[Pw_forum] ecut dependence of symmetry operations(?)

[Madhura Marathe]

 Dear Stefano,

 Thanks for your reply. I had not initially realized that discussions in
the forum about symmetry and FFT grid are directly related to its ecut
dependence. I browsed through forum after getting replies, and now have
got some idea about it.

 Sincerely,
 Madhura.


> 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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-- 
Madhura Marathe,
PhD student, TSU,
JNCASR, Bangalore.
India.
Phone No: +91-80-22082835