The Gurney-Butler approach to electron transferElectron transfer was expanded by Bockris, Dogonadze, Kuznetsov and Levich with a focus on symmetry factorsSymmetry factor. They concluded that the symmetry factor is ~1/2 at the reversible potential and trends to zero at high overpotentialsOverpotential. Savéant, Hammes-Schiffer and Stuchebrukhov, Koper, and Schmickler extended the Marcus picture to include Born–OppenheimerBorn-Oppenheimer bond potentials, bond breaking, solvent relaxation, and quantum mechanical treatment of electronic wavefunctions during proton transfer. The models were becoming sophisticated computationally but the dependencies of polarizationPolarization of the double layerDouble layer, polarization of adsorbate bonds, and adsorbate structures on applied potential were not yet incorporated into the theory.