A simple but accurate approach, the charge-on-site scheme, is introduced to calculate coupling matrix elements for electron transfer (ET) in donor-bridge-acceptor (d-b-a) systems. The electronic coupling is explicitly expressed through energies of relevant adiabatic states and charges on the donor and acceptor sites. The method is formulated within two- and multistate models. The proposed scheme can be directly employed in conjunction with any quantum mechanical method providing atomic charges in the ground and excited states. As an illustration, the charge-on-site method is applied to a pi stack GAG consisting of three nucleobases. (c) 2007 Elsevier B.V. All rights reserved.