2 where ℏω eg is the electronic excitation energy. The displacement along the α-th mode, d α , determines the strength of the electron-nuclear coupling as measured by the Huang-Rhys factor S α = 1 2ℏ m α ω α d 2 α or reorganization energy λ α = ℏS α ω α . The overall Stokes shift s is determined by s = 2λ = 2 α λ α , where λ is the overall reorganization energy.