Comparison of theoretical methods for resonant vibration-vibration energy transfer in liquids

The problem of resonant vibration-vibration (V-V) energy transfer in liquids is explored within a simple model in order to compare two calculational methods. Two bromine diatomics are constrained to move between two fixed argon solvent atoms in one dimension. The time-dependent probability for the transfer of a vibrational excitation between the bromine diatomics is computed semiclassically. The results of single-trajectory molecular dynamics methods are compared with those of multiple-trajectory surface hopping methods. It is found that the two methods give similar results, indicating that the simpler single-trajectory method adequately describes the resonant V to V transfer process. The proximity of the nearly degenerate potential energy surfaces leads to a phase coherence time that exceeds the time required for the transition probability to saturate for this model. As a result, the transition probability remains a nonlinear function of time, and this precludes the extraction of a rate constant from the slope of the resonant V-V transition probability curve for this simple one-dimensional model. (C) 1998 American institute of Physics.