[QE-users] About Gridding Complex-Valued Wavefunctions for Spectroscopy Calculations

[Paolo Giannozzi]

On 26/08/2024 03:58, Barry (Yangtao) Li wrote:

> I am writing to seek your guidance on generating and properly gridding 
> complex-valued wavefunctions. Our goal is to obtain 3D complex 
> wavefunctions, fully gridded on their defined FFT grid, which we intend 
> to use for advanced spectroscopy simulations as an added feature to 
> Quantum Espresso.

"Fully gridded" = "on a real-space grid" or what else? There are 1001 
places in QE where the wavefunctions are transformed from reciprocal 
space (plane-wave components) to real space, or vice versa

Paolo

> For instance, with a system defined by 2×2×2 k-points and no symmetry, 
> we expect to have 8 k-vectors (and therefore, we will obtain 8 
> wavefunctions binaries stored in .save after SCF). If we desire 40 bands 
> and an FFT grid of (20, 20, 20), our expectation is that 8 wavefunction 
> files (binaries or texts) would be written, each with the same size of 
> 16×20×20×20×40 = 5,120.000 bytes as we are aiming for each wavefunction 
> to be stored as complex*16 data, separated by bands looping from 1 to 
> 40, and in each band block its (1:20, 1:20, 1:20) a row of 3D complex 
> wavefunction data.
> 
> We would greatly appreciate any advice or documentation you can provide 
> on how to achieve this, especially how to ensure the wavefunctions are 
> accurately gridded on their FFT grid in the desired complex format, we 
> tried to count the bytes of the QE-generated .dat files, the bytes are 
> far away from the expected, so we don't know how to grid them. We also 
> tried pp.x postprocessing, however, since it is mainly designed for 
> orbital visualization, the phase information, i.e., the complex nature 
> of wavefunction is not stored after the post-processing. For Gamma-only, 
> we can do a square-root transformation, however, for non-Gamma cases, 
> the square-root won't work anymore.
> 
> We believe this capability could be highly beneficial for users 
> interested in extending QE's functionality into advanced spectroscopy, 
> and we would be more than happy to contribute to its development.
> 
> Thank you for your support and any assistance you can offer.
> 
> Sincerely
> Barry
> 
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 206, 33100 Udine Italy, +39-0432-558216
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