Hello all, I am confused about Huang-Rhys parameter and electron-phonon coupling. I read a chapter of a book The biophysics of photosynthesis, called "Electron–Phonon and Exciton–Phonon Coupling in Light Harvesting, Insights from Line-Narrowing Spectroscopies". I am confused about the following part (Line 27 of Page 55): If I understand correctly, Huang-Rhys parameter in the regime of Born-Oppenheimer approximation and Franck-Condon approximation, basically shows the equilibrium configuration displacement between initial and final state of a transition. Since Born-Oppenheimer approximation ignores the non-adiabatic part of the Hamiltonian, under this approximation, electron-phonon coupling is ignored. If electron-phonon coupling is to be considered, then it needs to be accounted for by considering the non-adiabatic part of the Hamiltonian in which there are several techniques available. And it is precisely these method that we can actually figure out how strong the electron-phonon coupling is. So then how can the Huang-Rhys parameter in the regime of Born-Oppenheimer approx. shows the strength of electron-phonon coupling? Am I fundamentally missing something here?