It should be noted that while many legacy works have extended their respective formalisms to incorporate anharmonic effects , for the time it was still a difficult task. Aside from publications by Siebrand and co-workers , only Burland & Robinson , Fischer , Standord & and Fischer , and Shimakura and co-workers reported ways to address anharmonic effects. While for many of these works anharmonicity was driven by cubic and quartic contributions to the oscillators, Shimakura in particular employed the use of a Morse potential. However, the dependence on the occupational quanta was yet to be addressed, and therefore the current state of the art was still lacking.