# Intrinsic Reaction Coordinate calculation— both sides of TS lead to same minimum

I am currently trying to find an addition transition state and verify it using an IRC calculation. I have found a good candidate, but the IRC calculation gives the same minimum on each side of the transition state. In my mind, this does not seem any different from a vibrational mode.

I think the TS is submerged, and I am sure this is complicating things, but it does not make sense that an IRC calculation would give the same minimum in both directions.

Any thoughts on what this signifies or how to remedy it?

Thanks

• Could you tell us more about the reaction you are looking into? – Martin - マーチン Jan 20 '15 at 3:51
• An information about the software package you use and the relevant pieces of the input file would also help the discussion. – Wildcat Apr 20 '15 at 18:13

Be sure you have really obtained TS, that means stationary point, i.e. all gradients are zero. Than, you should have one imaginary frequency corresponding to the motion along the reaction coordinate. If this is true and you still get both IRCs going to same minimum, your assumptions regarding the shape of Potential Energy Surface (PES) could be wrong.

It is easier to find the minima first, so be sure you have well optimized reactant, product, and if by any means reasonable, also intermediates. It is possible that there exist some high energy intermediates, which complicate the whole story (if you are not aware of them).

It could also be the case (impossible to tell, as you haven't told us what molecules you are using) that the two minima are equivalent by symmetry.

When I generate an IRC from a TS - using GAMESS (US) software - I calculate the IRC for the forward direction and the IRC for the backwards direction using MacMolPlt. Is it possible you've stitched together two forward (or two backward) direction IRCs by accident?

Also, I once tried to calculate and IRC from a TS but both the forward and backward IRCs went in the same direction. I don't know why ... I got around it by cribbing someone else's code.

A good place for code is the GAMESS (US) user group on google groups.

Hope the above helps...

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