[Pw_forum] ecut dependence of symmetry operations(?)

[Stefano Baroni]

Madhura: this has been discussed zilliions of times in this forum, and has to do with the way symmetry operations are found out by the code. When the symmetry group of the system is non-symmorphic, only those fractional translations that are commensurate with the FFT grid are actually found. This may sound a bit cryptic, and I leave it as such on purpose. You will find much more about this issue by browsing the archives of this mailing list. Hope this helps - SB

On Jan 5, 2010, at 12:24 PM, Madhura Marathe wrote:

> Dear all,
> 
> While performing convergence calculations w.r.t energy cut-off for a bulk
> system, I found that the number of symmetry operations detected by a code
> is different depending on the value of ecut. This is very surprising
> because for the same system, the symmetries should not depend on the ecut
> value.
> 
> For Ecut = 20, 25, 40, 50 and 55 Ry, the output says -
> "24 Sym.Ops. (with inversion)";
> whereas for ecut = 30, 35, 45 and 60 Ry, the output is -
> "12 Sym.Ops. (no inversion)"
> 
> I have performed the same calculations using versions 4.0.4 and 4.1 with
> the same results.
> The input file is as follows -
> *****
> &control
>    calculation = 'scf'
>    verbosity = 'high'
>    restart_mode = 'from_scratch',
>    prefix = 'RuGGA',
>    tstress=.true.
>    tprnfor=.true.
>    pseudo_dir = '/home/madhura-data/pseudo/',
>    outdir = '/home/madhura-data/tmp/'
> /
> &system
>    ibrav = 4, celldm(1) = 5.1831, celldm(3) = 1.584,
>    nat = 2, ntyp = 1,
>    ecutwfc = 20, ecutrho = 160,                     # ecutrho = 8*ecutwfc
>    occupations='smearing', smearing='mp',degauss=0.05
>    nbnd = 20
> /
> &electrons
>    diagonalization = 'david', mixing_mode = 'plain',
>    mixing_beta = 0.7, conv_thr = 1.0d-12
> /
> ATOMIC_SPECIES
> Ru 101.07 Ru.pbe-n-van.UPF
> ATOMIC_POSITIONS {crystal}
> Ru 0.00       0.00       0.00
> Ru 0.66666667 0.33333333 0.50
> K_POINTS automatic
> 8 8 8 0 0 0
> *****
> 
> Can anybody explain what is happening? Or do I need to file a bug report?
> Because except for this problem, energies and stress values seem to be
> converging.
> 
> Thanks and regards,
> Madhura.
> 
> -- 
> Madhura Marathe,
> PhD student, TSU,
> JNCASR, Bangalore.
> India.
> Phone No: +91-80-22082835
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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