[Pw_forum] problems of the K points of the unit cell and supercell

[Nicola Marzari]

Simple case - orthorombic.

If you double your unit cell in one direction, you need to half the
sampling in the corresponding direction in reciprocal space.

I.e., in absolute coordinates, the k-point spacing remains the same, but 
you account for the fact that when you increase your supercell, your
brillooin zone becomes smaller.

Please think carefully at these issues with the example of 1 band in 1 dim.

			nicola



On 25/04/2013 00:01, Chengyang Li wrote:
> Thanks a lot. I will try it.
>
> Chengyang
>
> ------------------------------------------------------------------------
>
>     *From: *"Yun Wang" <yun.wang at griffith.edu.au>
>     *To: *"PWSCF Forum" <pw_forum at pwscf.org>
>     *Sent: *Wednesday, April 24, 2013 3:17:38 PM
>     *Subject: *Re: [Pw_forum] problems of the K points of the unit cell
>     and        supercell
>
>     Hi Chengyang,
>
>         You can run a series of calculations with different k-point
>     meshes. The right k-point mesh density can be determined once the
>     total energies converge.
>
>     Cheers,
>     Yun
>
>
>     On Thu, Apr 25, 2013 at 5:18 AM, Chengyang Li
>     <li.chengyang at wmich.edu <mailto:li.chengyang at wmich.edu>> wrote:
>
>         Dear all
>
>               I have a question about the K points. When I try to repeat
>         the unit cell to get a supercell. I found someone said if the
>         structure is changed, the K points should be changed at the same
>         time if we want to keep the same result. I try to calculate the
>         phonon of the chromium, the atomic positions and cell parameters
>         are like this:
>
>         Unit cell
>
>         ATOMIC_POSITIONS crystal
>         Cr1      0.000000000   0.000000000   0.000000000
>
>
>         CELL_PARAMETERS cubic
>             0.500000000   0.500000000   0.500000000
>            -0.500000000   0.500000000   0.500000000
>            -0.500000000  -0.500000000   0.500000000
>
>         Supercell, there are two unit cells in this cell.
>
>         ATOMIC_POSITIONS crystal
>         Cr1      0.000000000   0.000000000   0.000000000
>         Cr1      0.000000000   0.000000000   0.500000000
>         Cr1      0.500000000   0.500000000   0.250000000
>         Cr1      0.500000000   0.500000000   0.750000000
>
>         CELL_PARAMETERS
>             1.000000000   0.000000000   0.000000000
>             0.000000000   1.000000000   0.000000000
>             0.000000000   0.000000000   2.000000000
>
>         If I set the unit cell like this
>
>         K_POINTS automatic
>            4 4 4 1 1 1
>
>         How should I change the k points in the supercell?
>
>
>         Thank you for the attention.
>
>         Chengyang Li
>
>         Western Michigan University,
>
>
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>
>
>
>     --
>
>     Dr. Yun Wang
>     Research Fellow
>     Centre for Clean Environment and Energy
>     Griffith School of Environment
>     Gold Coast Campus, Griffith University
>     QLD 4222, Australia
>     Tel:(61-7) 5552 8456
>     Fax:(61-7) 5552 8067
>
>
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-- 

----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL