How to properly treat low vibrational frequencies?

saeed_E · 2024-02-08 13:15:09

Dear Tian,
For big molecular systems including a large number of degrees of freedom Gaussian erroneously treats low rotational frequencies (associated with internal or hindered rotations) as vibrational frequencies. In such cases, Gaussian prints out the below warning message in the thermochemistry section:

Warning -- explicit consideration of XX degrees of freedom as vibrations may cause significant error

in which XX depends on the degrees of freedom. In consequence, C_V is not properly evaluated for these cases and, thus, thermochemical functions are not calculated in an exact manner. While using "freq=hinderedrotor" has been suggested to resolve this problem, I did not find any difference between thermochemical values with and without this keyword. Could you please let me know how one can resolve this problem and force Gaussian to compute thermochemical values in an exact way (low rotational frequencies are really and expectedly treated as rotational frequencies and not as vibrational ones). Do you think "freq=anharmonic" could be of helpful while such computation is highly time-consuming for big molecules?

P.S. Please ignore two emails I sent you regarding this matter since I found where such inequalization and inconsistency between E(Thermal)-ZPVE and T*CV comes from.

Sincerely,
Saeed

sobereva · 2024-02-08 22:15:57

Dear Saeed,

When your system contains low-frequencies, especially those lower than 100 cm-1, treating them in harmonic approximation manner may cause huge error in entropy and free energy. In this case (in fact in all cases), I strongly recommend using my Shermo code with ilowfreq=2 or 3 (corresponding to quasi-RRHO model proposed by Grimme and Minenkov, respsectively) to estimate these thermodynamic quantities. In this case the consideration of contribution of the low frequencies is much more reasonable and the result is much more reliable.

Using hindered rotor model is also OK and sometimes better, but much more expensive. Using the very expensive (usually only applicable to <30 atoms) anharmonic model does not necessarily lead to evidently better result.

Best,

Tian

saeed_E · 2024-02-08 23:06:03

Dear Tian,
Too many thanks for your highly valuable kindness to provide very informative and novel comments.
I already know the different abilities of Shermo and I also always use Shermo to obtain more accurate values at least for S. Thus, before sending you this problem, Shermo was employed using both ilowfreq=1 and 2 options but the difference between resulting values and those directly obtained by Gaussian are highly small so that CV and S are not still computed in exact manner. I checked this problem through below equations:

U_T-U_0=CV(T-0)

This equalization is satisfied only for the systems for which CV is quite exactly computed (such as H2). For other systems with an inaccurate CV value, there is a considerable difference, 2-5 kcal/mol, between (U_T-U_0) and CV(T-0).

Except employing Shermo and hindered rotor (which did not take effect in the case under my study), is there any other solution?

Please also let me state that there are only options 0, 1, and 2 for ilowfreq corresponding to Harmonic, Raising low frequencies, and Grimme's entropy interpolation, respectively. Is there any other update/release of Shermo with ilowfreq=3 as you stated and, I am not aware of that?

Sincerely,
Saeed

Last edited by saeed_E (2024-02-08 23:13:39)

sobereva · 2024-02-09 00:33:07

U_T-U_0=CV(T-0) is incorrect. CV is dependent of temperature, U_T'-U_0 should be expressed as integral of CV(T) over T from 0 to T'. See appendix of Shermo manual.

Minenkov's QRRHO is added to Shermo since version 2.4, see update history in Shermo website.

saeed_E · 2024-02-09 09:49:38

Dear Tian,
Too many thanks for your highly valuable and determining comments.
You are also deeply appreciated to make me aware of newest version of Shermo. This version was successfully downloaded.

In addition, please be aware that a moment ago I sent you an email including a pdf document in which some challenges about thermochemistry and Shermo are presented. I would be highly grateful if you kindly have a look at let me know your opinion.

Sincerely yours,
Saeed

Multiwfn forum

#1 2024-02-08 13:15:09

How to properly treat low vibrational frequencies?

#2 2024-02-08 22:15:57

Re: How to properly treat low vibrational frequencies?

#3 2024-02-08 23:06:03

Re: How to properly treat low vibrational frequencies?

#4 2024-02-09 00:33:07

Re: How to properly treat low vibrational frequencies?

#5 2024-02-09 09:49:38

Re: How to properly treat low vibrational frequencies?

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