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

[Ricardo Afonso]

dear Paolo,

Tsung-Lung Li explained the atoms will be moved idependently according to
the forces acting on them.
And you said that is correct. It seems to me that somehow contradicts to
say that this still preserves the symmetry.
Please clarify if I'm wrong about this. Because I understand right to keep
the symmetry the atoms should be moved not only according to their forces,
but also preserving their coordinates, which suggests that their should
move isotropically, right?

Thank you very much,

Ricardo.

On 5 October 2017 at 14:09, Paolo Giannozzi <p.giannozzi at gmail.com> wrote:

> 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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>



-- 
Ricardo Afonso
Student of Magnetism and Superconductivity Group
Federal University of Sao Carlos
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