6 and the Fermi level cannot be controlled independently from the solution phase. This indicates that there is no guarantee that the electrode potential can be controlled independently of solution phase species. This is not a problem for adiabatic electron transfer theories or adiabatic GCE-DFT simulations because the orbital filling depends explicitly on the electrode potential and should not be depend on other factors. However, simulations and theory of non-adiabatic electron transfer is based on diabatic states , which retain their charge state irrespective of the electrode potential. When using diabatic states and GCE-DFT to simulate electron transfer as a function of the electrode potential, both the reactant charge state and the electrode potential should be independently fixed. This cannot be achieved in constant We consider a classical outer-sphere redox couple using a diabatic description: a Ru[NH 3 ] 3+ 6 complex on the Au(111) surface as depicted in Figure . The Ru[NH 3 ] 3+ 6 charge is always +3 regardless of E DF T f because constrained DFT was used to regulate the Ru[NH 3 ] 6 charge state (see Section 4 for computational details). To understand the electrode potential, Fermi level, and inner potential variations in DFT simulations of outer-sphere reactions, the total charge of the simulation cell was varied to study the potential range of ±1.0 V around the reduction potential of Ru[NH 3 ] 3+ 6 . The molecular orbitals presented in Figure show that the redox couple and the electrode are strongly hybridized; this prevents separating the total system to two subsystems in GCE-DFT simulations. This is also seen in the DOS plot in Figure , which demonstrates that all electronic states near the Fermi level contain significant contributions from the Ru[NH 3 ] 3+ 6 complex despite the correct +3 charge on the redox couple. The electronic states residing on the redox couple are visible in the DOS plot around -0.2 eV at all charge states (or electrode potentials) except when the simulation cell carries a very high negative charge (line "-3.58" in Figure ). This means that E f is determined exclusively by the electrode only on very negatively charged gold surfaces. Table show that electrode potential variations inferred from either ϕ M or E f are small but systematically different. In particular, E f depends more sensitively on the charge state than ϕ Au . This observation is attributed to the large number of electronic states residing on the redox couple at the Fermi level and means that a certain charge in the simulation cell spreads across the electrode and solvent phase species. The reason for the different electrode potential differences obtained from ϕ Au and E DF T f can be understood based on the results in Figure . This data shows that the electrostatic potential maxima within Au and the Fermi level do not coincide, but there is a non-constant difference between the inner potential maxima and E DF T f at the same surface charge; this non-constant shift shows that E DF T f is not solely determined by µ 0