# Re: [Wien] Bulk vs supercell Plasma frequency

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

```
```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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