[QE-users] FFT order too large

[Aaron Friesz]

Hello,

I am receiving this error message:

*       Error in routine  good_fft_order (5076):
**        fft order too large*


I have found this post regarding the error:
https://lists.quantum-espresso.org/pipermail/users/2019-April/042636.html

I have a few more questions.

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.)

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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