[Pw_forum] Electric field in silicene

[Ari P Seitsonen]

Dear siham Sadki et co,

   Just to clarify Lorenzo's answer, I would not say that it is "wrong" to 
do so: If it is a layered material, there should be no dispersion in the 
bands in the direction perpendicular to the layers and thus you can choose 
any value. Since you have chosen the number of k points equal to one, it 
also does not increase the computational load (with some choices you 
might double the number of k points).

   Concerning the _lateral_ shift of the Monkhorst-Pack grid, it is better 
to use a grid that does include the Gamma point: Otherwise the symmetry of 
the k point grid is lower than the one of the lattice and more k points 
will be needed to recover the hexagonal symmetry; this was discussed in 
the literature soon after the original Monkhorst-Pack paper appeared (a 
Comment was written by some one, maybe Chadi and Cohen, I can check it out 
if needed), and I illustrated the reason/issue for this in my PhD thesis. 
A short answer, if using Monkhorst-Pack grid, it is better to use a set 
that does include Gamma point; if not computing a hexagonal system, then 
one might consider the grids with or without Gamma; but historically this 
shift was introduced in order to fix this issue of hexagonal grids and 
symmetry lowering (original Monkhorst-Pack grids do avoid Gamma point).

   Hopefully my long'ish answer clarifies more than confuses. :P

     Greetings from Brussels,

        apsi

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   Ari Paavo Seitsonen / Ari.P.Seitsonen at iki.fi / http://www.iki.fi/~apsi/
   Ecole Normale Supérieure (ENS), Département de Chimie, Paris
   Mobile (F) : +33 789 37 24 25    (CH) : +41 79 71 90 935


On Mon, 12 Jan 2015, Lorenzo Paulatto wrote:

>
> On 10/01/15 18:41, siham Sadki wrote:
>
>> K_POINTS {automatic}
>> 10 10 1 1 1 1
>
> Dear Siham,
> using a k-point grid shifted along the Z direction is wrong for 2D
> materials, it is also probably wrong for applying dipfield.
>
> kind regards
>
> -- 
> Dr. Lorenzo Paulatto
> IdR @ IMPMC -- CNRS & Université Paris 6
> +33 (0)1 44 275 084 / skype: paulatz
> http://www-int.impmc.upmc.fr/~paulatto/
> 23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05
>
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