Should octopus optimize the FFT dimensions? This means that the mesh to which FFTs are applied is not taken to be as small as possible: some points may be added to each direction in order to get a "good number" for the performance of the FFT algorithm. The best FFT grid dimensions are given by $2^a 3^b 5^c 7^d 11^e 13^f$ where $a,b,c,d$ are arbitrary and $e,f$ are 0 or 1. (ref). In some cases, namely when using the split-operator, or Suzuki-Trotter propagators, this option should be turned off. For spatial FFTs in periodic directions, the grid is never optimized, but a warning will be written if the number is not good, with a suggestion of a better one to use, so you can try a different spacing if you want to get a good number.
math/fft.F90 : 208
call parse_variable(namespace, 'FFTOptimize', .true., fft_optimize)