During electronic structure calculations I am getting a transition state which have imaginary frequency value 90 cm-1 corresponding mode distance is 3.15 cm-1. When I am trying to do IRC calculation getting an error every time Energy has increased. In this situation how do I validate/confirm about my transition state?
Thank you
Probably, the TS you have found is not close enough the the exact TS. It need to be optimized a bit tighter. The region of the TS maybe rather flat. You should check the imaginary mode of vibration if it is correct or not for your TS.
You should to re-optimize the TS using tighter convergence ("Opt=(TS,Tight)"). Also, increase the size of the numerical integration grid for DFT (e.g. "Integral=(Grid=Ultrafine)". Hope this help.
Dear Godara,
An alternative for IRC calculation is to displace the TS geometry towards reactants and products using the normal modes of the imaginary frequency. Gaussview software package has implemented a function to do that: after opening the output of the TS, right click the window and select frequencies. You will find the option to manually displace geometry and save displaced geometry.
Hope it was useful and clear,
Lluis
Dear LIuis,
Thanks for your suggestion. In my case the distance of reaction coordinate mode is high i.e. 3.15 cm-1, So normal mode visualization is not clear. My doubt is how to do IRC calculations for almost flat PES? Is it possible for flat PES or not?
Thanks to all
I'd suggest using large basis sets with diffuse functions and a new optimization as a preliminary action.
JL
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