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Re: 2+2 transition state search

Vladimir A. Mironov
vladimir.a.mironov@gmail.com


The point of maximum energy along the reaction coordinate is a good guess for TS search in Firefly. As far as I understand, you have already obtained it and want to find "saddle" with two negative frequencies. You can specify
method=schlegel nneg=2
in the $statpt group. Alternatively, you may use symmetry, if it is possible for your molecule.

The search of "true" saddle points in 2+2 reaction is tricky. Surface scan along the one of negative modes may help you. In this case you should specify the displacement vector from the hessian output and set all atom masses to 1.

On Tue Jun 29 '10 4:55am, Jonas Baltrusaitis wrote
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>I have these two wonderful molecule before and after reaction and the reaction is 2+2 internal reorganization. I would like to calculate transition state but to all fairness I fail due to several facts:
>-it's 2+2 organization so there is 4 total atoms involved. whenever I calculate initial guess Hessian (with some other software) it always gives 2 negative frequencies, one for symmetric one for antisymmetric stretch of those 4 atoms. That might mean that there is 2 TS where 2 atoms react...

>But I digress... Briefly, in FF I can't get decent initial guess. If I take constrained optimization between initial and final structures, calculate Hessian and start TS search, structure clearly goes the wrong way.

>How do I obtain good initial guess for this TS? In Gaussian there is several TS methods where one input initial and final coords. Is there such a thing in FF?

>Any other comments?

>thanks

>Jonas


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