Since K j is dependant on the electronic/nuclear terms, and G j ′ on the vibrational terms, this can be more simply explained as the rate of internal conversion between a promoting electronic state, and the accepting vibrational states of another electronic state; where the rate of coupling/overlap of these two components determines the speed of the mechanism. The evaluation of K j and G j ′ can be performed using Mehler's formula . Formally, the coupling matrix elements can be expanded with respect to the normal modes {Q j }, cast as :