MGGA does not seem to produce the correct bandstructure

[Peraz]

Dear Support,

I have struggled to get the bandstructure for a using MGGA for quite a while now,

The band structure of a relaxed nanowire without using MGGA seems alright but underestimated, when I use the MGGA the band gap seems to be not correct. Moreover the fermi-level is not in the middle of bandgap when using MGGA as oppose to bandstructure calculated without MGGA.

I have attached the band structure with and without MGGA as well as only coduction bands for more clarity.

Regards,

[Daniele Stradi]

Dear Peraz,

your procedure is correct, however MGGA is in principle designed for bulk systems, so one should always be a little careful when applying it to non-bulk systems. It might be that the c value needed to get a correct band structure in the GaAs nanowire is different from that of the bulk.

The Fermi level issue with MGGA is probably related to the fact that the band gap is much larger than with LDA, and you  need a much finer k-point grid to sample correctly the tail of Fermi function in the gap.

It would be great if you could attach or send us an input of your system, so we can have a closer look at it. Maybe some additional information could also help, e.g. which gap value and band structure do you expect.

Regards,
Daniele.

[Daniele Stradi]

Dear Peraz,

your input looks perfectly fine. I think the only issue is with the MGGA functional. I think the best way to determine the c parameter is to run a series of band structure calculations in which you vary the c parameter in a reasonable energy range, say 0.9 < c < 1.1 and then take the c parameter which gives the better band structure.

Regards,
Daniele.

[Peraz]

Dear Daniele,

While I was looking around I found the following paper which Quantumwise is a co-author:

http://www.sciencedirect.com/science/article/pii/S0038110115002580

Would you please let me know what was the approach to get the c-parameter value?

Thanks a million for your help,

Regards,

[Daniele Stradi]

Just forwarded to the authors.

[Peraz]

Thanks a million, looking forward to their reply.

[Daniele Stradi]

Hi,

after discussion with one of the authors of the paper, I have two main points:

(i) In the paper, they did the same as you did and fitted the c parameter to bulk GaAs. However, they did  calculations just for GaAs slabs, not nanowires. Therefore, it is not guaranteed  that the GaAs nanowire you are simulating is still a direct band gap material. I suggest that you to try to reproduce the results in the paper and see if a GaAs slab with the c parameter you fitted still has a direct band gap.

(ii) In the nanowire you are simulating, the passivating hydrogen atoms at the corners of the nanowire are in a threefold coordination. This might constitute an issue, as the fractional values for the H atoms might not be correct in this threefold coordination. I would suggest to try to calculate a different nanowire geometry in which all the H atoms have max twofold coordination.

Regards,
Daniele.

[Peraz]

Thanks a million Daniele,

I will try what you have suggested.

Regards,