[CP2K-user] [CP2K:19297] FFT problems with large unit cell

[Jürg Hutter]

Hi

there is an upper limit for the FFT length for the FFTSG library (1024).
I would switch to FFTW3 where no such limit exists.

regards
JH

________________________________________
From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Léon Luntadila Lufungula <Leon.luntadilalufungula at uantwerpen.be>
Sent: Tuesday, October 3, 2023 2:06 PM
To: cp2k
Subject: [CP2K:19284] FFT problems with large unit cell

Dear all,

I am running some tests with long chain adsorbates on a TiO2 slab and I'm running into some problems with the FFT libraries. I initially had the FFTSG set as preferred library, because the manual stated that "The use of PREFERRED_FFT_LIBRARY FFTSG is required" when using the WAVELET solver. However, I just read in this post<https://groups.google.com/g/cp2k/c/QEveWI7UqV0> from Prof. Hütter that you can use the FFTW3 library in combination with the WAVELET solver and I already read several times that using this library is generally faster compared to the internal FFTSG one. I also already had to enable the EXTENDED_FFT_LENGTHS option for calculations with smaller (10x10x30) cells as I ran into some "Index to radix array not found."-errors.

Now, with these long adsorbates I have a cell of around 10 x 15 x 60 Å and the combination of FFTSG/EXTENDED_FFT_LENGTHS again gives "Index to radix array not found."-errors, while switching to FFTW3/EXTENDED_FFT_LENGHTS says that "the FFT in the x direction is not allowed n01 dimension         175".

So I have two questions:
1) Is it possible to calculate such a large cell without getting these FFT errors or is it just too large?
2) Should I switch to FFTW3/EXTENDED_FFT_LENGTHS to speed up my calculations or is it fine to continue with FFTSG/EXTENDED_FFT_LENGTHS?

Hopefully someone can help me out with this problem.

All the best,
Léon


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