# [Pw_forum] Symmetry problem

Stefano de Gironcoli degironc at sissa.it
Sun Mar 22 04:26:22 CET 2009

Dear Yang Sun,

the variable cell relaxation keeps the symmetry that it finds at
the beginning of the run and stops with an error message if (for
numerical reasons) some symmetry is lost...
so your system WAS NOT base-centered orthorhombic in the first place.
The fact that you choose a certain Bravais lattoce does not mean
that the crystal has the corresponding symmetry. How many symmetry
operations are found
in your starting configuration ? Is this number what you expected ? if
not, look at your crystal structure with xcrysden or other structure
visualizer and see if there is some evident problem.
check the number of digits you are providing in your crystal
structure... symmetry is checked using a certain (small) tollerance.
Are you sure you have been using the same conversion factor from
Angstrom (used in the coordinates) and atomic units (used for celldm)
? in your case it is probably more convenient defining the structure
using the "a,b,c,cosab, cosbc,cosac" set, where the lengths are given
in Angstrom, or use crystal coordinates for the atomic positions

regards,
stefano

Stefano de Girocoli - SISSA and DEMOCRITOS

Quoting ?? <sunyang198498 at gmail.com>:

> Dear pw users:
>
> Hi, this is Yang. I am bothered by such a problem, described by the
> following.
>
> I have a system with twelve atoms contained in a unit cell:
>
> ATOMIC_POSITIONS {angstrom}
> C        0.000000000   0.000000000   0.000000000
> H        0.916999000  -0.568453000   0.000000000
> H       -0.916999000  -0.568453000   0.000000000
> C        0.000000000   0.726399000   1.339974000
> F        1.099429000   1.502392000   1.339974000
> F       -1.099429000   1.502392000   1.339974000
> C        0.000000000  -0.075546600   2.636186000
> H        0.916999000  -0.643999600   2.636186000
> H       -0.916999000  -0.643999600   2.636186000
> C        0.000000000   0.726399000   3.933239800
> F        1.099429000   1.502392000   3.933239800
> F       -1.099429000   1.502392000   3.933239800
>
> and the crystal parameter I chose is the base-centered orthorhombic, ie,
>
>  ibrav = 9,
>  celldm(1) = 16.11
>  celldm(2) = 0.57451
>  celldm(3) = 0.604393
>
> and I do a cell-variable relaxation. The following is the input file.
>
>  &control
>     calculation='vc-relax'
>     restart_mode='from_scratch',
>     pseudo_dir = '\$PSEUDO_DIR/',
>     outdir='\$TMP_DIR/'
>     prefix='PVDF'
>     etot_conv_thr=1.0e-6
>     forc_conv_thr=1.0e-5
>     nstep=100
>
> /
>  &system
>     ibrav = 9,
>     celldm(1) = 16.22
>     celldm(2) = 0.57226
>     celldm(3) = 0.59674
>
>
>     nat = 12
>     ntyp = 3
>     nspin = 1
>     ecutwfc = 25.0,
>     ecutrho = 500.0,
>     occupations='smearing',
>     degauss= 0.01
>  /
>  &electrons
>     conv_thr = 1.0e-8
>     mixing_beta = 0.7
>  /
>  &ions
>  /
>  &cell
>     cell_dynamics='damp-pr',
>  /
> ATOMIC_SPECIES
>  C 12.00 C.pbe-rrkjus.UPF
>  H 1.00  H.pbe-rrkjus.UPF
>  F 19.00 F.pbe-n-van.UPF
>
> ATOMIC_POSITIONS {angstrom}
> C        0.000000000   0.000000000   0.000000000
> H        0.916999000  -0.568453000   0.000000000
> H       -0.916999000  -0.568453000   0.000000000
> C        0.000000000   0.726399000   1.339974000
> F        1.099429000   1.502392000   1.339974000
> F       -1.099429000   1.502392000   1.339974000
> C        0.000000000  -0.075546600   2.636186000
> H        0.916999000  -0.643999600   2.636186000
> H       -0.916999000  -0.643999600   2.636186000
> C        0.000000000   0.726399000   3.933239800
> F        1.099429000   1.502392000   3.933239800
> F       -1.099429000   1.502392000   3.933239800
>
> K_POINTS (automatic)
>  4 8 8 0 0 0
>
>
> However, the relaxation result has different symmetry as far as I am
>  concerned.
> The cell parameter turns out to be the following:
>
> CELL_PARAMETERS (alat)
>    0.496579626   0.283698921  -0.001038829
>   -0.496579626   0.283698921  -0.001038829
>    0.000000000  -0.002166536   0.600253523
>
> ATOMIC_POSITIONS (angstrom)
> C        0.000000000  -0.119443593   0.057434432
> H        0.886464714  -0.767241790   0.054456517
> H       -0.886464714  -0.767241790   0.054456517
> C        0.000000000   0.694343614   1.348971837
> F        1.113214016   1.554765512   1.352609452
> F       -1.113214016   1.554765512   1.352609452
> C        0.000000000  -0.129168214   2.634282269
> H        0.885856007  -0.777144668   2.632570428
> H       -0.885856007  -0.777144668   2.632570428
> C        0.000000000   0.686823893   3.924824395
> F        1.113154270   1.547226828   3.928069069
> F       -1.113154270   1.547226828   3.928069069
>
> By looking at the CELL_PARAMETER, we see that the crystal loose the
> symmetry of
> orthorhombic but simply triclinic.
>
> Can we do VC-relax by keeping the previous symmetry?
>
> Thank you for any suggestion. :)
>
>
>
> Yang Sun
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