[Pw_forum] QE & Xcrysden

[Giovanni Cantele]

On Mar 10, 2010, at 9:44 AM, naphtaly moro wrote:

> Dear Q.E. & Xcrysden
> I am working on the various phase transitions of GaAs. I am trying to
> contact these structures. I ahve managed to construct the ZB and RS
> structures with success. I am using Xcrysden with Quantum Espresso.
> When i try to view the CsCl-GaAs structure,  I only see the Ga atoms
> and with no As atoms in the matrix. Can someone show me please where I
> am going wrong
> my input file is this
> &control
>   prefix='GaAs',
>   pseudo_dir='/home/physics/Desktop/espresso-4.0.5/pseudo'
>   outdir = './tmp',
> /
> &system
>   ibrav=  3, celldm(1) =10.2, nat=  2, ntyp= 2,
>   ecutwfc = 12.0,
> /
> &electrons
> /
> ATOMIC_SPECIES
> Ga  69.723  Ga.pbe-nsp-van.UPF
> As  74.9216 As.pbe-n-van.UPF
> ATOMIC_POSITIONS
> Ga 0.00 0.00 0.00
> As 0.50 0.50 0.50
> K_POINTS {automatic}
> 4 4 4 0 0 0
> 
> Thanks
> 
> 
> -- 
> Cecil Naphtaly Moro


The reason is that you specify ibrav = 3 that, according to what explained in Doc/INPUT_PW.txt is a bcc lattice.
At the same time you give the ATOMIC_POSITIONS in units of the alat (namely, a in bohr units).
Therefore, the As atom is placed at (a/2, a/2, a/2) which is, by chance (!), also a site of the bcc lattice.
In other words, what XCrysDen sees is a Ga atom in a/2,a/2,a/2 but ALSO an As atom at the same place and decides
to delete the latter.
Maybe you want to specify the fcc lattice (ibrav=2) and the As atom at a/4,a/4,a/4.

Giovanni


PS not very sure that ecutwfc = 12.0 whould give you converged results. Also, because you choosed
ultrasoft pseudopotentials, you might want also check the ecutrho parameter (by default = 4 * ecutwfc,
but larger values are needed when using US pseudo).

--

Giovanni Cantele, PhD
CNR-SPIN and Dipartimento di Scienze Fisiche
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
Phone: +39 081 676910 - Fax:   +39 081 676346
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
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