[CP2K-user] [CP2K:20681] Incorrect Band gaps from simple calculations

[Jürg Hutter]

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: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Thomas Kasel <tomkasel 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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