Temperature dependent spectral densities and quantum activated rate theory

The variability with temperature of spectral densities and rates calculated with quantum activated rate theory is investigated. Classical spectral densities at two temperatures are computed via molecular dynamics for a model of proton transfer in methyl chloride. In addition, quantum dynamics is computed for spectral densities which artificially boost variability at low frequency. We find significant variation in computed spectral densities at moderate frequency. These variations, however, have little effect on overall computed quantum dynamics. In contradistinction, artificial variation in spectral densities at the lowest frequencies can generate fairly significant effects on quantum dynamics. Detailed flux correlation function calculations are presented which illustrate this phenomenon. (C) 1998 American Institute of Physics. [S0021-9606(98)51237-8].