# Calculation of DOS from DFTB+?

I am trying to calculate the DOS and PDOS for a graphene-DNA structure. Upon referring the manual, I understand that we need to have charges converged before computing the DOS. Following this, I have a question:

I have computed the SCC in 4 sets of kpoint meshs (4x4x1 to 10x10x1), and see that the energies do not change till the third decimal place. Does this mean that I can use the lowest (4x4x1) grid for the DOS calculation? My experience from quantum espresso tells me that we need a denser grid for DOS calculations. Does it hold true here also?

• +1 but each post needs to have just one question. Commented Jan 25, 2023 at 10:51
• Sure Prof. Dattani. My thought process was like since they are connected, so I'll ask it together. I will keep this in mind. Commented Jan 25, 2023 at 10:52
• Yea, this happens to a lot of people! Commented Jan 25, 2023 at 10:53
• 1) Giving us kpoint meshes without telling actual dimensions is not really helpful. You may want to share the unit cell size. Is it 0.1 nm? 1? 10? 2) Without knowing specifics about your system, most probably a n x 1 x1-type mash makes more sense for a 1D infinite tube. Also, since DFTB works on local basis, you can infinity increase the cell size in the two other directions without increasing computational cost. 3) In principle, you need to converge the density for DOS calculations. However, single point and DOS/PDOS calculations are cheap, so you can check several meshes.
– Greg
Commented Jan 28, 2023 at 3:50
• @HemanthHaridas That is already a pretty big cell, especially if big part of it vacuum. Sorry, for a reason I mistaken your system, and though you are talking about nanotubes, not graphene sheet. If you have tested it, it is reasonable that your band energies are already converged with such large cell, and you can do the DOS calculation.
– Greg
Commented Jan 28, 2023 at 10:23