[CP2K-user] [CP2K:22083] Rotational modes of methanol have high frequencies (> 550 cm-1), and they do not look like rotations

[Torstein Fjermestad]

Dear Prof. Hutter, 

Thanks a lot for your help. Using the GAPW method improved the situation 
significantly (see table below).

Dear all, 

I would like to share the updated results with you in case it may be useful 
for some of you. 
In the table below, Calculation 6 is the same calculation as shown in the 
table sent on January 8, 2026. I include it to show the connection to the 
previous results.

The difference between Calculation 8 and Calculation 7 shows the effect of 
including smoothing when using the GPW method. 
There is a notable change in the vibrational frequencies, but I would not 
say that the situation improved. 

The change from Calculation 8 to Calculation 9 shows the effect of changing 
the method from GPW to GAPW while keeping the smoothing. 
This improved the result significantly in that the vibrational frequencies 
of the modes corresponding to rotations became a lot closer to zero. 

The change from Calculation 9 to Calculation 10 shows the effect of 
removing the smoothing while using the GAPW method. 
This had a small influence on the vibrational frequencies. 

Best regards,
Torstein 

[image: Screenshot 2026-02-03 225522.png]






fredag 9. januar 2026 kl. 09:44:52 UTC+1 skrev Jürg Hutter:

> Hi
> This behaviour is related to the way CP2K integrates XC functionals. The 
> integration is
> done directly on the PW grid (in real space). This integration causes 
> both, translational and
> rotational invariance to be violated for molecules. Usually, the amount of 
> translation and
> rotation seen depends on the functional and the pseudopotential.
> The effect on molecular vibrations is mostly small (see frequencies before 
> and after removing
> of translation/rotation).
>
> 1) You can use FULLY_PERIODIC T to remove 'rotations'
> 2) You can use GAPW and/or smoothing methods in XC. This improves results 
> somewhat.
>
> regards
> JH
>
> ________________________________________
> From: cp... at googlegroups.com <cp... at googlegroups.com> on behalf of 
> Torstein Fjermestad <tfjer... at gmail.com>
> Sent: Thursday, January 8, 2026 8:44 PM
> To: cp2k
> Subject: [CP2K:22038] Rotational modes of methanol have high frequencies 
> (> 550 cm-1), and they do not look like rotations
>
> CP1K version: 2023.1
>
> Dear all,
>
> I am doing a frequency calculation on a methanol molecule in a cubic unit 
> cell of length 20.0 Å, using the M06 functional.
> I have noticed that the three lowest frequencies, corresponding to 
> rotational modes, are very high (> 550 cm-1, see table below). I am aware 
> that the frequencies of rotational modes will not be exactly zero in a 
> numerical calculation, but I was at least expecting frequencies lower than 
> 100 cm-1 for these modes.
> Furthermore, the modes do not look like rotations, but rather like angle 
> bending and twisting of a torsion.
>
> I investigated the convergence with respect to the following parameters:
> EPS_SCF, EPS_DEFAULT, CUTOFF, REL_CUTOFF (see table below), but it had a 
> negligible effect on the vibrational frequencies.
> Are there other crucial parameters that may affect the vibrational 
> frequencies that I am not aware of?
> Please find the output files attached (The input is given at the start of 
> the *.out files).
> Thanks in advance for your help.
>
> Best regards,
> Torstein
>
>
>
> [Screenshot 2026-01-08 204106.png]
>
>
>
>
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