[Pw_forum] phonon calc. for fcc at X

Mon Dec 11 00:05:37 CET 2006

  Dear Stefano,
  Thank you, I will study the relevant part for pert. potential., do the suggested tests and modifications,
  and send the result back to close the case.
  But a fast point that I wanted to note that is,
  InN turns out to be a metal according to my calculations,
  without several corrections used, like GW or SIC.
  That was the reason of the k-mesh.
  Thank you for your help.

Emine Kucukbenli

Stefano de Gironcoli <degironc at sissa.it> wrote:

If I remember correctly there used to be a term in the phonon 
calculation in which the perturbing potential was written as DeltaV(r) = 
exp(+iqr)*\DeltaVperiodic(r) and \DeltaVperiodic was expanded up to
the charge density cutoff ... this term was therefore slightly different
for two q wavevectors that would otherwise be equivalent. This never
gave any noticeable difference in real calculations and I would be 
surprised if this would be the cause of your results but a way to check
this is to increase the charge density cutoff and see what happens as
in this case the problem should disappear when the potential description
become complete.

>From your input it looks to me that you are using the default value for 
the selfconsistent threshold (conv_thr=1.d-6 in the &electrons namelist).
This is usually fine for total energy which is variational, but already 
for structural relaxation it is often reduced so as to get more accurate 
forces.

Phonon calculations are even more sensitive to the quality of 
self-consistency of your unperturbed system. Try to reduce conv_thr to 
1.d-8 in the scf (and nscf) steps and see if this improves things.

InN is a (wide band gap) semiconductor: there is no real need to use a 
metallic setup which, by the way, is incompatible with the calculation
of dielectric constant and effective charges, that you will probably need
to get correct phonons frequencies at gamma.
I would expect that 2 or 10 Chadi-Cohen points should be enough for this 
system (automatic grids 2 2 2 1 1 1 and 4 4 4 1 1 1, respectively).

hope this helps,

stefano

On Sun, 10 Dec 2006, e kb wrote:

