In the theory of Sumi and Kakitani on electron transfer (ET) via a midway m
olecule, the overall ET was formulated using the second-order perturbation
method and phenomenologically incorporating the renormalization factor in t
he intermediate state to escape from the divergence of the rate. In this Le
tter, we developed a nonperturbative theory of the ET via a midway molecule
and we have succeeded in naturally deriving the above renormalization fact
or under certain approximations. (C) 1998 Elsevier Science B.V. All rights
reserved.