The dependence of electron transfer (ET) coupling element, V-AB, on the length of rigid-rod-like systems consisting of bicyclo[1.1.1]pentane (BCP), cubane (CUB), and bicyclo[2.2.2]octane (BCO) monomers, has been investigated with the use of ab initio Hartree-Fock (HF) method employing Marcus-flush two-state (TS) model. The value of V-AB decreases exponentially with increase in the number of the cage units of the sigma-bonded molecules. The calculated decay constant, beta, shows good agreement with previously reported data. For molecular length greater than or equal to15 Angstrom, the value of V-AB becomes negligibly small, suggesting complete suppression of the through bond direct tunneling contribution to ET process. (C) 2002 Elsevier Science B.V. All rights reserved.