[Pw_forum] "q mesh breaks symmetry" error even with q mesh being the same as k mesh

[stefano de gironcoli]

it does not look so much longer than the other to me ...
Anyhow the point is that a grid in order to respect the symmetry must 
transform into itself under symmetry operations and yours does not 
satisfy this condition for reflexion x -> -x, z -> -z and rotations of 
pi around z ...

unless nk1=nk3

for the fft grid ... I would try to keep nr1=nr3 for the same reason. 
actually my suggestion was based on a real-space argument but I'm pretty 
sure it is valid in reciprocal space too.

stefano

On 09/08/2016 11:34, Thomas Brumme wrote:
> Thanks for the suggestion Stefano!
>
> Yet, lattice vector 3 is in my cell the largest and correspondingly nk3
> must be larger than nk1=nk2, or? That's why I choose 10 10 6 (even if I
> might have to increase nk3 slightly).
>
> Or do I miss something? I meanwhile also played with the cutoff of the
> wave functions (i.e. finding also an fft mesh which can be used with the
> symmetries which include a translation) but even if the fft mesh is
> 96x96x120 the symmetries which include the translation are neglected.
> I'll try with 96x96x128...
>
> Thomas
>
> On 08/09/2016 02:59 AM, stefano de gironcoli wrote:
>> I think that given your Bravais lattice nk3 must be equal to nk1
>> so
>> 10 nk2 10 0 0 0
>> or
>> 6 nk2  6 0 0 0
>> should be fine
>>
>> 10 nk2 6 0 0 0
>> is not
>>
>> the automatic unfolding of a grid generate additional points but this
>> does not mean they form a regular grid. think about what happens for an
>> hexagonal lattice with a shifted grid.
>>
>> stefano
>>
>> On 08/08/2016 17:51, Thomas Brumme wrote:
>>> Dear all,
>>>
>>> I want to calculate the phonon frequencies on a regular q mesh specified
>>> by using
>>> ldisp=.true. and the nq1, nq2, nq3 variables. Yet, I always get the error
>>>
>>> q-mesh breaks symmetry
>>>
>>> even if the grid has the same dimensions as the k mesh in the PWscf
>>> calculation.
>>> This is weird as the q mesh is exactly the same as the k mesh, which was
>>> created
>>> by applying the symmetries to a regular grid. The system is LCO and the
>>> input for
>>> PWscf (in the reduced cell) is:
>>>
>>> &control
>>>      calculation   = 'scf',
>>>      restart_mode  = 'from_scratch',
>>>      prefix        = 'LCO',
>>>      pseudo_dir    = './',
>>>      outdir        = './tmp/',
>>>      nstep         = 300,
>>>      wf_collect    = .true.,
>>> /
>>> &system
>>>      ibrav       = 0,
>>>      nat         = 14,
>>>      ntyp        = 3,
>>>      ecutwfc     = 200,
>>>      occupations = 'smearing',
>>>      smearing    = 'gauss',
>>>      degauss     = 0.01,
>>>      nspin       = 2,
>>>      starting_magnetization(3) = 0.02,
>>>      nr1 = 128,
>>>      nr2 = 128,
>>>      nr3 = 128,
>>> /
>>> &electrons
>>>      electron_maxstep = 250,
>>>      diagonalization  = 'cg',
>>>      conv_thr         = 1.0d-10,
>>> /
>>> CELL_PARAMETERS (angstrom)
>>>        5.261112503   0.000000000   0.000000000
>>>        0.000000000   5.330947322   0.000000000
>>>       -2.630556252  -0.000000000   6.548827231
>>> ATOMIC_SPECIES
>>>      La   138.90547   La1.UPF
>>>      O    15.999   O.pz-n-mt.UPF
>>>      Cu  63.546   Cu1.UPF
>>> K_POINTS automatic
>>>      10 10 6 0 0 0
>>> ATOMIC_POSITIONS (crystal)
>>> ...
>>>
>>> I defined the FFT grid by hand since then the code does not drop the
>>> symmetries
>>> including fractional translations... Could this be the problem? Or that
>>> I did choose
>>> a FFT grid which has the same number of points in x/y/z directions? Or
>>> could this
>>> be due to a similar problem" as in hexagonal crystals where shifting the
>>> k mesh
>>> away from Gamma is a bad idea? Or is there an obvious error in my input
>>> for ph.x:
>>>
>>>      &inputph
>>>       tr2_ph   = 1.0d-18,
>>>       prefix   = 'LCO',
>>>       amass(1) = 138.90547,
>>>       amass(2) = 15.999,
>>>       amass(3) = 63.546,
>>>       outdir   = './tmp/',
>>>       fildyn   = 'LCO.dynG',
>>>       ldisp=.true.,
>>>       nq1=10, nq2=10, nq3=6,
>>>       fildvscf = 'LCO.dvscf',
>>>       verbosity = 'high',
>>>      /
>>>
>>> I searched the archive however I couldn't find a solution to my problem.
>>> Thus, any
>>> help would be very much appreciated :)
>>>
>>> Regards
>>>
>>> Thomas
>>>
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