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

[Paolo Giannozzi]

On Sun, Oct 8, 2017 at 9:03 PM, Ricardo Afonso <ricardo at df.ufscar.br> wrote:

>
> Tsung-Lung Li explained the atoms will be moved independently according to
> the forces acting on them.
>

if they are moved "according to the forces", they are not moved
"independently", because the forces are constrained by the symmetry of the
system.

Paolo


> 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 <+39%200432%20558216>, fax +39-0432-558222
>> <+39%200432%20558222>
>>
>>
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>
>
>
> --
> Ricardo Afonso
> Student of Magnetism and Superconductivity Group
> Federal University of Sao Carlos
>
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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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