[CP2K-user] FFT library for wavelet Poisson solver

[Nicklas Österbacka]

Hi!

I'm working towards doing some calculations on adsorption energies on slabs 
using CP2K 6.1 and would prefer to use non-periodic boundary conditions in 
the surface normal direction. As I have little experience with aperiodic 
DFT I ran some small tests to compare with periodic calculations.

The wavelet Poisson solver requires the use of the FFTSG FFT library 
according to the manual, but using FFTW3 instead yields virtually the same 
result for the geometry optimization of an oxygen molecule in a 20x20x20 Å 
box with no periodicity in any direction. (I realize that the box is much 
larger than necessary.) The energy difference between the two solvers is 
about 2e-13 Ha, while the difference in O-O bond length is about 3e-9 Å.

As the calculation using FFTW3 ran about 20 % faster it seems preferable to 
using FFTSG, but I want to make sure that CP2K isn't doing anything 
unexpected when combining FFTW3 with the wavelet Poisson solver. Is the 
manual incorrect on the FFTSG requirement, or is it perhaps only required 
for XZ periodicity?

Any insight would be appreciated!
Nicklas
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