Depending on how the atomic and electronic dynamics of the system and the medium interact, the ET reaction can be either diabatic or adiabatic. In the diabatic case, an electron makes a vertical jump between the two states while the atomic configuration remains frozen. The system is then moved to its final state configuration by nuclear motions. The adiabatic reaction pathway forms a continuum of free energy between two states due to electronic coupling effect. As a result, the system evolves along with the configuration coordinate while remaining in its electronic ground state.