Density Functional Theory (DFT) and Time Dependent DFT (TDDFT) calculations have been carried out on a series of Donor-Bridge-Acceptor (DBA) systems, where the bridges are pi-conjugated molecular wires. In these systems a lack of strong dependence of the charge-transfer (CT) rate on large variations in the D-A distance has been observed. The results show that a suitably defined length, R-eff, shows significantly smaller variation over the different bridges which have widely varying geometrical lengths. The distance R-eff is determined on the basis of application of a modified Mulliken's formula for the CT energies and comparison to those calculated by TDDFT. (C) 2016 Elsevier B.V. All rights reserved.