  We examine electron transfer between two quantum states in the presence of a
dissipative environment represented as a set of independent harmonic
oscillators. For this simple model, the Marcus transfer rates can be derived
and we show that these rates are associated to an explicit expression for the
environment correlation time. We demonstrate that as a manifestation of the
Goldilocks principle, the optimal transfer is governed by a single parameter
which is equal to just the inverse root square of two.
