Dynamical decoupling in the calculations of transport coefficients

The study of several transport and relaxation cross-sections for the title systems has been carried out, for the first time, using full quantum calculations in the low-energy region. The potential functions employed correspond to those given recently by a parameter-free model of the forces at play (J. chem. Phys., 1988, 88, 5465) and have been labelled the BTT potential energy surfaces. Comparison of the various generalized cross-sections, at fixed energy values, is given between the essentially exact close coupling calculations and the decoupled forms of the dynamics described currently as the coupled state and infinite order sudden approaches. Different properties perform very differently when more approximate treatments of the dynamics are employed and physical reasons are given for such behaviour. Further comparisons between calculated quantities and measured values as function of temperature are carried out, very effectively, by combining quantum calculations at low energies and classical calculations at energies well above the value of the well depths for each system. The effect of quantum treatments with respect to classical trajectory calculations, as well as the importance of using either classical or quantum partition functions, is also discussed and analysed for both molecular mixtures. We clearly find that, especially in the low-temperature region, marked differences exist between classical and quantum results for several properties.