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

[Lorenzo Paulatto]

Dear Barry,

you could have a look to the utility "pw2bgw.f90" which is used to 
interface with a different code, but does also input/output of the 
wavefunctions, which you could recycle.

! pw2bgw subroutines:
!
! write_wfng  - generates complex wavefunctions in G-space (normalized to 1)
! real_wfng   - constructs real wavefunctions by applying the Gram-Schmidt
!               process (called from write_wfng)
! write_rhog  - generates real/complex charge density in G-space
!               (units of the number of electronic states per unit cell)
! calc_rhog   - computes charge density by summing over a subset of occupied
!               bands (called from write_rhog), destroys charge density
...


Be careful, that if you use any ultrasoft or paw pseudopotentials, the 
wavefunctions are not normalized.


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.
>
> 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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-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC - CNRS UMR 7590 & Sorbonne Université
phone: +33 (0)1 442 79822 / skype: paulatz
http://www.impmc.upmc.fr/~paulatto/ - https://anharmonic.github.io/
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