[QE-users] Does order of atoms matter in Quantum Espresso?

[Stefano de Gironcoli]

the order of the entries in the pseudopotential list or in the atomic 
position list should not affect the result.

however you are not changing only the order.

1) you are also swapping the two atoms... this too should not change 
anything but is not the same as changing the order only :)

2) the first calculation is metallic (with a quite large gaussian 
smearing, 0.05 Ry is ~ 0.7 eV) the second uses the default insulating 
setting. THIS may have a big impact, for two reasons: i) the fundamental 
gap in AlSb, if I remember correctly, is not very large .. comparable to 
the smearing width used; ii) being AlSb a polar material the vibrational 
properties have a non analytic component ruled by the macroscopic 
dielectric properties (Z*, epsilon_infty) that are disabled in the 
phonon calculation of metals (where epsilon_infty diverges).

Additionally, in the phonon calculation step you should make sure you do 
compute Z* and epsilon and include them in the subsequent phonon 
dispersion calculation step.

HTH

stefano

On 30/12/19 23:07, Sheikh Ziauddin Ahmed wrote:
> I am trying to calculate the phonon dispersion of AlSb. I am getting 
> two different results depending on how I am introducing the atoms in 
> the SCF calculation. Will the order of the atom change the symmetry of 
> the zinc blende AlSb?
>
> In the first case the input deck is:
> &system
>     ibrav=2, nat=2, ntyp=2,
>     ecutwfc = 50, ecutrho = 500,
>     occupations='smearing',
>     smearing='gauss',
>     degauss=0.05,
>     celldm(1)=11.5944146,
>  /
>  &electrons
>     conv_thr    = 1.0e-12
>     mixing_beta = 0.7
>     mixing_mode = 'local-TF'
>  /
> ATOMIC_SPECIES
>  Al 26.98 al_lda_v1.uspp.F.UPF
>  Sb 121.76 sb_lda_v1.4.uspp.F.UPF
> ATOMIC_POSITIONS (alat)
>  Al 0.0   0.0   0.0
>  Sb 0.25  0.25  0.25
> K_POINTS automatic
> 6 6 6 0 0 0
>
> In the second case the input deck is:
>  &system
>     ibrav=2, nat=2, ntyp=2,
>     ecutwfc = 50, ecutrho = 500,
>     !occupations='smearing',
>     !smearing='gauss',
>     !degauss=0.05,
>     celldm(1)=11.5944146,
>  /
>  &electrons
>     conv_thr    = 1.0e-12
>     mixing_beta = 0.7
>     mixing_mode = 'local-TF'
>  /
> ATOMIC_SPECIES
>  Sb 121.76 sb_lda_v1.4.uspp.F.UPF
>  Al 26.98 al_lda_v1.uspp.F.UPF
> ATOMIC_POSITIONS (alat)
>  Sb 0.0   0.0   0.0
>  Al 0.25  0.25  0.25
> K_POINTS automatic
> 6 6 6 0 0 0
>
> Thanks in advance!
> -- 
> *Sheikh Ziauddin Ahmed *
> PhD Candidate
> Charles L. Brown Department of Electrical and Computer Engineering
> University of Virginia
> Email:sza9wz_ at virginia.edu_ <mailto:sza9wz at virginia.edu>
> LinkedIn: https://www.linkedin.com/in/sheikh-ziauddin-ahmed
>
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