Bandgap vs. DOS curve

[Hassan]

Hi, I am trying to determine the bandgap of graphene nanoribbons. I am also trying to compare with the density of states with the bandstructure diagram, and something doesn't add up. The system under question: This is a nanoribbon 20 atoms wide in the armchair configuration. This is the bandstructure diagram I get with the following settings:

Code

            bandstructure = Bandstructure (
                configuration=configuration_obj,
                route=band_route,
                points_per_segment=100,
                bands_above_fermi_level=5
)

As you can see the bandgap is around ~100meV. Now, the Density of States curve with the following settings:

Code

dos = DensityOfStates(
                configuration=configuration_obj,
                kpoints=MonkhorstPackGrid(4,4,4),
                bands_above_fermi_level=5,
                energy_zero_parameter=FermiLevel,
             )

In this photo you can see the bandgap to be around 2eV. Why is this difference so large? Is there anything wrong in my setup? The settings for the DFT calculation were PBE.GGA Double Polarized, 4x4x4 k point grid and 65 Rydberg E cutoff. Any help is appreciated.  

[Shinji Usui]

Dear Hassan

Try to increase k point value of Z direction like

dos = DensityOfStates(
                configuration=configuration_obj,
                kpoints=MonkhorstPackGrid(1,1,101),
                bands_above_fermi_level=5,
                energy_zero_parameter=FermiLevel,
             )

In my experience it should be over one hundred to get good result.
And kx, ky should be one because the direction is not periodic.

BR

[Hassan]

Perfect, the results actually make sense now
Thanks!

[Luis M. Villamagua C.]

Hello all,

Can you clarify why Mr. Hassan is using YXZG route. In the forum (http://quantumwise.com/forum/index.php?topic=2479.0#.U1qDefl5N1Y) Mr Dipankar Saha and Dr. Blom say that G,Z is sufficient. what is the drawback/advantage of using this route?

Thanks,
Luis M. VIllamagua

[Anders Blom]

The drawback of using YXZG is that the calculation of the band structure takes longer time and doesn't give any useful information for the X-Y segment (the band structure is flat), while X-Z is equivalent to G-Z so it's redundant.