where J j mn is the pure electronic term, and F k (mr, ns) are the Franck-Condon factors. This formalism can be simplified via selection rules, which are similar to those for vibronic coupling . The pure electronic matrix elements becomes non-vanishing for modes corresponding to similar representations within the molecular point group, via the direct product representation ϕ m × ϕ n . Specifically , the internal conversion selection rule is determined by ψ ns ∂ ∂Qj ψ mr and can be calculated in a similar fashion to that of the optical transition dipole moment . In the case of non-degenerate electronic states, only one vibrational normal mode could be expected to interfere with this interaction, in which case the matrix element can be reduced to a single electronic-Franck-Condon product term. These coupling matrix elements can also be displayed as a sum of products between electronic and Franck-Condon overlap terms .