[Pw_forum] k points in epsilon.x

[Gabriele Sclauzero]

dev sharma wrote:
> Sir, i want to calculate dielectric constant as a function of 
> frequency.  I have read the manual And it is written there that
> Epsilon.x doesn't support the reduction of the k-points grid into the 
> unreducible Brillouin zone, so the previous PW runs must be performed 
> with a uniform k-points grid and all k-points weights must be equal to 
> each other, i.e. in the k-points card the k-points coordinates must be 
> given manually in \emph{crystal} or \emph{alat} or \emph{bohr}, but not 
> with the \emph{automatic} option.

In your first mail you were speaking about the automatic keyword as if it was an option 
for a k-point list to be supplied to epsilon.x. As you have now reported from the manual, 
epsilon.x does not need a k-point grid, since it uses the k-points from the underlying 
data produced by the previous pw.x run.
For a cubic symmetry system, for instance, you can supply manually a N x N x N mesh of 
k-points (using crystal option might be the easiest solution, in my opinion), which must 
provide a good sampling the full BZ (it is not difficult to produce one, see the source 
code of pwtools/kpoints.f or PW/kpoint_grid.f90).
Be careful and check the code does not reduce the mesh by symmetry (which is the default, 
good behaviour of the code, that in this specific case you want to avoid). Use the nosym 
option if needed (as written in the manual).
You can also try to generate the grid with the automatic option of K_POINTS after setting 
to true nosym and noinv (see Doc/INPUT_PW.txt), but I'm not sure if this always works.


Regards,

GS



> Thanks
> 
> On Mon, Oct 12, 2009 at 12:40 PM, Gabriele Sclauzero <sclauzer at sissa.it 
> <mailto:sclauzer at sissa.it>> wrote:
> 
> 
>     dev sharma wrote:
>      > hi 2 all,
>      >
>      >  I have one question. In epsilon.x, the k points is not automatic
>      > option. Sir, means how to give the K points option i.e. we have
>     to give
>      > only irreducible k points or like for the calculation ='bands' ,
>     we have
>      > give the path (like G{0,0,0}-->X(0.5,0,0)). If we have to give the
>      > irreducible points , how to calculate those points ???? can i do
>     this ,
>      > that run the pw.x with a mesh like 4*4*4 grid  and the number of k
>      > points in the scf output are the required k points
> 
>     Never used this utility, so maybe my advice is useless: are you sure
>     that epsilon.x does
>     need a k-point grid on input? For what purpose are you using the
>     program? Do you know what
>     it computes? If not, please look at the documentation in
>     Doc/eps_max.tex.
> 
> 
>     GS
> 
> 
>     PS: Please always sign your mails with your name and affiliation. Thanks
> 
> 
> 
>     --
> 
> 
>     o ------------------------------------------------ o
>     | Gabriele Sclauzero, PhD Student                  |
>     | c/o:   SISSA & CNR-INFM Democritos,              |
>     |        via Beirut 2-4, 34014 Trieste (Italy)     |
>     | email: sclauzer at sissa.it <mailto:sclauzer at sissa.it>              
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o ------------------------------------------------ o
| Gabriele Sclauzero, PhD Student                  |
| c/o:   SISSA & CNR-INFM Democritos,              |
|        via Beirut 2-4, 34014 Trieste (Italy)     |
| email: sclauzer at sissa.it                         |
| phone: +39 040 3787 511                          |
| skype: gurlonotturno                             |
o ------------------------------------------------ o