I m doing PES for a substitution reaction using Gaussian 16. I did scan(relaxed(redundant coord)) for bond length between neucleophile and electrophile. When I open output file is Gaussview, in the result section i can only see scan but not vibrations. I want to see vibration frequencies. How can I get vibration frequencies in PES scan?
Dear Sudeera
As far as I know, if you want the vibrational frequencies at each step of the scan you will have to perform separate single point frequency calculations for all geometries, the scan does not provide that information (at least in Gaussian 09). If you are trying to locate a transition state, just take the energy maximum of the scan and perform the calculation there (and then optimize the TS structure)
Finally, just take into consideration that performing frequency calculations outside a minimum/TS/saddle point is not reliable due to the failure of the harmonic approximation
Enrique Manuel Arpa Thank you very much for the answer. I will take the saddle point of the PES and do frequency optimization to see the vibrations. Then I will optimize the saddle point coordination to get 'TS'. Thanks a lot.
Significant part of computational time in frequency calculation is spent on calculation of force constants, that are also needed to run transition state search.
So after preliminar Frequency calculation one can Opt=(TS,readFC) from copied chk, instead of CalcFC. Actually for a lot of software (like Turbomole or VASP) this is obligatory step in transition state search. The difference is that while in for example VASP user has to decide what vibration transition state search should follow (and help to program in this way, or to see directly that initial guess is too bad), in Gaussian user can just have a look on the Frequencies in starting point and can do rather little to improve the starting guess.
In short:
Freq followed by Opt=TS gives you a bit more info about how calculation gonna suffer.
Opt=TS is little bit faster.
In both ways after TS is found Frequencies calculation has to be runned again to prove nature of this point.
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