  We study the excitation energy transfer (EET) for a simple model in which a
massless scalar particle is exchanged between two molecules. We show that a
finite-size effect appears in EET by the interaction energy due to overlapping
of the quantum waves in a short time interval. The effect generates finite-size
corrections to Fermi's golden rule and modifies EET probability from the
standard formula in the Forster mechanism. The correction terms come from
transition modes outside the resonance energy region and enhance EET
probability substantially.
