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I've run an IRC (intrinsic reaction coordinates) calculation from my TS (transition state) and got two minima for each side. I am concerned about them.

One of them took about 100 steps to be achieved and looks reasonable. But the other is only one step from a TS and doesn't look reasonable, it more resembles a TS itself rather than the product although it lies about 15 kcal/mol lower.

So, do I need to optimize the structures I've got from IRC or can I just take their energies?

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I'm surprised that a single geometry step could result in a converged structure that's 15 kcal/mol lower in energy. That is odd, although not impossible. I would recommend carrying out a frequency calculation on the structure. It should, of course, have no imaginary modes. I'd also consider running a "regular" geometry optimization on the resulting structure to make sure that it's not some artifact of the optimization algorithm associated with the IRC calculation. If both of those point to this indeed being a local minimum, then perhaps this is just a very early or late TS closely resembling the reactant or product, respectively. This is not terribly unusual. As a final note, if your system can exhibit multiple plausible spin multiplicities, make sure that you have found the ground state solution for the reactant, TS, and product.

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  • $\begingroup$ I've setup geometry optimization and frequencies calculation to verify if it's a minimum. I'm more concerned if it is a "global" minimum along the reaction path, but I'm not sure if it matters. Also my system (regular organic molecule) is definitely a singlet (at least I hope so). $\endgroup$ Jan 30 at 18:54
  • $\begingroup$ So I've run optimization on both minima from IRC, one of them, which I'm concerned about, got lower by ~0.1 kcal/mol, and the other got lower by ~0.3 kcal/mol. I think it's not a big difference. $\endgroup$ Jan 31 at 6:37
  • $\begingroup$ @romaichenko -- it sounds like you may just have a very early/late TS then. Nothing inherently wrong with that. $\endgroup$ Jan 31 at 7:20
  • $\begingroup$ I have aromatic nucleophilic substitution, so I have and intermediate lying in between of reagents and products. Maybe it's the reason why the IRC for the first transition state (between reagents and intermediate) is strange. So, it more or less works and I'll stick with it. $\endgroup$ Jan 31 at 10:04

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