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

[Giuseppe Mattioli]

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)
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    E-mail: <giuseppe.mattioli at ism.cnr.it>