) defined through SIFS simulation results agree well with the results of its direct determination by the mean-square displacement of the particles in the entire range of concentrations and temperatures.

Nonequilibrium molecular dynamics simulations of vibrational energy relaxation of HOD in D2O.

Vibrational energy relaxation of HOD in deuterated water is investigated performing classical nonequilibrium molecular dynamics simulations. A flexible SPC/E model is employed to describe the intermolecular interactions and the intramolecular potential of the D(2)O solvent. A more accurate intramolecular potential is used for HOD. Our results for the OH stretch, OD stretch, and HOD bend vibrational relaxation times are 2.7, 0.9, and 0.57 ps, respectively. Exciting the OH stretching mode the main relaxation pathway involves a transition to the bending vibration. These results are in agreement with recent semiclassical Landau-Teller calculations. Contrary to this previous work, however, we observe a strong coupling of bending and OH stretching mode to the HOD rotation. As a result almost half of the total vibrational energy is transferred through the HOD rotation to the bath. At the same time the most efficient acceptor mode is the D(2)O rotation indicating the importance of resonant libration-to-libration energy transfer. We also find significant vibrational excitation of the D(2)O bending mode of the D(2)O solvent by V-V energy transfer from the HOD bending mode.

Mode-specific energy absorption by solvent molecules during CO2 vibrational cooling.

Non-equilibrium molecular dynamics (NEMD) simulations of energy transfer from vibrationally excited CO(2) to CCl(4) and CH(2)Cl(2) solvent molecules are performed to identify the efficiency of different energy pathways into the solvent bath. Studying in detail the work performed by the vibrationally excited solute on the different solvent degrees of freedom, it is shown that vibration-to-vibration (V-V) processes are strongly dominant and controlled by those accepting modes which are close in frequency to the CO(2) bend and symmetric stretch vibration.

Molecular dynamics modeling of cooling of vibrationally highly excited carbon dioxide produced in the photodissociation of organic peroxides in solution.

Non-equilibrium (NEMD) and equilibrium (EMD) molecular dynamics simulations are performed to investigate the vibrational cooling and asymmetric stretch spectral evolution of highly excited carbon dioxide produced in the photodissociation of organic peroxides in the solvents dichloromethane, carbon tetrachloride and xenon. Due to strong Fermi resonance the symmetric stretching and bending modes of carbon dioxide in CH2Cl2 and CCl4 jointly relax on a ten and hundred picosecond timescale, respectively, which is in accordance with experiment. However, the high frequency CO2 asymmetric stretch vibration relaxes on a considerably longer time scale because of weak interaction with the other modes. The relaxation rate coefficients of (and works done by) different modes obtained from NEMD and the Landau-Teller rate coefficients calculated through equilibrium force time correlation functions are in reasonable agreement. The analysis of these results leads to the conclusion that, in contrast to xenon where the relaxation takes about 20 ns, the shorter time scales in CH2Cl2 and CCl4 are caused by efficient near resonant vibration to vibration energy transfer from carbon dioxide to solvent molecules. The results of the non-equilibrium simulations are used to monitor the quasi-stationary asymmetric stretch infrared spectra of carbon dioxide during the cooling process. Comparison of the corresponding experimental results suggests that carbon dioxide initially is produced with a broad distribution of energy disposed in its bend and symmetric stretch modes while the asymmetric stretch mode remains unexcited.