[Wien] Bulk vs supercell Plasma frequency

[Peter Blaha]

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 
> <pblaha at theochem.tuwien.ac.at <mailto: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 <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
>>     Muhammad Sajjad
>>     Post Doctoral Fellow
>>     KAUST, KSA.
>>
>>
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>
>
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>
> -- 
> Kind Regards
> Muhammad Sajjad
> Post Doctoral Fellow
> KAUST, KSA.
>
>
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