  A Hamiltonian based approach using spatially localized projection operators
is introduced to give precise meaning to the chemically intuitive idea of the
electronic energy on a quantum subsystem. This definition facilitates the study
of electronic energy transfer in arbitrarily coupled quantum systems. In
particular, the decomposition scheme can be applied to molecular components
that are strongly interacting (with significant orbital overlap) as well as to
isolated fragments. The result leads to the proper electronic energy at all
internuclear distances, including the case of separated fragments, and reduces
to the well-known Forster and Dexter results in their respective limits.
Numerical calculations of coherent energy and charge transfer dynamics in
simple model systems are presented and the effect of collisionally induced
decoherence is examined.
