Relationship between the barrier (ΔG‡) to thermal electron transfer, the energy of a corresponding optical charge-transfer (CT) transition (Δ E op), and the overall change in standard Gibbs energy accompanying thermal electron transfer (Δ G o). Assuming a quadratic relation between the energy of the system and its distortions from equilibrium (harmonic oscillator model) the expression obtained is: \[\Delta G^{\ddagger} = \frac{\Delta E_{\text{op}}^{2}}{4\ (\Delta E_{\text{op}}\,-\,\Delta G^{o})}\] The simplest form of this expression obtains for degenerate electron transfer (Δ G o) in e.g. symmetrical mixed valence systems: \[\Delta G^{\ddagger} = \frac{\Delta E_{\text{op}}}{4}\] Note that for this situation the Marcus equation reads: \[\Delta G^{\ddagger} = \frac{\lambda }{4}\]