As discussed previously, Hornburger and co-workers noted that a non-Condon type method was acceptable, disagreeing with many previous studies , and concluded that a Condon treatment was acceptable for most systems. However, for the case where a first-order treatment was not acceptable, the theory was still lacking. Following this, Nitzan & Jortner pushed for a more generalised formalism, outside of the Condon approximation. While the theory proposed by Herzberg & Teller was available at the time, no one yet understood exactly how this could be applied to internal conversion. By assuming each molecular frequency was roughly identical (an average frequency), a generating function can be integrated with the steepest descent method, yielding a decay rate formalism in terms of the vibration-less :