RRKM calculations harmonic oscillators

Only in the high-energy limit does classical statistical mechanics give accurate values for the sum and density of states terms in equation (A3.12.15) [3,14]. Thus, to determine an accurate RRKM k(E) for the general case, quantum statistical mechanics must be used. Since it is difficult to make anharmonic corrections, both the molecule and transition state are often assumed to be a collection of harmonic oscillators for calculating the... 

RRKM theory has been used widely to interpret measurements of unimolecular rate constants. However, harmonic state counting procedures are usually used in the RRKM calculations. This is not because enharmonic effects are thought to be unimportant, but because they are difficult to account for. The only comprehensive attempt to include the effect of anharmonicity has involved treating the vibrational degrees of freedom as separable Morse oscillators. However, since this correction is an obvious oversimplification it has not been widely used. The importance of anharmonicity is illustrated by comparing the trajectory unimolecular rate constant for C2H5 H + C2Hi dissociation at 100 kcal/mol (Fig. 4b), which is about 4.7 X 10 with that predicted by harmonic classical RRKM... 

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