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

[lung Fermin]

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
>
>
> --
> ********************************************************
> - 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)
>     Tel + 39 06 90672836 - Fax +39 06 90672316
>     E-mail: <giuseppe.mattioli at ism.cnr.it>
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20150602/9cd530c2/attachment.html>