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

[Thomas Brumme]

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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-- 
Dr. rer. nat. Thomas Brumme
Max Planck Institute for the Structure and Dynamics of Matter
Luruper Chaussee 149
22761 Hamburg

Tel:  +49 (0)40 8998 6557

email: Thomas.Brumme at mpsd.mpg.de