[CP2K-user] [CP2K:14939] FFT library for wavelet Poisson solver

[Nicklas Österbacka]

Hi!

Great - thank you the hasty reply!

All the best,
Nicklas
fredag 12 mars 2021 kl. 09:58:01 UTC+1 skrev jgh:

> Hi
>
> there should never be inconsistencies, but you might run into
> incompatibilities. The code will stop in this case. If your
> grid has more than 1000 points in one direction, we will have 
> to hack the code or use some "cutoff magic" to get it running.
>
> regards
>
> Juerg Hutter
> --------------------------------------------------------------
> Juerg Hutter Phone : ++41 44 635 4491 <+41%2044%20635%2044%2091>
> Institut für Chemie C FAX : ++41 44 635 6838 <+41%2044%20635%2068%2038>
> Universität Zürich E-mail: h... at chem.uzh.ch
> Winterthurerstrasse 190
> CH-8057 Zürich, Switzerland
> ---------------------------------------------------------------
>
> -----c... at googlegroups.com wrote: -----
> To: "cp2k" <c... at googlegroups.com>
> From: "Nicklas Österbacka" 
> Sent by: c... at googlegroups.com
> Date: 03/11/2021 05:56PM
> Subject: Re: [CP2K:14939] FFT library for wavelet Poisson solver
>
> Hi!
>
> Excellent. Thank you for the reply!
>
> Follow-up question: Some of the slabs I will be considering are quite 
> large, so I will need to set EXTENDED_FFT_LENGTHS to True. This leads to a 
> shift in total energy, at least with FFTW3, as expected per the 
> documentation. Could this lead to inconsistencies in results with the 
> wavelet solver using FFTW3?
>
> All the best,
> Nicklas
>
> torsdag 11 mars 2021 kl. 12:59:58 UTC+1 skrev jgh:
> Hi 
>
> I think the manual is not correct in this case. The wavelet solver 
> uses its own FFT library. The wavelet FFT library is the same as the 
> FFTSG library in CP2K and they share the allowed FFT roots. 
> That is the reason behind the statement in the manual. 
> However, we restricted the allowed roots when using the FFTW3 to more 
> or less the same as in FFTSG (not exactly). But I think 
> they will only diverge for very large grids. 
> So, you can use FFTW3 in CP2K with the wavelet solver. Should there 
> be an inconsistency of grid sizes, this will be checked in the wavelet 
> solver and the code will stop with an error message. 
>
> regards 
>
> Juerg Hutter 
> -------------------------------------------------------------- 
> Juerg Hutter Phone : ++41 44 635 4491 <+41%2044%20635%2044%2091> 
> Institut für Chemie C FAX : ++41 44 635 6838 <+41%2044%20635%2068%2038> 
> Universität Zürich E-mail: h... at chem.uzh.ch 
> Winterthurerstrasse 190 
> CH-8057 Zürich, Switzerland 
> --------------------------------------------------------------- 
>
> -----c... at googlegroups.com wrote: ----- 
> To: "cp2k" <c... at googlegroups.com> 
> From: "Nicklas Österbacka" 
> Sent by: c... at googlegroups.com 
> Date: 03/10/2021 11:42AM 
> Subject: [CP2K:14920] FFT library for wavelet Poisson solver 
>
> 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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