[Pw_forum] phonon at Gamma point

[Eyvaz Isaev]

Hi,

>From Users Guide (it was discussed before):

pw.x determines first the symmetry operations
(rotations) of the Bravais lattice; then checks which
of these are symmetry operations of the system
(including if needed fractional translations).
This is done by rotating (and translating if needed)
the atoms in the unit cell and verifying if the
rotated unit cell coincides with the original one.

If a symmetry operation contains a fractional
translation that is incompatible with the FFT grid,
it is discarded in order to prevent problems with
symmetrization. Typical fractional translations are
1/2 or 1/3 of a lattice vector. If the FFT grid
dimension along that direction is not divisible
respectively by 2 or by 3, the symmetry operation will
not transform the FFT grid into itself.

Bests,
Eyvaz.

--- lan haiping <lanhaiping at gmail.com> wrote:

> Thank you  , Eyvaz.
>  I still have a confusion : why does the modified
> ecutrho  affect the
> system's symmetry ?
> 
> Regards
> 
> Hai-Ping
> 
> On 8/20/06, Eyvaz Isaev <eyvaz_isaev at yahoo.com>
> wrote:
> >
> > OK, let me hint more.
> >
> > Anatase has a Pearson symbol tI12 or space group
> > I4_1/amd. So, ibrav should be 7, but you have
> chosen a
> > monoclinic lattice, i.e. ibrav=14. The space group
> has
> > 16 symmetry elements, but your choice after
> correcting
> > ecutrho gives only 4 symmetry elements .
> >
> > Imagine  FCC lattice and its conventional basis
> > vectors now. According to you now we have to put
> > ibrav=4 or 5. But ibrav=2 for FCC lattice.
> >
> > In addition, I also checked your input file via
> > XCrysDen before I replied previously. Yes, there
> is no
> > problem with it, but a figure does not look like
> > anatase though  you used  nonzero ibrav. XcrysDen
> > usually replicates atomic positions  if 
> ibrav.neq.0.
> > For example, if you put ibrav=0  for  FCC cell and
> > specify conventional CELL_PARAMETERS you will find
>  a
> > rhombohedron, but using ibrav=2 and  default basis
> > vectors you find a cubic cell.
> >
> > Bests,
> > Eyvaz.
> >
> >
> > --- lan haiping <lanhaiping at gmail.com> wrote:
> >
> > > Dear Eyvaz,
> > > Thank you.
> > > I checked the strcuture again using XCRYSDEN,
> > > and found no problems related to the  lattices.
> > >
> > > Regards,
> > >
> > > Hai-Ping
> > >
> > >
> > >
> > > On 8/20/06, Eyvaz Isaev <eyvaz_isaev at yahoo.com>
> > > wrote:
> > > >
> > > > Hi,
> > > >
> > > > It seems your lattice choice is not correct.
> > > Please
> > > > have a look at
> > > >
> http://cst-www.nrl.navy.mil/lattice/struk/c5.html
> > > >
> > > > Bests,
> > > > Eyvaz.
> > > >
> > > > --- lan haiping <lanhaiping at gmail.com> wrote:
> > > >
> > > > > Dear All,
> > > > > I also have calculated gamma phonon
> frequencies
> > > of
> > > > > anatase TiO2 .
> > > > > I run ph.x caculation after relaxing the
> > > structure,
> > > > > but  9 of 18 frequencies
> > > > > are minus.
> > > > >
> > > > > Would you please give me some hints and
> help?
> > > > >
> > > > > input settings are below, thank you in
> advance !
> > > > >
> > > > > *phonons of TiO2 at Gamma
> > > > >  &inputph
> > > > >   tr2_ph=1.0d-14,
