[Wien] Bulk vs supercell Plasma frequency

[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 <pblaha at 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 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
> Muhammad Sajjad
> Post Doctoral Fellow
> KAUST, KSA.
>
>
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-- 
Kind Regards
Muhammad Sajjad
Post Doctoral Fellow
KAUST, KSA.
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