dependence of electron transfer rates on temperatures and energy gaps is investigated using more realistic spectral density for solvent fluctuation, such as ohmic, sub-ohmic, super-ohmic dissipation, and the Debye models. We will systematically examine the entire range of temperature and energy-gap dependence, covering the quantum-tunneling regime, the classical Marcus regime, and the intermediate crossover regime. (C) 1997 American Institute of Physics.