Dear colleagues,
I am studying half-metal ferromagnetic bulk material. So by definition half-metal means presence of a band-gap for minority (spin-down) states. Both VASP and Quantum espresso (plane-wave DFT packages) reproduce perfectly half-metallic DOS with a band-gap at the Fermi level (E-Ef = 0 eV).
While in the case of Siesta the DOS shape is nicely preserved BUT it's systematically shifted to the right at ~ 0.5 eV. As a result, no band gap at the Efermi, see the figure attached.
Shortly it means that Siesta doesn't allow to consider my bulk as half-metal in contrast to e.g. VASP and QE.
So why Siesta tends to shift half-metallic DOS?
Maybe it's some technical artefact (some equation in the Siesta code) cased by half-metallicity and, therefore, wrong evaluation of equilibrium Fermi level when spin-up states are presented and spin-down are not.
Maybe the reason is that VASP and QE are plane-wave packages while Siesta employs atomic orbitals as a basis set. But can it explain my problem?
Some obvious things I tried to check:
- Use LDA/GGA and other functionals
- Change k-points/cutoff
- Change basis (SZP, DZP, etc.)
- Implement +U (for the transition metal Co and Fe atoms but it only broadens Eg to the right)
- Change smearing
In all these cases Siesta DOS is shifted ~0.5 eV to the right in comparison with VASP & QE. And I have no idea what to do...
I would highly appreciate any suggestions or advices. Thank you!
Dear Alrik Stegmaier
Thank you a lot for the comprehensive answer.
I fully agree that there are differences in the valence states if we compare all three DOSs peak-by-peak. Basically, I put all of them together only to show that some 2 packages put Egap to the Fermi level, while Siesta has a shift.
So, if we forget about VASP and QE, and think only about Siesta: if there are some reasons why Siesta may has a shift of Fermi level.
If I got it right, your basic opinion that it's might be possible to shift Egap towards zero if I play more with parameters (e.g. basis set) in Siesta? And you did not hear or meet any intrinsic problem in Siesta that prohibit true description of half-metals?
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