Modelling barrier-less reactions

I am currently exploring a few dissociative reactions whose PES is barrierless (no first order saddle point). I have found a few approaches in literature like flexible nudged elastic band theory, Newton string method, URVA etc to identify the Transition state structure. Say I use one of these methods to find the structure and obtain its vibrational frequencies, can I use it in standard RRKM calculations to find the rate constants ? Or should I follow theories like VRC-VTST which finds the rate constant directly ? What is the preferred approach in problems like this.




I would suggest you to use VRC-VTST for rate constant calculations especially if you want to calculate it for a large temperature range, and the reason is that in barrierless reactions the position of the Transition state along the reaction path change dramatically by changing the temperature (canonical) or the energy (microcanonical), thus using one structure for TS would may be lead to a good result only for very small temperature range.


 0