[Wien] How to find the best KPOINTS?

[Rocquefelte]

Dear Jihoon Park,

Yes there is a method, which is the one you are using.
More precisely, it always depends on the property you are interested in.
For instance, the kmesh for optical properties will be denser than for 
total energy (or a DOS).
Thus, we should always check the convergency of the property we are 
calculating in terms of k-points, at least.

For magnetic properties, you need a good k-mesh for sure. One solution 
could be to take the smallest unit cell (ferromagnetic order) with the 
full-symmetry of the crystal and to check the k-mesh convergency. Then 
you will know how many k-points you need in the FULL-Brillouin ZONE 
(FBZ) to properly describe the FM state .

Based on this information, you can extrapolate for a larger cell.
For instance, if you need 800 k-points in the FBZ in the 
crystallographic cell calculation (FM-state).
For a supercell with a volume, which is 8 times bigger, you will need 
100 k-points.

Such an approach is a pragmatic one and you always need to check the 
coherence of your assumptions.
In particular, the k-mesh also depends on the magnetic order (to a 
smaller extent than volume of the cell).

Finally, if your interest is to compare different magnetic orders, it 
means that you are interested in the convergency of energy differences.
In many cases, "energy difference" converge faster than "total energy" 
... It means that your convergence criteria could be less restrictive. 
But here also you have to do it with care.

Best Regards

Xavier






Le 25/10/2012 18:03, Jihoon Park a écrit :
> Dear Users,
>
>
> I am trying to calculate magnetic properties for a large unitcell (64 
> atoms).
> For this kind of large unitcells, it take very long time to do the 
> KPOINTS convergence test.
> Should I anyway "have to" do the KPOINTS convergence test or is there 
> any other method to find a good KPOINTS?
>
>
> All my best,
> Jihoon Park
>
>
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> Wien at zeus.theochem.tuwien.ac.at
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