R ), it is difficult to have a simple reaction mechanism and derive an analytical rate constant expression. This is indeed the case for Kramers turnover when the friction parameter is in between the energy and spatial diffusion limits . A similar situation also occurs for the theory of electron transfer under the non-adiabatic limit (golden rule, Marcus Theory) or adiabatic limit (Born-Oppenheimer, Hush Theory), where well-defined rate theories are available in both regimes , but there is no analytic theory for the entire crossover region. Nevertheless, one can apply an ad hoc approach by interpolating the two FGR expressions in Eqs. 2 and 17 as follows