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When I use VASPKIT tool to generate K-path KPOINTS file for Bandstructure calcualtion of a 4x4x1 supercell of a pure material, it gives a PRIMCELL.vasp file (which contains the unit cell of that material) and asks me to replace my original supercell with it. BUT, when I use VASPKIT with a doped supercell structure, the PRIMCELL.vasp file keeps the structure a supercell and doesn't convert it to a unit cell.

Could you please tell me what the meaning of that file ? and what's the difference between the two procedures ?

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I have never used VASPKIT, so my answer is of a general nature.

If you have a $4\times4\times1$ supercell of a perfect crystal, you have redundant information because you could always rebuild the supercell starting from the primitive cell by applying a supercell matrix. My guess is that the PRIMCELL.vasp file simply contains this primitive cell as a computer code will normally want to use the primitive cell, because calculations are cheaper.

If you first build a $4\times4\times1$ supercell of a perfect crystal, and then add a defect into the system, your new system can no longer be described as a supercell of a smaller primitive cell. I imagine this is the reason why in this case the PRIMCELL.vasp file does not contain a smaller cell: no smaller cell exists that can be transformed into your actual defect cell, so you have to do your calculations in the larger defect cell.

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  • $\begingroup$ Will the bandstructure of 4x4x1 pure supercell will be the same of the primitive cell ? $\endgroup$ – Chi Kou Jan 21 at 13:55
  • $\begingroup$ You will have the same number of states at the same energy, so the density of states will be the same between the two. However, the band structures will look somewhat different, because of band folding: you will have many more bands in the supercell calculation. $\endgroup$ – ProfM Jan 21 at 14:27
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I assume that you are considering 2D materials. In short, you can use the undoped primitive cell to generate a k-path to plot the band structure of your $4\times4\times1$ supercell with doping.

Why can you do this?

enter image description here

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