[QE-users] FFT order too large

[Paolo Giannozzi]

On Thu, Jan 21, 2021 at 11:36 PM Aaron Friesz <friesz at usc.edu> wrote:

1 ) In order to fix this it is stated that a change can be made in
FFTXlib/fft_param.f90.
> This implies that it must then be recompiled. Is this correct? (I'd rather
> avoid this as it will make me responsible for maintaining the software,
> instead of our computing resources division.)
>

if the origin of your problem is the one descripted in the thread ( guess
so), the proper fix is this one:

https://lists.quantum-espresso.org/pipermail/users/attachments/20190425/2584418a/attachment.obj
and yes, you have to recompile. Alternatively: &system ...
force_symmorphic=.true. ... / will not look for  symmetry operations with
associated fractional translations and should solve the problem.
Alternatively, install a newer version.

If you really want to allow FFT dimensions > 2048 (the hard-coded value;
5076 is what you have) you have to modify the code and recompile. Note that
the typical real-space resolution is something of the order 0.1 A, so 2048
corresponds to approx. 200 A cell side: quite a big cell

The FFT is three-dimensional, so each one-dimensional FFTis very fast, but
you need a bunch of them!

Paolo

2 ) Is the number in parentheses, (5076), the number of terms in the fft?
> Are there any heuristics to reducing this number short of reducing the
> number of atoms in the system?
>
> 3 ) Why is this check made? Is the FFT computed in every loop of the
> algorithm, thus incurring a large computational cost? (From what I remember
> FFTW should be very fast on arrays of this size, but if it is calculated
> often that could be an issue.)
>
> Thank you for your help!
>
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 206, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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