[Pw_forum] Run QE with space_group and "ATOMIC_POSITIONS crystal_sg"

[Paolo Giannozzi]

On Thu, Oct 5, 2017 at 4:03 PM, Tsung-Lung Li <quantum at mail.ncyu.edu.tw>
wrote:

It seems to me QE does the following.
>
> 1. Initially, QE uses space_group and the 7 inequivalent atoms in
>     ATOMIC_POSITIONS to produce 24 atoms in the crystal unit cell.
>

correct


> 2. In each subsequent geometric step of the 'relax' calculation,
>     the 24 atoms are moved independently according to the forces
>     acting on each atom.
>

correct


> Therefore, at the end of the 'relax' calculation, the space_group=15
> symmetry is not always preserved.
>

not correct. Structural optimization based on forces does not break the
symmetry, with occasional unfortunate exceptions due to numerical noise


> Can QE only move the 7 inequivalent atoms and use space_group symmetry to
> construct the other 17 atoms for next geometric step?
>

It could be done in principle, but it would require some extensive changes
to the structural optimization algorithm, with dubious advantages in return.

Paolo

This way, the space_group symmetry will be preserved at the end of the
> simulation.
>
> Thank you in advance.
>
> Best Regards,
> Tsung-Lung Li
>
> ------------------------------------------------------------------
> &control
>    calculation = 'relax'
>    restart_mode = 'from_scratch'
> /
> &system
>    space_group = 15
>    celldm(1) = 11.86181088389489 ! bohr
>    celldm(2) = 2.41851202803887
>    celldm(3) = 0.903616377250279
>    celldm(5) = -0.408489773780522
>    ntyp = 4
>    nat = 7
> /
>
> ATOMIC_SPECIES
>     Ca   40.0780000000  Ca_pbe_v1.uspp.F.UPF
>      S   32.0650000000  S_pbe_v1.2.uspp.F.UPF
>      O   15.9994000000  O.pbe-n-kjpaw_psl.0.1.UPF
>      H    1.0079000000  H.pbe-rrkjus_psl.0.1.UPF
>
> ATOMIC_POSITIONS crystal_sg
>    Ca  0.00000   0.17050   0.25000
>    S   0.00000   0.32727   0.75000
>    O   0.08319   0.27218   0.59103
>    O   0.19997   0.38195   0.91298
>    O  -0.20823   0.06826  -0.07831
>    H  -0.25800   0.08700  -0.23400
>    H  -0.24400   0.02000  -0.07700
>
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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