  I investigate energy transfer in a donor-acceptor pair beyond weak
system-bath coupling. I identify a transition from coherent to incoherent
dynamics with increasing temperature, due to multi-phonon effects not captured
by a standard weak-coupling treatment. The crossover temperature has a marked
dependence on the degree of spatial correlation between fluctuations
experienced at the two system sites. For strong correlations, this leads to the
possibility of coherence surviving into a high temperature regime.
