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I want to get the phonon density of states for a monolayer MoS2. For this, I need phonon dispersion curves, and to choose a path where to compute phonons.

VASPKIT recommends Γ-M-K-Γ

SeeK-path recommends Γ—M—K—Γ—A—L—H—A|L—M|H—K|Γ—M'—K'—Γ—A'—L'—H'—A'|L'—M'|H'—K'

Why such different paths? Which one to choose to calculate the phonon density of states?

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If you check out this picture of the Brillouin zone of a hexagonal cell, you can see points like L, A and H are defined for a 3D Brillouin zone. This is because the k-path finding algorithms are generally made to work for general 3D cells, which is what SeeK-path is giving you. I'm guessing that for VASPKIT, there would be a point at which you indicate that you are simulating a monolayer, so it only gives you the high-symmetry k-points for a 2D Brillouin zone. So in short, the VASPKIT one is the one you should be using because there is no periodicity in the z-direction for a free-standing monolayer (even though, from a simulation standpoint, the DFT codes we use mostly impose periodic boundary conditions in all directions).

As a little side note that might be useful to people with similar concerns and are looking for alternatives besides VASPKIT, I like to use Pymatgen to get high-symmetry k-points, but it runs into the same problems for slab type structures. This MPInterfaces package has a nice function in this file of their GitHub repo called remove_z_kpoints() that can be used to remove the high-symmetry k-points with a z-dimension so you're only left those that would be found in a 2D Brillouin zone.

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  • $\begingroup$ Could you also add information about 1D materials (nanowires)? If the material is modeled in the z-direction in a tetragonal supercell with a=b=something large, and c=interatomic distance, is gamma to Z k-path sufficient then? $\endgroup$ Apr 30, 2023 at 14:41
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    $\begingroup$ I haven't thought about it carefully, maybe someone else can add on, but for such a case, I think the k-path could just go from (0, 0, 0) to (0, 0, 1) without having to rely on any additional tools and code that we'd use for the more complex 3D k-paths. $\endgroup$
    – CW Tan
    Apr 30, 2023 at 15:01
  • $\begingroup$ Thanks. I think you are right. I found several papers that analyzed 1D material (along Z axis) where they decided to use the gamma(0,0,0) to Z(0,0,1) k-points to plot the band structure. It confirms your suggestion $\endgroup$ Apr 30, 2023 at 16:26
  • $\begingroup$ @CWTan Indeed, at some point VASPKIT asked me, whether it is 2D $\endgroup$ May 1, 2023 at 1:34

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