[Wien] Different Plasma Frequency (PF) for bulk and supercell Na

[Lyudmila Dobysheva]

13.10.2015 09:42, Muhammad Sajjad wrote:
> I have computed PF for bulk Na and its value is *5.943 eV* with k-mesh
> *18*18*18*.
> Then I constructed a supercell 1*1*6:
> *K-mesh                        PF (x/z components in eV)*
> *24*24*24                  5.7097   /   5.8192*
> *36*36*24                  5.7614   /   5.8098
> *50*50*10                  5.9358   /   5.6280
> *50*50*30                  5.9365   /   5.8079*
> My question is why PF is very sensitive to k-mesh selection?

I do not know about PF, but general principles are that the larger is a 
supercell the less number of points you need. So, the best described 
direction is z, it corresponds to 180 points in bulk (30x6). Let it be 
that you have reached a convergence in it in supercell, this means that 
you didn't obtain it in a bulk calculation. If you want a comparable 
number of k-points you should have more points in bulk. Maybe 24 in z 
direction is already enough. So, IMHO, try up to 120x120x120 for bulk, 
at least, and further 180x180x180. And for supercell it means 180x180x30
> and which
> value of PF is required for correct dielectric constant calculation? Do
> I need *PF = 5.943 eV* for the supercell

In terms of accuracy, this is the worst.

Best wishes
   Lyudmila Dobysheva
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