[CP2K-user] [CP2K:20687] Incorrect Band gaps from simple calculations

[Thomas Kasel]

Thanks to your suggestions I managed to get the SCF to converge for PBE 
based upon playing with the mixing. The only problem is now that CP2K is 
not printing the HOMO-LUMO gap. Is there a setting I can specify to force 
print the gap?

Regards,
Tom

On Tuesday, September 10, 2024 at 4:45:21 AM UTC-5 Jürg Hutter wrote:

> Hi
>
> For silicon: if you are using the unit cell (with 2 atoms) you need a 
> large number of k-points.
> In this case SCF needs diagonalization and an appropriate mixing scheme. I 
> would also add smearing
> of occupation numbers. If you have this working, you can start 
> investigating basis sets for convergence.
>
> For graphene: The problem is the Dirac cone at the K special point. 
> Selecting a matching cell and k-point
> sampling is mandatory to get correct results. There is ample literature 
> addressing this problem.
>
> regards
> JH
>
> ________________________________________
> From: cp... at googlegroups.com <cp... at googlegroups.com> on behalf of Thomas 
> Kasel <tomk... at gmail.com>
> Sent: Monday, September 9, 2024 10:50 PM
> To: cp2k
> Subject: [CP2K:20677] Incorrect Band gaps from simple calculations
>
> Hello All,
>
> As a test I am trying to accurately (relatively) reproduce the band gaps 
> of silicon and graphene structures using PBE. I have however, run into an 
> issue where graphene gives a band gap of 1.2 eV while silicon gives a band 
> gap of 0.18 eV, the exact opposite of what one would expect. I would like 
> to ask what it is I am doing wrong in my calculations. Attached is a zip 
> file containing all my input files including basis set and pseudopotentials.
>
> Additionally, I cannot seem to get the SCF to converge unless I specify:
>
> &OT
> PRECONDITIONER FULL_ALL
> MINIMIZER CG
> &END OT
>
> Why is this setting so critical to SCF convergence in these cases?
>
> Cheers and thanks in advance,
> Tom
>
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