Vibrational energy relaxation in liquid oxygen from a semiclassical molecular dynamics simulation

The semiclassical theory of vibrational energy relaxation (VER) developed in the preceding paper is extended to the case of molecular liquids, and used for calculating the VER rate constant in neat liquid oxygen at 77 K. We employ a semiclassical approximation of the quantum-mechanical force-force correlation function (FFCF), which puts it in terms of the Wigner transforms of the force and the product of the Boltzmann operator and the force. The multidimensional Wigner integrals are performed via a novel implementation of the local harmonic approximation (LHA). The methodology is extended to include centrifugal forces, and effective ways are developed in order to make it applicable to molecular liquids. The fact that VER of high-frequency solutes is dominated by the solvent molecules in their close vicinity suggests that the semiclassical treatment is restricted to the small cluster of molecules around the relaxing molecule. The rest of the molecules are frozen and serve as a static cage that keeps the cluster from falling apart. The method is applied to the challenging problem of calculating the extremely slow (k(o<--l) = 395 s(-1)) and highly quantum-mechanical (homega/k(B)T = 29) VER rate constant in neat liquid oxygen at 77 K. The results are found to be in very good quantitative agreement with experiment and suggest that this semiclassical approximation can capture the 4 orders of magnitude quantum enhancement of the experimentally observed VER rate constant over the corresponding classical prediction. As for the simpler models considered in the preceding paper, we find that VER in liquid oxygen is dominated by a purely quantum mechanical term, which vanishes at the classical limit. These results further establish the semiclassical method as an attractive alternative to the commonly used approach which is based on ad hoc quantum correction factors.