  We present a novel {\em ab initio} approach for computing intramolecular
charge and energy transfer rates based upon a projection operator scheme that
parses out specific internal nuclear motions that accompany the electronic
transition. Our approach concentrates the coupling between the electronic and
nuclear degrees of freedom into a small number of reduced harmonic modes that
can be written as linear combinations of the vibrational normal modes of the
molecular system about a given electronic minima. Using a time-convolutionless
master-equation approach, parameterized by accurate quantum-chemical methods,
we benchmark the approach against experimental results and predictions from
Marcus theory for triplet energy transfer for a series of donor-bridge-acceptor
systems. We find that using only a single reduced mode--termed the "primary"
mode, one obtains an accurate evaluation of the golden-rule rate constant and
insight into the nuclear motions responsible for coupling the initial and final
electronic states. We demonstrate the utility of the approach by computing the
inelastic electronic transition rates in a model donor-bridge-acceptor complex
that has been experimentally shown that its exciton transfer pathway can be
radically modified by mode-specific infrared excitation of its vibrational
mode.
