Rotational modes of methanol have high frequencies (> 550 cm-1), and they do not look like rotations

[Torstein Fjermestad]

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

[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...@googlegroups.com <cp...@googlegroups.com> on behalf of Torstein Fjermestad <tfjer...@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

[Screenshot 2026-01-08 204106.png]




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