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Flat bands around E = 0 eV
Date: 2020/09/27 05:23
Name: Malone


I have another question, this time regarding band structure calculations.
When I take a slab consisting of 20 atoms, for example, and conduct a calculation, in the end
I get flat lines on a band diagram around 0 eV, where the program puts the chemical potential.

At the other hand, if I count occupied bands by hand, the result is more acceptable.
For example, I have 150 valence electrons in a spin-polarized calc, I have 150 occupied bands,
so the LUMO will be 151st from the bottom. I extract energies for this 151st bands directly
from the .Band file and it occurs to be around +60 eV, but bands do look like... bands, not just flat
lines. I had encountered similar behaviour in quantum espresso, so this is not a complete surprise.

Nevertheless, when I take a flat graphene sheet, I get the Dirac point exactly at 0 eV at K point.
This is what confuses; maybe I do not understand completely what do the codes do when they place the E=0 mark.
Could you describe what is going on here, please?
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Re: Flat bands around E = 0 eV ( No.1 )
Date: 2020/09/30 03:07
Name: Malone

It is a bug in GNUPLOT's Autoscale. By default, bandgnu13 generates energy range from Emin to Emax, rather large values on the order of 100 eV. When you try to select a section of the plot with right click, it produces such "flat" bands as they appeared to be at the full scale from -200 to 200 eV.

If one manually sets 'yra [-5, 5]', for example, the problem does not appear.
Re: Flat bands around E = 0 eV ( No.2 )
Date: 2020/09/30 18:12
Name: Zsolt

I also observed this behavior of gnuplot 5.2 (patchlevel 8.)

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