I have two A and B reagents. what is the frequency calculation protocol for Gaussian to classify stationary points (minimum and transition state)?
Hi Brahim Hachlaf
You should optimize molecules to obtain ground state of molecules. Then, you carried out frequency calculation using following keyword:
optimization: opt Method/basis set
frequency: freq Method/basis set
Transition state (TS): The number of atoms in molecules must same. Then, you must plot both reactant and product in one window of Gauss view. then you do following path:
calculate---->Gaussian calculate setup, select opt+freq in job type, and select TS in optimize to a:
Then select TD-SCF in Method section
and finally Submit----->Save
then OK
Using frequency calculations, the structure should be optimized. Then, frequencies mention in the output file of this calculation. For transition state, there must be one negative (imaginary) frequency values and other all frequencies are positive. Therefore, we can think that it should be a TS structure. However, IRC calculation must use to conform whether this structure is TS. For minimum structure, all frequencies are positive.
IMAGINARY FREQUENCIES: Negative frequencies indicate instability in the molecule or, in other words, saddle points on the potential energy surface. A stable molecule should have no imaginary frequencies, a transition state should have one (1st order saddle point), while more than one imaginary frequency means that there is a problem with your molecule’s geometry
As an example, if we consider four atoms molecules (HONO), there should six positive frequency values for a minimum structure. For TS, one must be an imaginary frequency, all others are positive values.
Further information;
http://gaussian.com/freq/
https://www.youtube.com/watch?v=fh_CcF8gFDg
http://sf.anu.edu.au/~vvv900/gaussian/ts/locating-ts.html
http://gaussian.com/qst2/
https://m.tau.ac.il/~ephraim/glab-4.pdf
http://collum.chem.cornell.edu/documents/Gaussian_optimization.pdf
https://www.youtube.com/watch?v=ufbIbjGNOac
https://www.youtube.com/watch?v=Za8ihr5BRwk
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