> Dear Stefano , QE users,
> Thank you for your interest,
>
> Here are the scf, nscf and ph.x input files respectively, for X(0,0,1).
> For X(0,1,1) the scf file is the same, in nscf input I just changed the phonon namelist to (0.00 1.00 1.00) so not to make the mail longer and unreadable, I am skipping it and just adding ph.X input for X(0,1,1).
>
> I should note that this was a part of test calculations for the new pseudo for Indium that I recently generated using the 'atomic' code. The cut off energy that I used for these calculations (34 Ryd) is chosen to be enough to represent correct lattice constants, but still the pseudo and the value of cut off energy needs further examination. That is why I would not be surprised to see any discrepiencies in the frequencies. But again, I could not understand the differences that occured for the same point.
> With a verbose output I checked the phonon polarization vectors,
> As far as I understand, the calculation is done for the same modes for the two points.
>
> I do not want to bother you with the whole output files,
> if this is a very simple problem that an experienced/careful user can notice from the inputs,
> otherwise I can attach the output files.
>
> Buona domenica,
> Emine Kucukbenli
> ------------
> &control
> calculation = 'scf'
> restart_mode='from_scratch',
> prefix='34_vib',
> tprnfor = .true.
> pseudo_dir = '$PSEUDO_DIR' ,
> etot_conv_thr = 1.0D-5
> /
> &system
> ibrav= 2,
> celldm(1)=9.593140000
> nat= 2,
> ntyp= 2,
> ecutwfc =34.0,
> ecutrho =272.0,
> occupations = 'smearing'
> smearing = 'gaussian'
> degauss = 0.004,
> /
> &electrons
> mixing_mode = 'plain'
> mixing_beta = 0.7
> /
> ATOMIC_SPECIES
> In 114.818 In_us_2.upf
> N 14.0067 N.pbe-van_ak.UPF
> ATOMIC_POSITIONS { crystal }
> In 0.00000000000 0.00000000000 0.0000000000000
> N 0.25000000000 0.25000000000 0.2500000000000
> K_POINTS { automatic }
> 8 8 8 0 0 0
> ------------
> &control
> calculation = 'phonon'
> restart_mode='from_scratch',
> prefix='34_vib',
> tprnfor = .true.
> pseudo_dir = '$PSEUDO_DIR' ,
> etot_conv_thr = 1.0D-5
> /
> &system
> ibrav= 2,
> celldm(1)=9.593140000
> nat= 2,
> ntyp= 2,
> ecutwfc =34.0,
> ecutrho =272.0,
> occupations = 'smearing'
> smearing = 'gaussian'
> degauss = 0.004,
> /
> &electrons
> mixing_mode = 'plain'
> mixing_beta = 0.7
> /
> &phonon
> xqq(1) = .00, xqq(2) = .00, xqq(3) = 1.00
> /
> ATOMIC_SPECIES
> In 114.818 In_us_2.upf
> N 14.0067 N.pbe-van_ak.UPF
> ATOMIC_POSITIONS { crystal }
> In 0.00000000000 0.00000000000 0.0000000000000
> N 0.25000000000 0.25000000000 0.2500000000000
> K_POINTS { automatic }
> 8 8 8 0 0 0
> -------------------------
>
> &inputph
> tr2_ph=1.0d-14,
> amass(1)=114.818,
> amass(2)=14.0067,
> prefix='34_vib',
> fildyn='34_vib.dyn',
> /
> 0.00 0.00 1.00
> --------------------------------
>
>
> &inputph
> tr2_ph=1.0d-14,
> amass(1)=114.818,
> amass(2)=14.0067,
> prefix='34_vib',
> fildyn='34_vib.dyn',
> /
> 0.00 1.00 1.00
> ----------------------------------
> X(0,0,1)
> omega( 1) = 3.163928 [THz] = 105.537987 [cm-1]
> omega( 2) = 3.163928 [THz] = 105.537987 [cm-1]
> omega( 3) = 6.030058 [THz] = 201.142433 [cm-1]
> omega( 4) = 14.775784 [THz] = 492.870383 [cm-1]
> omega( 5) = 14.775784 [THz] = 492.870383 [cm-1]
> omega( 6) = 16.372030 [THz] = 546.115769 [cm-1]
> X(0,1,1)
> omega( 1) = 2.153190 [THz] = 71.823161 [cm-1]
> omega( 2) = 2.153190 [THz] = 71.823161 [cm-1]
> omega( 3) = 6.239891 [THz] = 208.141736 [cm-1]
> omega( 4) = 14.879198 [THz] = 496.319913 [cm-1]
> omega( 5) = 14.879198 [THz] = 496.319913 [cm-1]
> omega( 6) = 16.354833 [THz] = 545.542147 [cm-1]
> ----------------------------------
>
> Stefano Baroni wrote:
> Dear Emin, dear all: all my apologies!
>
> Nicola was of course right, and I was WRONG! (011) IS indeed equivalent to (001), because the latter is equivalent to (-1,0,0) and the difference between (011) and (-100) is (111) which IS an FCC reciprocal-lattice vector. The coordinates of the K point are (3/4,3/4,0). With reference to the figure mentioned in the link below, K is the intersection among the planes x+y+z=3/2, z=0, and x=y, hence the above coordinates.
>
> To come to your original question. Are the symmetries/degeneracies of the results what you expect? Can you make the input/output of your calculation accessible to us?
>
>
> Stefano
>
> On Dec 10, 2006, at 2:46 PM, e kb wrote:
>
> Thank you for your answer Stefano,
> Of course this explains the different frequencies,
> and that also means that I misunderstand the kpoints given in example 5,
> about band structure for an fcc,
> and Dr. Marzari's explanation to another QE user,
> who was confused like me.
> http://www.democritos.it/pipermail/pw_forum/2006-June/004365.html
>
> Thank you again,
> Emine Kucukbenli
>
> Stefano Baroni wrote:
> Emine: (001) and (011) are *NOT* two "X" points in the FCC Brillouin zone. The difference between the second and the first (010) is *NOT* a reciprocal-space vector. If it was, X would also be, which you will agree it is not. As a matter of fact, (011) has its own nickname (it is called "K"). No surprise, therefore, that frequencies @K are different from frequencies @X. Are these arguments clear and convincing enough? Stefano
>
> On Dec 9, 2006, at 11:10 PM, e kb wrote:
>
> Dear QE users,
> I am performing phonon calculations for a zinc blende structure,
> I calculated the frequencies at high symmetry points, evaluating a single point at each time.
> So I followed the procedure of scf, nscf and ph.x calculation one by one for L(0.5 0.5 0.5) - Gamma - X(0,0,1) - X(0,1,1) points.
> In these calculations every parameter (else than q-vector in &phonon list in nscf calculation and phonon card for ph.x input of course) is kept fixed.
>
> Although I expect the frequencies at two X points - (0,0,1) and (0,1,1) - to be the same,
> the difference in TA modes' frequencies are around 30 cm-1,
> in the other ones, around 5 cm-1.
>
> I would like to attribute this to unconverged lattice parameters, cut off energies or thresholds etc. but what I do not understand is why the program treats these two points differently, although they should be equivalent. So I expect to see the same value for frequency at these points, even if it is wrong, highly negative, or sth else that it should not be.
>
> I will appreciate any answers.
> Thank you.
>
> Emine Kucukbenli
>
>
>
>
>
> ---------------------------------
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> ---
> Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste
> [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
>
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> ---
> Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste
> [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
>
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> Please, if possible, don't send me MS Word or PowerPoint attachments
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