This work reports a careful systematic analysis of the validity of the ab initio two-state model for electron transfer between halides and various transition metal single crystal electrodes induced by an external electric field. The surfaces are represented by cluster models and the electron transfer is modeled by compact ab initio wave functions, which are a linear combination of donor-acceptor valence bond structures (DA and D+ A(-)) written in an appropriate orbital basis. It is found that the two-state model is able to successfully describe the ET for all those adsorbate/surface systems that feature a sufficiently large vertical equilibrium distance (orbitals well localized on the fragments and VB structures properly defined), and that exhibit at least a partially ionic character. The former condition occurs always when the solvent is explicitly taken into account, whereas the latter is fulfilled when not complete charge transfer occurs upon chemisorption. (c) 2006 Elsevier B.V. All rights reserved.