where 𝐻 78 = Y𝑖[𝐻 \ 78 [𝑓^ and 𝑆 *. = ⟨0|𝜈⟩ are the electronic coupling term between the electronic states of the reactant (|𝑖⟩) and the product (|𝑓⟩) and the overlap integral between the vibrational states of the reactant (|0⟩) and the product (|𝜈⟩), respectively. While neither of these terms is directly accessible experimentally, surrogates could be identified with reasonable approximations. 𝐻 78 is the matrix element of electronic transition with a time-independent and non-oscillating perturbing field and can be expressed as: