frequencies

[Sihem AZIZI]

Dear all,

I'm trying to calculate the frequencies of an organic molecule, and each time I get two or three negative values. I don't understand why. When I do the same thing with Gaussian, all the values ​​are positive. Can you help me? Thank you.

[Frederick Stein]

Dear Sihem,

Can you provide more details on your computational setup? Have you optimized all parameters of your setup (box size, CUTOFF, EPS_DEFAULT, ...)? Had you run a geometry optimization before you started the frequency calculation with CP2K?

Best,

Frederick

[Jürg Hutter]

Hi

are you running using a supercell (periodic) or have you set
PERIODIC NONE for CELL and POISSON Solver ?

Default is to only remove 3 translations in periodic calculations,
no rotations will be removed (3 further frequencies).

regards
JH

________________________________________
From: cp...@googlegroups.com <cp...@googlegroups.com> on behalf of Sihem AZIZI <sihema...@gmail.com>
Sent: Thursday, March 13, 2025 10:26 PM
To: cp2k
Subject: [CP2K:21281] frequencies

Dear all,
I'm trying to calculate the frequencies of an organic molecule, and each time I get two or three negative values. I don't understand why. When I do the same thing with Gaussian, all the values ​​are positive. Can you help me? Thank you.

--
You received this message because you are subscribed to the Google Groups "cp2k" group.
To unsubscribe from this group and stop receiving emails from it, send an email to cp2k+uns...@googlegroups.com<mailto:cp2k+uns...@googlegroups.com>.
To view this discussion visit https://groups.google.com/d/msgid/cp2k/04e9b6b3-fb06-4d60-b861-9546dcf83fe5n%40googlegroups.com<https://groups.google.com/d/msgid/cp2k/04e9b6b3-fb06-4d60-b861-9546dcf83fe5n%40googlegroups.com?utm_medium=email&utm_source=footer>.

[Sihem AZIZI]

Dear Jürg and Frederick, Thank you very much for your reply. Yes, I perform a geometry optimization calculation before starting the frequency calculation, optimizing all the parameters. In response to Jürg : I perform an optimization calculation on an isolated organic molecule, setting PERIODIC NONE and using POISSON_SOLVER ANALYTIC, followed by a frequency calculation.

For now, I'm only studying the isolated molecule; the periodic system will follow as soon as I understand how it works.  
The coordinates in the input file are obtained from a geometry optimization calculation performed with Gaussian. I took the coordinates as-is from the Gaussian output file.

I have attached all the files. Thanks again for your help.

Sihem

[Frederick Stein]

Hi Sihem,

I checked your geometry and your forces. Is your system expected to be planar? If yes, try to set the y-coordinates of your atoms to 0.

Generally, a vibrational analysis requires tighter thresholds. Your forces need to have an higher accuracy than the force differences which are used by CP2K to run the analysis. With your value of the DX parameter, I see from your force differences that you need an accuracy significantly better than 1e-4 to 1e-5, i.e. at least 1e-6 to 1e-7. Especially an EPS_SCF of 1e-6 is usually to high. Please double-check the convergence of the parameters EPS_DEFAULT, EPS_SCF and CUTOFF and in your case maybe even the box-size in x-direction.

Best,

Frederick

[Sihem AZIZI]

Good evening Frederick, Thanks again.

No, the molecule is not planar.

I obtained the molecule's coordinates by performing a geometry optimization with Gaussian.

Do I need to use another software to draw the molecule and get the xyz coordinates?  
For the rest, I'll try it out. Thanks again,

Sihem

[Jürg Hutter]

Hi
you have to reoptimize the structure in CP2K with the identical settings as will be used for
the vibrational analysis.

regards
JH

________________________________________
From: cp...@googlegroups.com <cp...@googlegroups.com> on behalf of Sihem AZIZI <sihema...@gmail.com>

Sent: Sunday, March 16, 2025 9:56 PM
To: cp2k
Subject: Re: [CP2K:21288] frequencies

To view this discussion visit https://groups.google.com/d/msgid/cp2k/3924deef-0744-4155-885e-fe0bdc346662n%40googlegroups.com<https://groups.google.com/d/msgid/cp2k/3924deef-0744-4155-885e-fe0bdc346662n%40googlegroups.com?utm_medium=email&utm_source=footer>.