> > > > >   recover=.true.,
> > > > >   epsil=.true.,
> > > > >   prefix='tio2',
> > > > >   fildyn='TiO2-gamma.dyn',
> > > > >   outdir='/home/haiping/tmp/'
> > > > >  /
> > > > > 0.0 0.0 0.0*
> > > > > **
> > > > > *Relaxing calculation input :*
> > > > > *&CONTROL
> > > > >              title = 'TiO2' ,
> > > > >        calculation = 'relax',
> > > > >          verbosity = 'default',
> > > > >            tprnfor = .true.,
> > > > >             outdir = '/home/haiping/tmp/',
> > > > >             prefix = 'tio2',
> > > > >            disk_io = 'default',
> > > > >              nstep = 200,
> > > > >         pseudo_dir =
> > > > > '/home/haiping/espresso/pseudo/',
> > > > > /
> > > > > &SYSTEM
> > > > >                 ibrav= 14,
> > > > >
> > > > >
> celldm(1)=10.2856,celldm(2)=1.0,celldm(3)=1.0,
> > > > >
> > > > > celldm(4)=-0.759357,celldm(5)=-0.759357,
> > > > >                celldm(6)=0.518714
> > > > >                nat = 6 ,
> > > > >               ntyp = 2 ,
> > > > >               nbnd = 30 ,
> > > > >            ecutwfc = 30.0000000000,
> > > > >            ecutrho = 160.000,
> > > > >        occupations = 'fixed' ,
> > > > > /
> > > > >
> > > > >
> > > > > &ELECTRONS
> > > > >   electron_maxstep = 100,
> > > > >           conv_thr = 1.0e-07,
> > > > >        mixing_mode = 'plain' ,
> > > > >        startingpot = 'atomic' ,
> > > > >        startingwfc = 'atomic' ,
> > > > >        mixing_beta = 0.300000000,
> > > > >        mixing_ndim = 8,
> > > > >    diagonalization = 'david_overlap' ,
> > > > >   diago_david_ndim = 8,
> > > > > /
> > > > >
> > > > > &IONS
> > > > >              ion_dynamics = 'bfgs',
> > > > >              pot_extrapolation =
> 'first_order',
> > > > >              wfc_extrapolation =
> 'first_order',
> > > > >
> > > > > /
> > > > >
> > > > > ATOMIC_SPECIES
> > > > > O  15.999 O.pw91-van_ak.UPF
> > > > > Ti 47.867  Ti.pw91-nsp-van.UPF
> > > > > ATOMIC_POSITIONS crystal
> > > > > Ti    0.00000   0.00000   0.00000
> > > > > Ti    0.75000   0.25000   0.50000
> > > > > O     0.20800   0.20800  -0.00000
> > > > > O     0.95800   0.45800   0.50000
> > > > > O     0.54200   0.04200   0.50000
> > > > > O     0.79200   0.79200  -0.00000
> > > > > K_POINTS   automatic
> > > > > 6 6 7   0 0 0
> > > > >
> > > > > *
> > > > >
> > > > >
> > > > > On 8/19/06, Ezad Shojaee
> > > <ezadshojaee at hotmail.com>
> > > > > wrote:
> > > > > >
> > > > > > hi
> > > > > > i am trying to calculate phonon
> frequencies at
> > > > > gamma point of
> > > > > > TiO2(anatase)
> > > > > > and i have relaxed the structure enough (i
> > > think
> > > > > )but performing PH.Xafter
> > > > > > scf, 2of 18 frequencies  are minus!
> > > > > > i want to know that what technical problem
> do
> > > i
> > > > > have? does anyone
> > > > > > calculate
> > > > > > these beore?
> > > > > > any suggestion will be appreciated
> > > > > >
> > > > > >
> > > > >
> > > >
> >
> > --- lan haiping <lanhaiping at gmail.com> wrote:
> >
> > > Dear Eyvaz,
> > > Thank you.
> > > I checked the strcuture again using XCRYSDEN,
> > > and found no problems related to the  lattices.
> > >
> > > Regards,
> > >
> 
=== message truncated ===


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