Non‐radiative transitions: Fundamental difficulties of the adiabatic base approach

The adiabatic base approach is extensively used in literature (“Condon” approximation, “non‐Condon” approximations). Two fundamental difficulties are indicated: (a) for large Huang‐Rhys factors it is not the non‐adiabaticity operator which governs the non‐radiative decay but tunneling; (b) for a small transitive coupling parameter Δ very high potential barriers (with divergent area) are built up as a non‐adiabatic consequence of non‐crossing. A possible way out is shown by suggesting a release of the non‐crossing requirement.