How to decide Freuency (-f and --fs) while using GoodVibes (I am new in Quantam)

[nileshjchoudhary]

Dear Prof. Paton lab, Thanks for the great package, I am using it for Quasi-harmonic entropy correction and enthalpy correction" for gaussian output(transition state) Now, I am confused about frequency.
-f FREQ_CUTOFF Cut-off frequency for both entropy and enthalpy (wavenumbers) (default = 100) --fs S_FREQ_CUTOFF Cut-off frequency for entropy (wavenumbers) (default =100) --fh H_FREQ_CUTOFF Cut-off frequency for enthalpy (wavenumbers) (default = 100)

How to decide it should be 100 150 or any number While we are having many vibrational frequency including -negetive one(for ts)

[jvalegre]

Dear Nilesh,

Thanks for using the program! The default number used as the frequency cut-off (100 cm-1) comes from the original paper of the quasi-harmonic correction from Grimme (https://doi.org/10.1002/chem.201200497). You can read more about the impact of using different frequency cut-offs in the GoodVibes paper (https://f1000research.com/articles/9-291).

To be honest, as far as the cut-off is within reasonable limits you'll have similar results (i.e. using 50 or 150 cm-1 will lead to very similar results most of the time).

Also, be aware that the quasi-harmonic correction for enthalpy from Head-Gordon and coworkers was designed for measuring adsorption enthalpies (original paper for more information --> https://pubs.acs.org/doi/10.1021/jp509921r). If you are studying a reaction mechanism, I would not include it, and I would just use the quasi-harmonic entropy correction. The imaginary frequency you mentioned coming from the TS is ignored for calculating thermodynamic corrections (imaginary frequencies are also ignored in Gaussian).

Hope this helps! Juan

[nileshjchoudhary]

Dear jvalegre, Thanks for answering. also adding comment about, a reaction mechanism, This is great help for me. I am a learner. I will read suggested papers.

[nileshjchoudhary]

Dear Jvalegre, I am searching reason behind this statement
"If you are studying a reaction mechanism, I would not include it, and I would just use the quasi-harmonic entropy correction."

Is that mean reaction mechanism does not require entahlpy correction? and they why ? Why only we need quasi-harmonic entropy correction not entropy

Can you please explain or give some reading material for same.(I am new in this field)

[jvalegre]

Dear Nilesh,

I believe the three papers suggested in my first comment will be quite useful for your questions : 1) Quasi-harmonic correction from Grimme (https://doi.org/10.1002/chem.201200497) 2) GoodVibes paper (https://f1000research.com/articles/9-291) 3) Quasi-harmonic correction for enthalpy from Head-Gordon and coworkers (https://pubs.acs.org/doi/10.1021/jp509921r)

Regarding your questions: Is that mean reaction mechanism does not require entahlpy correction? - Commonly we don't use quasi-harmonic enthalpy corrections to study reaction mechanisms, we only use quasi-harmonic entropy corrections.

and they why ? - The quasi-harmonic correction for enthalpy was originally designed for measuring adsorption enthalpies, treating one component as a surface (which is different from most of the reaction mechanisms that happen in solution phase). Please, check reference number 3 from Head-Gordon and coworkers for a more detailed explanation.

Hope this helps! Juan

[nileshjchoudhary]

Thank Juan,

I am following the papers