What Factors Influence the Band Gap in Band Structures?

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Discussion Overview

The discussion centers around the factors influencing the band gap in band structures, particularly in the context of Density Functional Theory (DFT) calculations and their comparison with experimental values. Participants explore the parameters that may affect band gap values in theoretical models.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the broadness of the initial inquiry about band gap parameters.
  • A participant reports a calculated band gap value of 5 eV for rutile TiO2 using DFT, which contrasts with an experimental value of 3.2 eV, suggesting a potential overestimation in their calculation.
  • Another participant notes that DFT calculations can overestimate band gaps if unreasonably large U values are used in the DFT+U framework and mentions that hybrid functionals may also lead to overestimation for certain materials.
  • Concerns are raised about the testing of pseudopotentials used in calculations, with a suggestion to verify their reliability based on the developers' tests.
  • A later reply emphasizes that testing a pseudopotential is complex and typically relies on the credibility of the developers' prior tests, mentioning the VASP team's pseudopotentials as an example.

Areas of Agreement / Disagreement

Participants express differing views on the reliability of DFT calculations and the factors affecting band gap values, indicating that multiple competing perspectives exist without a clear consensus.

Contextual Notes

The discussion highlights the potential for discrepancies between calculated and experimental band gap values, the influence of specific parameters like U values and pseudopotentials, and the complexity of validating these computational tools.

Who May Find This Useful

Researchers and students interested in computational materials science, particularly those working with DFT and band structure calculations.

Arun Prasath
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What are the parameters affects the band gap in a band structure?
 
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Too broad question. Try to be specific a bit.
 
I have calculated band gap value of rutile TiO2 using Density Functional Theory with the help of
Quantum ESPRESSO package.I got the band gap value around 5 eV. But from experimental reference, I could confirm that the band gap value is 3.2 eV. Then DFT will always underestimate the band gap value. But in my calculation it overestimate the band gap value? so I would like to correct the error. So kindly let me know what are the parameters have an impact on bandgap value.
 
In a DFT calculation this can happen if one uses an unreasonably large U values within DFT+U framework. Also I believe that hybrid functionals in their standard form (i.e. 25% of exact exchange) can overestimate the band gap for some materials.
Has the pseudopotential you are using been tested carefully?
Other factors I can think of is straining the lattcie parameter of the simulation cell.
 
Thank you Useful nucleus. could you please tell me how to test the pseudopotential?
 
Testing a pseudopotnetial is a nontrivial task and I would not get into it unless I'm the one who generated it. Usually one trusts the tests that the developers of the psuedopotential did. I used those developed by VASP team and they seem to be well-trusted (except may be for their ultrasoft ones which the VASP team is not developing any more.).

To learn more about the kinds of tests they typically do , you may for example download and have a look at some ppt slides on VASP website which they prepared for a workshop in the past. If you have more time you may look up some of their papers as well. Or even better look at the website of the code you are using.
 

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