# Minimum number of kpoints for calculation of properties of nanoparticles in vasp

I am a little confused about what should be the correct number of kpoints that I wanna use. I am currently working on a small cluster so picking a high value(even around 10) gives me a memory fault. My first question is how does choosing the k points vary for nanostructures and for bulk.

For our geometry optimization we use a k-points

Automatic generation
0
Monkhorst-pack
1 1 1
0 0 0


And for calcualtion of DOS we have been using 3 3 3 which I think is less. I am currently exploring how the DOS vary for various values of K Points. And the values vary a lot when I take 4 4 4 instead of 3 3 3 . Why is this happening!

• Mar 14, 2022 at 13:44
• A nanocluster is not a periodic system, so the correct way of simulating it with periodic boundary conditions is to use a very large supercell (how large is a convergence parameter) such that you suppress the interactions between the nanocluster and its periodic images. When things are properly converged, you should only use a single $\mathbf{k}$-point as the "bands" will be flat. Mar 15, 2022 at 8:14
• @ProfM thanks for your reply and in the past day I have understood what you mean by using only a single k point. I have also gone through some literature and I am seeing people using only the 1 1 1 for ground state minimization but I am still a little unsure about using 1 1 1 for different properties. Mainly for DOS and Linear optical properties. I have also read that increasing the number of KPoints actually gives me wrong results. Is that true ?? Mar 15, 2022 at 8:24
• The same argument applies for DOS or optical properties. In a true nanocluster there are no bands, just isolated energy levels. When we simulate it using periodic boundary conditions, a well-converged calculation should lead to completely flat "bands", which means that you only need one $\mathbf{k}$-point, and this could be any $\mathbf{k}$-point as they should all be equivalent. In this context, it is best to think of a nanocluster as a "big molecule" rather than as a bulk material. Mar 15, 2022 at 10:42
• @ParmeetSinghEP066 why don't you write a self-answer? Future users might find it helpful! Sep 24, 2022 at 15:38