Electronic coupling Hush approximation

There has been considerable confusion in the literature about what the ET distance, the dab of Equations (2) and (3), actually represents. For transition metal-centered systems the metal-metal distance (estimated from model compounds such as a non-IV oxidation level of the compound of interest) was traditionally used as 7ab. However, as first pointed out clearly by Cave and Newton in their generalized Mullik-en-Hush theory,53,54 dab refers to the diabatic surfaces, while real molecules exist on adiabatic surfaces with electronic couplings intact, so it cannot be directly measured experimentally. Cave and Newton also pointed out that one can convert an ET distance measured on the adiabatic surface, a du value, to <2ab using the optical spectrum with Hush s Gaussian approximation of Equation (3), by employing the following equation ... 

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