A theoretical model is presented aimed to provide a detailed microscopic description of the electron transfer reaction in an electrochemical environment. The present approach is based on the well-known two state model extended by the novelty that the energy of the two states involved in the electron transfer reaction is computed quantum mechanically as a function of the solvent coordinate, as defined in the Marcus theory, and of the intensity of an external electric field. The solvent conformations defining the reaction coordinate are obtained from classical molecular dynamics and then transferred to the quantum mechanical model. The overall approach has been applied to the electron transfer between a chloride anion and a single crystal Cu(100) electrode. It is found that the solvent exerts a strong influence on the equilibrium geometry of the halide and hence on the relative energy of the two states involved in the electron transfer reaction. Finally, both solvent fluctuations and external field facilitate the electron transfer although solvent effects have a stronger influence. (C) 2004 American Institute of Physics.