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
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
Phys.-Techn. Institute of Ural Br. of Russian Ac. of Sci.
426001 Izhevsk, ul.Kirova 132
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