# [Pw_forum] Symmetry problem

Sun Mar 22 02:58:46 CET 2009

```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

```