[Pw_forum] vc-relax of R-3c structure does not converge

[Giuseppe Mattioli]

Dear Fermin
Sorry for the late answer to your last question, but yes! You must use the {crystal} option in this case.
Regards
Giuseppe

On Tuesday, June 02, 2015 11:34:53 AM lung Fermin wrote:
> Thanks Giuseppe for the advises.
> 
> I had a closer look at the {alat} and {crystal} option for the atomic
> position input. For the {crystal} option, the basis vectors are the
> primitive lattice vectors (a_i), i.e. x a_1 + y a_2 +z a_3. For the {alat}
> option, the basis are the unit vectors in the Cartesian coordinates
> (i^{\hat}), i.e. x(a*x^{\hat})+y(b*y^{\hat})+z(c*z^{\hat}), where a, b, c
> are the lattice constants. Is my understanding correct?
> 
> For the current case, in which I obtain the atomic positions from the
> Wyckoff positions in the rhombohedral axes, I should use the {crystal}
> option, right?
> 
> Regards,
> Fermin
> 
> 2015-06-01 15:55 GMT+08:00 Giuseppe Mattioli <giuseppe.mattioli at ism.cnr.it>:
> > Dear Fermin
> > You are using ultrasoft pseudopotential (I hope that you are aware of
> > the difference between norm-conserving and ultrasoft...). The ecutrho
> > variable does not appear in your input, so you are using the default
> > ecutwfc*4 value which is suitable for NC but not for US. Reasonably
> > safe values for your pseudopotentials should be ecutwfc=40.0 and
> > ecutrho =320.0~480.0, but always check! In difficult cases (which
> > yours should not be) you can lower mixing_beta down to 0.05 0r 0.02,
> > or also carefully try to use the scf_must_converge=.false. setup.
> > Remember also that error on stress is one order of magnitude higher
> > than error on forces, and that it is therefore safe to start a
> > vc-relax calculation with at least conv_thr=1.0D-7. *BUT* your
> > calculation does not converge because *you did not check* the
> > structure... If you try to open your input file with xcrysden you will
> > touch it with your hands! A hint: do you know the difference between
> > {alat} and {crystal} coordinates?
> > HTH
> > Giuseppe
> > 
> > Giuseppe Mattioli
> > ISM-CNR
> > Italy
> > 
> > lung Fermin <ferminlung at gmail.com> ha scritto:
> > > Dear QE community,
> > > 
> > > I want to calculate the phonon diseprsion relation of PbTiO3 in the
> > > strucutre of R-3c spacegroup. First I did the vc-relax using the
> > > experimental lattice parameters. However, the scf calculation fails to
> > > converge within 100 iteration and the job exits with error. I have tried
> > > changing the mixing_beta to 0.3 and mixing_ndim to 10, but still fails. I
> > > am a newbie to QE. May someone please give me some advices on this issue?
> > > 
> > > INPUT file:
> > >  &control
> > >  
> > >     calculation='vc-relax',
> > >     restart_mode='from_scratch',
> > >     prefix='PbTiO3',
> > >     outdir = './tmp/',
> > >     pseudo_dir = '$PSD_DIR',
> > >     max_seconds = 4.1D+6
> > >  
> > >  /
> > >  &system
> > >  
> > >     nat = 10
> > >     ntyp = 3
> > >     ibrav = 5
> > >     celldm(1) = 10.037,
> > >     celldm(4) = 0.53,
> > >     ecutwfc =60
> > >  
> > >  /
> > >  &electrons
> > >  
> > >    mixing_beta=0.3D0,
> > >    mixing_ndim=10
> > >  
> > >  /
> > >  &ions
> > >  /
> > > 
> > > &cell
> > > 
> > >  /
> > > 
> > > ATOMIC_SPECIES
> > > Pb  207.2  Pb.pbe-spn-rrkjus_psl.1.0.0.UPF
> > > Ti  47.867  Ti.pbe-spn-rrkjus_psl.1.0.0.UPF
> > > O  15.999  O.pbe-n-rrkjus_psl.1.0.0.UPF
> > > ATOMIC_POSITIONS {alat}
> > > Pb  0.25 0.25 0.25
> > > Pb  0.75 0.75 0.75
> > > Ti  0.00 0.00 0.00
> > > Ti  0.50 0.50 0.50
> > > O  0.62351014  0.87648986
> > > 0.25000000
> > > O  0.37648986  0.12351014  0.75000000
> > > O  0.87648986  0.25000000  0.62351014
> > > O  0.12351014  0.75000000  0.37648986
> > > O  0.25000000  0.62351014  0.87648986
> > > O  0.75000000  0.37648986  0.12351014
> > > K_POINTS AUTOMATIC
> > > 15 15 15 0 0 0
> > > 
> > > Thanks in advance.
> > > Fermin
> > 
> > --
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> > 
> >     Giuseppe Mattioli
> >     CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
> >     v. Salaria Km 29,300 - C.P. 10
> >     I 00015 - Monterotondo Stazione (RM)
> >     Tel + 39 06 90672836 - Fax +39 06 90672316
> >     E-mail: <giuseppe.mattioli at ism.cnr.it>
> > 
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********************************************************
- Article premier - Les hommes naissent et demeurent
libres et ègaux en droits. Les distinctions sociales
ne peuvent être fondèes que sur l'utilitè commune
- Article 2 - Le but de toute association politique
est la conservation des droits naturels et 
imprescriptibles de l'homme. Ces droits sont la libertè,
la propriètè, la sùretè et la rèsistance à l'oppression.
********************************************************

   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
   v. Salaria Km 29,300 - C.P. 10                
   I 00015 - Monterotondo Stazione (RM), Italy    
   Tel + 39 06 90672836 - Fax +39 06 90672316    
   E-mail: <giuseppe.mattioli at ism.cnr.it>
   http://www.ism.cnr.it/english/staff/mattiolig
   ResearcherID: F-6308-2012