[QE-users] Details of Automatic FFT Grid Size Calculation

[Stefano de Gironcoli]

it's in SUBROUTINE realspace_grid_init  in  FFTXlib/src/file fft_types.f90

         !
          ! ... calculate the size of the real-space dense grid for FFT
          ! ... first, an estimate of nr1,nr2,nr3, based on the max values
          ! ... of n_i indices in:   G = i*b_1 + j*b_2 + k*b_3
          ! ... We use G*a_i = n_i => n_i .le. |Gmax||a_i|
          !
          dfft%nr1 = int ( sqrt (gcutm) * sqrt (at(1, 1)**2 + at(2, 
1)**2 + at(3, 1)**2) ) + 1
          dfft%nr2 = int ( sqrt (gcutm) * sqrt (at(1, 2)**2 + at(2, 
2)**2 + at(3, 2)**2) ) + 1
          dfft%nr3 = int ( sqrt (gcutm) * sqrt (at(1, 3)**2 + at(2, 
3)**2 + at(3, 3)**2) ) + 1

stefano



On 27/02/25 20:11, Dyer, Brock wrote:
> Hello all, I have been working quite a bit lately on automating my QE 
> workflow, and as part of that workflow I check the automatically 
> calculated FFT grid sizes for the level of theory that I have been 
> using in order to improve my parallelization.
>
> I have tried tracing down and reading the code that calculates the FFT 
> grid sizes, however I cannot find/understand the actual code to 
> calculate the grid sizes. My current understanding is that the initial 
> parameters to calculate the grid size are 'ecutwfc' and/or 'ecutrho', 
> and the unit cell size, and then there seems to be some more math, and 
> perhaps at the end the final dimensions get calculated in 'fft_ggen.f90'.
>
> What I am looking for ideally is a mathematical formula that includes 
> all of the input parameters and operations required to calculate the 
> FFT grid sizes so that I can implement it into my workflow and 
> hopefully not have to run double calculations to properly parallelize.
>
> Thanks in advance for the help!
>
> Brock Dyer, Ursinus College Class of 2025
>
>
>
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