[Pw_forum] Problem in VC-relaxation with fixed atomic positions

[N. Plugaru]

Dear Marcos

I remember a similar issue posted on this forum maybe a few months ago,
and the answer was that the code makes a difference between fixing the
atomic positions with reference to the Cartesian system but still they are
free to change in the crystal axes system.

 HTH,
Nucu

On Fri, June 6, 2014 7:21 am, Marcos Veríssimo Alves wrote:
> Thanks, Arles, but I opened the relaxation on xcrysden and on the first
> step the cell was hexagonal.
>
> Best,
>
> Marcos
>
> On Thursday, June 5, 2014, Arles V. Gil Rebaza <arvifis at gmail.com> wrote:
>
>> Dear Marcos, please review your CELL_PARAMETERS tag..!! I thing that
>> there
>> are some mistakes... you are using a square 2D lattice and not a
>> hexagonal
>> one.
>>
>> Best
>>
>> PhD. Arles V. Gil Rebaza
>> Instituto de Física La Plata
>> La Plata - Argentina
>>
>>
>> 2014-06-05 21:40 GMT-03:00 Marcos Veríssimo Alves <
>> marcos.verissimo.alves at gmail.com
>> <javascript:_e(%7B%7D,'cvml','marcos.verissimo.alves at gmail.com');>>:
>>
>>> Hi all,
>>>
>>> I am trying to perform a cell optimization for graphene in which I
>>> would
>>> like to fix the atomic positions so as to have arbitrary bond lengths,
>>> and
>>> see what happens to the in-plane cell vectors. Thus I would like to
>>> keep
>>> the atomic positions fixed and let the in-plane cell vectors change.
>>>
>>> I am using vc-relax with cell_dofree="2dxy" and I am (theoretically)
>>> fixing atomic positions (specified in Angstrom) with "0 0 0" after the
>>> cartesian coordinates. The problem is, the coordinates do not remain
>>> fixed
>>> during the cell optimization:
>>>
>>> ATOMIC_POSITIONS (angstrom)
>>> C        0.000000000   0.000000000   7.000000000    0   0   0
>>> C        1.508583432   0.000000000   7.000000000    0   0   0
>>> --
>>> ATOMIC_POSITIONS (angstrom)
>>> C        0.000000000   0.000000000   7.000000000    0   0   0
>>> C        1.553137965   0.000000000   7.000000000    0   0   0
>>> --
>>> ATOMIC_POSITIONS (angstrom)
>>> C        0.000000000   0.000000000   7.000000000    0   0   0
>>> C        1.583982256   0.000000000   7.000000000    0   0   0
>>> --
>>> ATOMIC_POSITIONS (angstrom)
>>> C        0.000000000   0.000000000   7.000000000    0   0   0
>>> C        1.592677072   0.000000000   7.000000000    0   0   0
>>> --
>>> ATOMIC_POSITIONS (angstrom)
>>> C        0.000000000   0.000000000   7.000000000    0   0   0
>>> C        1.597484048   0.000000000   7.000000000    0   0   0
>>> --
>>> ATOMIC_POSITIONS (angstrom)
>>> C        0.000000000   0.000000000   7.000000000    0   0   0
>>> C        1.599092143   0.000000000   7.000000000    0   0   0
>>> --
>>> ATOMIC_POSITIONS (angstrom)
>>> C        0.000000000   0.000000000   7.000000000    0   0   0
>>> C        1.598936919   0.000000000   7.000000000    0   0   0
>>> --
>>> ATOMIC_POSITIONS (angstrom)
>>> C        0.000000000   0.000000000   7.000000000    0   0   0
>>> C        1.598936919   0.000000000   7.000000000    0   0   0
>>>
>>> My input is as follows:
>>>
>>> &control
>>>     calculation='vc-relax'
>>>     restart_mode='from_scratch',
>>>     prefix='graphene',
>>>     pseudo_dir = '/home/mverissi/pseudos_espresso',
>>>     outdir='./'
>>> /
>>> &system
>>>     ibrav=0,
>>>     celldm(1)=4.073139044,
>>>     nat=2,
>>>     ntyp=1,
>>>     nspin = 1,
>>>     ecutwfc = 28.0,
>>>     ecutrho = 252.0,
>>>     occupations='smearing',
>>>     smearing='methfessel-paxton',
>>>     degauss=0.001,
>>>     nbnd=10,
>>> /
>>> &electrons
>>>     conv_thr = 1.0e-9,
>>>     mixing_beta = 0.7
>>> /
>>> &ions
>>>     ion_dynamics='bfgs'
>>> /
>>> &cell
>>>     cell_dynamics = 'bfgs',
>>>     cell_dofree = '2Dxy',
>>> /
>>> ATOMIC_SPECIES
>>> C   12.0107    C.pbe-rrkjus.UPF
>>> ATOMIC_POSITIONS {angstrom}
>>> C    0.0     0.0   7.0    0  0  0
>>> C    1.42    0.0   7.0    0  0  0
>>> K_POINTS {automatic}
>>>        24   24   1  0  0  0
>>> CELL_PARAMETERS {alat}
>>>    0.866025404   0.5  0.0
>>>    0.866025404  -0.5  0.0
>>>    0.000000000   0.0  6.0
>>>
>>> Am I making some silly mistake here? The version of Espresso in use is
>>> 5.0.2. Sorry if this has already come up, but I couldn't find anything
>>> similar to my problem.
>>>
>>> Best regards,
>>>
>>> Marcos
>>>
>>> _______________________________________________
>>> Pw_forum mailing list
>>> Pw_forum at pwscf.org <javascript:_e(%7B%7D,'cvml','Pw_forum at pwscf.org');>
>>> http://pwscf.org/mailman/listinfo/pw_forum
>>>
>>
>>
>>
>> --
>> ###--------->   Arles V.   <---------###
>>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum


*********************************************************
  N. Plugaru
  National Institute of Materials Physics
  Atomistilor Str. 105 bis,
  077125 Bucharest-Magurele, Ilfov, Romania.

  Phone: 0040 21 2418 221 (office)
  Fax:     0040 21 3690 177
  e-mail: plug at infim.ro
  http://www.infim.ro/    skype: nplugaru
*********************************************************