Electrochemical electron-transfer reactions form the basis of such important devices as fuel cells and sensors. Previous theories of these reactions were limited either to the case of weak electronic interaction between the electrode and the reactants, or to strong interactions. In this work the rate of electron exchange is calculated by a combination of quantum mechanics and computer simulations. This method is valid for all strengths of the electronic interaction, so that the dependence of the reaction rate on the interaction strength could be obtained. Our results encompass three different regimes; in the order of increasing interaction these are: (i) a linear region, in which first-order perturbation theory holds; (ii) a weakly adiabatic region, in which the rate is limited by solvent dynamics; (iii) a strongly adiabatic region, in which the interaction lowers the energy of activation.