Density of states for bulk cleaved Si

[Kaspar]

I have read the post about using 25, 50 or even 100 k-points in the z-direction when doing transmission and DOS calculations on devices.
But what about when doing calculations on a bulk structure?

I am calculating DOS and Transmission on Si for different surfaces (100, 110, and 111) for simple unit cells.
I want to compare this with the DOS and transmission for a block of silicon with one dopant in (Eg. 100 Si atoms with one B or P atom)

I have done a convergence test on a bulk Si unit cell which showed that 8,8,8 k-points was fine. Now since the unit cell of the cleaved structures is not cubic but eg. 3.8 x 3.8 x 5.3, I use 8x8x6 k-points. (please tell me if this is wrong)
For the block of Si with a dopant in, the dimensions are 7.7 x 7.7 x 27, so I believe that I could use 4x4x2 and have same resolution.
(By mistake I actually used 8x8x6 also for the block calculations, but I don't think results could be wrong, right? I showed convergence already at 8x8x8 for the unit cell and this is higher resolution)

The question is that, the DOS plot for the undoped unit cell, cleaved in any surface orientation, looks way less detailed, and the band gap is severely off ( ~ 0.7 eV band gap) even though I use MGGA. There are some peaks spaced 1.3 eV apart around Ef, which makes me  believe that they are the real band edges.
Do I need 25 - 100 k-points in the c ( or z ) direction also for bulk/unit cell calculations?

[Anders Blom]

There is only one way to know for sure, and that is to test it, and compare the results. But in general I would expect that you are rather safe with the numbers you indicate - at least for the self-consistent loop. For the DOS calculation itself, I would imagine you often need much higher sampling, like 21x21x21 or so for the minimal cell, in order to get smooth curves.

For certain materials and in particular graphene and other 2D hexagonal materials the common rules of thumb don't hold, however, since you must hit the relevant k-point (K) and have a good sampling around it.