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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.

Arun
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  • This is slightly related, but from a different time, where the methods you mention were not as wide spread. https://chemistry.stackexchange.com/q/9957/4945 I would recommend explaining the abbreviations you have used. This enhances not only searchability but it'll help other people get familiar with the terms. I personally found NEB a good tool to look in theses things. Iirc it is available with orca, so you might be able to do that for your level of theory already. In total, I don't really think that there is a preferred method (yet). – Martin - マーチン Aug 27 '23 at 16:27
  • Another note: [Mattermodeling.se] would also have been a good home for this question. It is too old to migrate now though. Maybe you find some more help there. Just to avoid confusion: I'm not saying it is off-topic here. If you would want to ask it there again, you may do so, but please delete the question here first. – Martin - マーチン Aug 27 '23 at 16:31

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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.

Nadjib
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