# Re: [Wien] Bulk vs supercell Plasma frequency

Usually, different properties require different k-meshes. So it should be ok to converge the scf with a smaller k-mesh (it still must be a "good" one), but use more k-points for a plot of the DOS or optics.

Please repeat the optics calculation with a series of k-meshes and plot w_p as funktion of the k-mesh. You should find out in this way the converged value.

PS: I know that for Al one needs MANY more k-points to get a converged value. No idea about Na.

Am 29.10.2015 um 21:50 schrieb Muhammad Sajjad:
Dear Prof. P Blaha
Thank you very much for yous fruitful suggestions. However, I did not change RKmax. For cubic Na only W_p_xx is enough but after making supercell (1 1 6) still I need only w_p_xx and forget about w_p_zz, though the system is tetragonal now. Is it right I understood? Second thing is it principally right that one converges SCF with small k-mesh say 30*30*5 and then use the same scf ao get OPTICAL properties with k-mesh say 90*90*15? I mean no need to converge scf with 90*90*15.
Kind Regards

On Thu, Oct 29, 2015 at 9:03 PM, Peter Blaha <pbl...@theochem.tuwien.ac.at <mailto:pbl...@theochem.tuwien.ac.at>> wrote:

In principle you are doing thinks now correct.
In reality, however, we use the tetrahedra-method in the
integration of the BZ to calculate a joint-density-of-states. And
for a metal !!! here there is a difference between k-meshes in the
small or large cell. (it is related to the "back-folding" problem
and in the supercell there are less tetrahedra and thus less
possibilities to interpolate for the tetrahedron method than in
the small cell).

In any case, you need to consider two things:

a) You have to decide to which accurcy a certain number should be
calculated. Typically, a plasma frequency would be quoted with one
or two digits after the comma (i.e. 5.9 or at most 5.94 eV).

b) Then you have to converge the numbers with respect to k-mesh
(but also Rkmax !!)
Definitely you should use a ratio of 6:1 for the supercell, but
eventually you need to increase the mesh until there are no
changes within the desired accuracy. In your case, 90x15 is almost
sufficient, but not quite if you want two digits accuracy.

c) in a cubic system, of course it is useless to quote two tensor
components and one is sufficient.

Am 29.10.2015 um 10:35 schrieb Muhammad Sajjad:
Dear Prof. Blaha

I did some calculation for Bulk Na (bcc, space gp #229_Im-3m)
wiht k-mesh 30*30*30 and found the plasma frequency (in
case.outputjoint) as
Plasma frequencies:

w_p_xx      w_p_zz    [eV]

5.9446      5.9446

Now I constructed a supercell 1*1*6 (definitely symmetry reduced
and now sp.gp <http://sp.gp> is 1_P1), used k-mesh 30*30*5 and  found
Plasma frequencies (much deviating from bulk values):

w_p_xx      w_p_zz    [eV]

5.76146      5.3446

Then used k-mesh 90*90*15 and found
Plasma frequencies (w_p_zz is deviating from bulk w_p_zz ):

w_p_xx      w_p_zz    [eV]

5.9485      5.8903

I have read the previous post
(https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg09782.html)
but it is not speaking about w_p_zz. So when I plot epsilon data
for w_p_xx then it is matching with that of bulk but when I use
w_p_zz  then it is definitely  away from bulk. Should I consider
w_p_zz  or use "number of choice = 1 in case.inop" and plot
epsilon only with w_p_xx ?

Kind Regards
Post Doctoral Fellow
KAUST, KSA.

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--
Kind Regards
Post Doctoral Fellow
KAUST, KSA.

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