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

[Barry (Yangtao) Li]

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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