# [Pw_forum] k_points

Stefano Baroni baroni at sissa.it
Mon Jun 14 14:13:18 CEST 2010

```On Jun 14, 2010, at 1:33 PM, raheleh vaziri wrote:

> Dear Paolo
>
> Thanks very much for your reply. I read the lectures and I understand that offset shift only points and not change the total energy. Am I wrong?

yes

> I calculated energy for a system with two kinds of k_points (2 2 5 0 0 0 and 2 2 5 1 1 1) and I observed different energy for each of them. How to determine the offset for any kpoints?

I do not quite understand what the question is. whatever it may be, the short answer is: kpoint sets are uniform grids in reciprocal space. for a given density of points (inreciprocal space), you have the freedom to chose the coordinates of one point of the grid (the others are fixed by the periodicity of the grid. the "offset" is simply the position of such point. Usually, it is choosen in such a way that the origing is located in the middle of one elementary paralleliped of the grid (rather than at one corner)

SB

> Is there need for convergence?
>
> Mahkameh Vaziri
>   PhD student of physical chemistry
> K. N. Toosi university of Technology, Tehran, Iran
>
>
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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