Stretch-bend partition function

The partition functions and numbers of accessible states can all be calculated from the reaction path data, (j) and F(s), assuming harmonic vibrations and separation of vibration and rigid-body rotation. The vibrational partition function may be improved in accuracy by accounting for anharmonicity in some modes. This has been done simply in a separable mode approximation (e.g., Morse stretches and quartic terms in the bending potentials) [129-131]. There are now a number of examples of applications of various forms of canonical variational transition-state theory using ab initio reaction path calculations [15,106,108,112,131-142]. 

Thus, in contrast to the stretching degree of freedom, small changes in the bending frequency or moment of inertia do not result in large changes in the ratio of partition functions at low temperatures. 

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