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

[Sheikh Ziauddin Ahmed]

Thank you all for your response. For computing the Z* and epilson for polar
materials, I saw in a couple of tutorials that I have to first compute the
dynamic matrix using dynmat.x at the Gamma point setting  epsil=.true.. For
the remaining q-points, I have to add the non-analytical part in the input
of the dynmat.x. Is this the correct for version 6.4.1 or should I just
have to set some flags?

 Also, is there a difference if I use matdyn.x instead of dynmat.x?

Regards,
Sheikh

On Mon, Dec 30, 2019 at 5:34 PM Stefano de Gironcoli <degironc at sissa.it>
wrote:

> 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*@virginia.edu* <sza9wz at virginia.edu>
> LinkedIn: https://www.linkedin.com/in/sheikh-ziauddin-ahmed
>
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-- 
*Sheikh Ziauddin Ahmed *
PhD Candidate
Charles L. Brown Department of Electrical and Computer Engineering
University of Virginia
Email:sza9wz*@virginia.edu* <sza9wz at virginia.edu>
LinkedIn: https://www.linkedin.com/in/sheikh-ziauddin-ahmed
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