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I read that ORCA runs a QuasiRRHO calculation based on Grimme approximation, at the end of a freq calculation.

Unfortunately, I wasn't able to find the default value for the CutOffFreq. In Grimme's article, I can't figure out which is the suggested value. Comparing the value to the value that comes out from GoodVibes and from Shermo the difference is very little (some decimals of kcal/mol or the 5th decimal of Hartree).

Is possible to assume that the default value is $100\ cm^{-1}$ as in these two scripts?

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I have access to ORCA's source code, and I can assure you that ORCA indeed uses $\omega_0 = 100 \rm{~cm^{-1}}$. However $\omega_0$ is hard-coded in ORCA and not controlled by the input parameter CutOffFreq; if the user specifies the latter, it means that all frequencies smaller than CutOffFreq are neglected in the calculation of the partition function, which is a completely different thing than what $\omega_0$ does.

In Grimme's paper, it has actually been explicitly mentioned that "We chose a value of $\omega_0 = 100 \rm{~cm^{-1}}$ as a default" (right after Eq. (8)). Therefore the people who wrote the corresponding chapter in the ORCA manual probably didn't bother to stress that again. That being said, the same paper also said that $\omega_0$ can be anywhere between $50 \rm{~cm^{-1}}$ and $150 \rm{~cm^{-1}}$ without deteriorating the results, and indeed some papers did choose other values (such as this one which chose $50 \rm{~cm^{-1}}$). Given this, I may consider mentioning $\omega_0 = 100 \rm{~cm^{-1}}$ in the ORCA manual so that future users can exactly reproduce the procedure that ORCA is using.

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  • $\begingroup$ Thanks very much. I have another little question. Do you know if the imaginary frequencies are excluded from the entropy calculation? $\endgroup$ May 25 at 14:15
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    $\begingroup$ @AndreaPellegrini Yes they are. I believe the same is true for all quantum chemistry programs, because there is no justification to do otherwise. $\endgroup$
    – wzkchem5
    May 25 at 15:41
  • $\begingroup$ Thanks so much. I though about that, but I wanted to be sure $\endgroup$ May 25 at 18:50

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