While calculating IRC Path for HCN isomerization reaction inspite of calculating it for both directions it is giving the path only for forward. direction.
I am using QST3 optimization option for transition state search and then calculating its IRC Path but it is only giving for one direction??
Dear colleague
It may be possible that you are starting the IRC calculation in a geometry that is not strictly the transition state. In that case you are starting in a point of the potential energy surface whose gradient is not zero, and there exists only one direction to follow when going downhill.
Have you checked the vibrational frequencies in the structure obtained with the QST3 calculation? There should be only one imaginary frequency, corresponding with the isomerization.
Anyway, before the IRC calculation try to optimize the transition state normally, taking the QST3 geometry, just to ensure that you really are in the transition state. Once the calculation is finished an your TS structure is flawless, submit the IRC (or split it into two calculations, one forward and one reverse)
Hope you find it helpful
in this case I will recommend Enrique Manuel Arpa answer by doing separate IRC calculations" one forward and one reverse "
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