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August 17, 2019

Transition state optimization Log Out | Topics | Search
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David Kochman
Posted on Tuesday, June 24, 2003 - 10:09 pm:   

How would one go about figuring out transition state geometries/energies for Claisen rearangements.
I just wanted to test the 3-allyloxy-propene system as a start to see the calculated distance between the oxygen and the new bond being made to the carbon.
I know this is possible as I'm looking at JACS v121 pg10866 with this in it, however its a little light on the procedural details.
Posted on Wednesday, June 25, 2003 - 10:40 am:   

Disclaimer: I am not a computational chemistry expert, merely a competent user.

You can use the QST2 calculation in Gaussian (called 'SADDLE' calculation in WebMO) or a SADDLE calculation in MOPAC to generate an initial guess for the transition state. As an input, these calculations require optimized geometry for the starting reactant(s) and the final product.

The SADDLE calculation tries to generate a guess geometry between the reactant and the product. Take this guess and then do a transition state optimization on (minimize the gradient, not the energy) to find the actual transition state.

Obviously, this guess has to be verified by doing a frequency calculation to check for one imaginary (negative) frequency, and then do forward and backward IRC calculations to make sure that the imaginary mode brings you to reactant in one direction and product in the other.

This certainly is not a guaranteed way to find the transition state, but if you don't have any idea what the TS looks like, this is a good starting point.
Anne Jensen
Posted on Tuesday, November 09, 2004 - 5:23 am:   

I need to know how to mak transition state optimization on bimolecular systems, using Gaissian or Jaquar. Qst2 optimazation fail - reactant and product structures are too different. How do I find a good guess for the additional structure for the qst3 calculation or a suitible structure for other types of TS optimisations????
Posted on Tuesday, November 09, 2004 - 9:44 am:   

In general, it can be EXTREMELY difficult to find a transition state. QST calculations are merely one way to get a good guess for what the transition state might look like. In general, you have to use a combination of chemical intuition and trial/error to find something 'close' to the transition state. From there, a transition state optimization will usually bring you to the top.

You might try posting to the CCL.net newsgroup for more detailed help.
Posted on Wednesday, March 18, 2009 - 11:23 am:   

Dans le but de déterminer un état de transition, j'ai fait un calcul QST3. Pour vérifier mon résultat j'ai fait un calcul de fréqence puis un IRC. Seulement, l'IRC s'arrête avec une "normal termination" avant d'arriver aux isomères de départ ( les reactifs). J'aimerai savoir comment le programme fait le découpage du chemin entre l'état de transtion et les deux reactifs ? est ce que si on donne un nombre de points insufisant, il s'arrête avant d'arriver aux reactifs ?
g ar
Unregistered guest
Posted on Tuesday, May 05, 2009 - 11:14 pm:   

i was currently doing my transition state search for phosphocholine compound. But, i always get error like number of steps exceeded and the optimization is stoppped. can u explain what is that and how can i find my transition state?
thank youu..
JR Schmidt
Username: Schmidt

Post Number: 94
Registered: 11-2006
Posted on Tuesday, May 05, 2009 - 11:18 pm:   

Increase the maximum number of iterations. The keyword to do this depends on the program you are using.
Posted on Saturday, October 28, 2017 - 11:32 am:   

how do i increase the number of maximum iterations? I keep on getting an error for geometry optimization for benzene

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