[QE-developers] Possible bug in PDOS projections for PAW pseudopotentials?

[Adam Denchfield]

Hello,

This is Adam Denchfield, PhD student of Hyowon Park at UIC. My advisor
suggested there's a bug in QE's projections for PAW pseudopotentials and
that I should reach out to the QE devs.

Your time is valuable, so here is the short version: our main system has
qualitative differences between the default and PAWproj=.true. projection
schemes:
[image: image.png]
(The bottom is zoomed in for visibilities' sake)
We are not sure which to trust for physical insight, considering test
calculations on simpler systems like LuH3 seem to suggest the
PAWproj=.true. option is faulty. A comparison to VASP, which also uses PAW
projectors by default, seems to see disagreement with the QE default and QE
PAWproj=.true. options as well. See below for an extended account & details
on pseudopotentials, etc. This is all with QE version 7.1.

----------------------------------

Myself and my advisor have compared PDOS calculations of cubic LuH3 with QE
[default], QE [pawproj=.true.], and VASP default [PAW] projection schemes.
There's some disagreement here. Would love your input as we're trying to
prepare to submit this but can't seem to agree on what's going wrong here.

Below is an image comparing a hydride (LuH3) with the Wentzovich PS for Lu,
and the H.pbe-kjpaw_psl.1.0.0.UPF for H. I use PBE here. There shouldn't be
any PAW sphere overlap issues since this structure isn't under pressure,
and I've used large PW cutoffs and k-meshes to confirm this isn't just a
convergence issue. Also using the optimized tetrahedron method here.
[image: image.png]

It appears PAWproj=.true. massively underestimates the PDOS contributions
of the PBE case, from all orbitals. I may have expected it to occur for
some orbitals (H-s), but even localized ones like Lu-f are very
misrepresented. The error in occupations isn't a simple scale factor
either, it's orbital dependent. I would have suspected the Wentzovich
pseudopotentials but the problem occurs for the hydrogen too.

My advisor ran a VASP-PBE calculation on the same material for comparison
and found the VASP default projection scheme (which as far as I can tell
also uses the PAW projectors) doesn't have this massive underestimation.

[image: image.png]
While the VASP results for this system are close enough to the QE default
scheme, this isn't true for our actual system of study, which includes
Nitrogen too. This is just a reference system.

The above is a reference system. Here is a snippet of our main system and
its qualitative discrepancy between the two methods.
[image: image.png]
As you can see, one method predicts one orbital character dominant at Ef
over another.

Back to the reference system; When I ran an HSE06 calculation on LuH3 using
the input_dft='hse06' (as well as the ACE), the problem seemed to mostly go
away for PAWproj=.true., and both projection schemes basically agree other
than a small normalization to be expected from the PAW spheres. My advisor
was skeptical of the HSE06 results for other reasons (increasing the
f-orbital energies), but that's another story [would love a comment on this
from you though if you have the time]. See below.
[image: image.png]


Essentially, my advisor suspects a bug in the QE projections for PAW
projectors, and possibly also in the default projection scheme [since it
disagrees with VASP's output].

I can send over the input files if needed.

-- 
Regards,
Adam Denchfield
UIC Physics Ph.D Student
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