Despite the success the Robinson-Bixon-Jortner model displayed, it's derivation was based on an oversimplified physical model. Bixon & Jortner discuss that an exact physical solution can only be modelled using the exact Born-Oppenheimer states, which at the time was impractical. However, it is also worth noting that the exact distribution of energy levels is not crucial for a full analysis of internal conversion. Further, the derived formalism was developed specifically to treat an isolated molecule in the gas phase, and therefore has minimal applicability to modern quantum chemistry.