# Modelling 2D semiconductor heterostructures including one magnetic layer with QE

I want to compute the electronic structure of a heterostructure composed of two 2D monolayers. I am stacking a TMD monolayer on top of a magnetic 2D monolayer. Both of them are semiconductors.

Do I need to include smearing when I compute the electronic structure or not?

## 1 Answer

Regardless of the type of material, using small smearing often helps in the convergence procedure. However, there are some caveats to look for.

• Always make sure that the smearing value is neither too small nor too large. Run a few convergence tests with different types of smearing to find the appropriate degauss value and smearing type.
• smearing is somewhat coupled to k-point sampling. So, maybe you want to converge ecutwfc, ecutrho, and K_POINTS before converging the degauss value.
• Smearing is needed for metallic systems to obtain convergence. And it doesn't harm to have a small smearing in semiconductors or insulators too! It affects the electronic energy a little bit but by carefully choosing the degauss value, you can reduce such an effect. (see answers to this question for details)
• As far as I know, choosing the smearing type or degauss value has nothing to do with whether the system is magnetic or not. Usually for any kind of system gaussian smearing is safe to choose. But as I said previously, maybe test the convergence first. Even though gaussian is safer, but mp or mv smearing often gives a substantial advantage.
• If you need to do the DOS calculation, occupations='tetrahedra' or occupations='tetrahedra_opt' methods are preferred instead of smearing.

So, to answer your question precisely, you don't need to use smearing. But probably you will have a hard time achieving convergence if you don't use it. That's why, as a practical method, you can use a small smearing to accelerate the convergence procedure. The code automatically tries to remove any effects due to smearing (fictitious finite temperature effects) after the calculation. Just make sure that this smearing value is not too large. Otherwise, you will get inaccurate ground state properties since the code will have a hard time removing the effects.