[CP2K-user] [CP2K:19284] FFT problems with large unit cell

[Léon Luntadila Lufungula]

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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