# [Pw_forum] Questions on supercell calculation of MgB2

Fri Sep 2 10:07:10 CEST 2005

```Dear Paolo:
Thank you very much for your reply. However, I am not familiar with
supercell calculation, especially how "several k-points of the original
cell" refold into the Gamma point of the supercell. I can't find any
documents related to this topic and I cannot image it at all.Further more,
on how to "compare calculations that use the same grid to sample the
respective Brillouin zones", would you give me some further advice? What
does "the same grid" meaning? With the same deltak? Or something else?
With my kindest regards.

Hongbin Zhang

>Message: 5
>From: Paolo Giannozzi <giannozz at nest.sns.it>
>Organization: Scuola Normale Superiore di Pisa
>To: pw_forum at pwscf.org
>Subject: Re: [Pw_forum] Questions on supercell calculation of MgB2
>Date: Thu, 1 Sep 2005 12:13:22 +0200
>Reply-To: pw_forum at pwscf.org
>
>On Monday 29 August 2005 11:40, =D5=C5 =BA=E9=B1=F2 wrote:
>
> > I have done a calculation to check the convergence of supercell
> > method by calculating a 3*3*1 supercell without atom loss, but=20
> > the band energy is very different with those of original unit cell
>
>you mean "the band energies" ? and what does it mean that=20
>"they are different" ? of course they are: the Gamma point of=20
>the supercell contains several k-points of the original cell that=20
>are refolded into the Gamma point of the supercell.
>In order to establish an exact mapping of the band energies for=20
>the supercell into those of the original cell, you need to compare
>calculations that use the same grid to sample the respective=20
>Brillouin zones. Do this and you will find that the supercell has=20
>exactly 9 times the total energy of the original cell and that all
>eigenvalues at Gamma of the supercell can be mapped into
>eigenvalues at Gamma + refolded k-points of the original cell.
>
> > Further more, when I want to plot the band structure using
> > bands.x to get the .dat file, there is an error like:
> > [...]
> >      from punch_band : error #         5
> >       increase maxdeg
>
>the degeneracy of energy bands in a supercell may be quite high.=20
>If you really want to plot bands in a supercell, do what it says and
>recompile
>
> > And at the beggining of the scf calculation procedure, there are
> > "additional transition found"
>
>good: this means that the identity plus a fractional translation=20
>is a symmetry operation, i.e. that your cell is a supercell. In a
>supercell all translation vectors that are lattice vectors of the=20
>original cell but not of the supercell will appear as fractional
>translations.=20
>
>Paolo
>
>=2D-=20
>Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
>Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513=20
>Piazza dei Cavalieri 7      I-56126 Pisa, Italy
>
>--__--__--
>

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