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I am just learning DFT using the BURAI GUI of Quantum Espresso.I took the CIF file of a structure from the Materials project database. It is well vc-relaxed and conversed easily but gives the band gap much less than the published value, however, PPs are already added in CIF. Please suggest possible regions

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  • $\begingroup$ You need to clarify some things. For example, material type, reference you are using to compare with, functional used in both cases, etc. $\endgroup$
    – Camps
    Dec 14, 2023 at 16:42

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Since you used BURAI, it's safe to assume that you used either an LDA or a GGA functional for your pseudopotentials (PP). In order to confirm, check if you have pz or pbe in the name of your PP. It is expected that you would get a much smaller band gap, around 40% underestimation from the experimental band gap using these functionals. This is a well-known problem of DFT, the band-gap problem.

In order to improve your band gap, there are several strategies that you can follow. You can use hybrid functionals such as HSE06 or gaupbe. Check out this and this questions to learn more about using QE with hybrid functional. If your system is a d-block metal, you will get a improved band gap using DFT+U. I believe you need to move from BURAI and start to use Quantum ESPRESSO using command line to implement these strategies since BURAI is a really old GUI and uses a very old QE version. The syntax of that old version changed significantly in the newer version.

There are even further approach that you can use such as GW approximation or using Koopmans functional. Hybrid functionals tend to overestimate the band gap a little. Using Koopmans functional, it is claimed to be found the band gap for Si exactly as the experimental values. Beware that these methods are computationally very inefficient compared to the simple LDA or GGA case. Also, they require huge memory.

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