Dear Cem,

I suggest you to try to take the imaginary frequency, and save two structures + and - 0.30 for example (manual displacement). Then, with these structures, perform a optimization. Please, let us know if the results are going in the correct direction or not.

Dear Bartra,

Thank you for your answer. Actually, this is the method which I do mostly before IRC calculations. In this case, it also turned to the same intermediate from 0.25 + and - motion of TS structure. In a different structure case, however, I got different compounds that I wanted  from both side althought IRC shows same intermediate from both side.

Here is another interesting thing is that the geometry which has highest energy on IRC is not on the point of zero. The highest one is formed after one forward step of TS structure. 

Many thanks,

Cem

Dear Cem,

Then, it is possible that (even having 1 imag. freq) the structure that you are using, it is not the TS that you are expecting, for example, maybe is the TS referred to a conformer movement (that could explain because the IRC it is symmetric).

If you want, via private message, you could send the TS to give you a more accurate opinion about it.

Best regards,

Joaquim Rius

Hello,

This kind of behavior is sometimes seen in reactions like Cope rearrangements where following the steepest decent path from the TS leads to what is called a valley ridge inflection (VRI) point.  Effectively both IRC directions follow the same path until they reach the VRI at which we would ideally branch off in two different directions to lead to different minima.  Unfortunately since the VRI is an inflection point the gradient at that point is zero and there is no means for the program to choose different directions and so both IRC paths take the same descent path ultimately.

Things you can use to combat this are to use a very tight convergence criterion for both the wavefiunction and geometry optimization of your TS and to use shorter step sizes in the IRC so that you might skirt close to the VRI but ultimately around it, but there's no guarantee that these may work, due to the very nature of the VRI itself.

Take care,

Peter Warburton

Dear Dr. Warburton and Bartra, 

Thank you for the convincing explanations which are in agreement with each other. From the answers, I think I will follow the gradient options. 

Many thanks,

Cem

Good day, try to use a proper "stepsize" in the "opt" command; if it doesn't work, you might need to consider "CalcALL" command instead of "CalcFC".

Regards,

Mansour