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

[Dyer, Brock]

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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