[Pw_forum] question for phonon calculation

[Eyvaz Isaev]

Hi,

> But I got the minus value at the lowest three branch.

> how to avoid the minus values 

Here you are incorrect, you got imaginary frequences.

5 years ago I did such kind calculations for MgB2  using my own PBE US pseudopotentials and met no problem. As far as know phonons are sensitive to parameters used to generate. Most likely, you can ignore these frequences, as applying ASR will correct them.  As concerns how to apply ASR, just use matdyn.x in QE/PH or QE/bin (with keyword asr='crystal') file to generate phonon dispersion relations with correct acoustic modes at Gamma point. See example file /examples/example06.

>   tr2_ph=1.0d-25
Looks extremly low. As 1.d-10 is default, the use of d-12 or d-14 should be sufficient.

Bests,
Eyvaz.

-------------------------------------------------------------------
Prof. Eyvaz Isaev, 
Theoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia, 
Department of Physics, Chemistry, and Biology (IFM), Linkoping University, Sweden 
Condensed Matter Theory Group, Uppsala University, Sweden 
Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com


--- On Tue, 4/21/09, 潘登 <panda.deng.pan at gmail.com> wrote:

> From: 潘登 <panda.deng.pan at gmail.com>
> Subject: [Pw_forum] question for phonon calculation
> To: "pw forum" <pw_forum at pwscf.org>
> Date: Tuesday, April 21, 2009, 10:23 PM
> Dear All Users,
> 
> I was going to calculating phonon frequency of MgB2 at
> Gamma point.But I got
> the minus value at the lowest three branch.
> 
>      q = (    0.000000000   0.000000000   0.000000000 )
> 
> 
> **************************************************************************
>      omega( 1) =      -1.333147 [THz] =     -44.469304
> [cm-1]
>      omega( 2) =      -1.333147 [THz] =     -44.469304
> [cm-1]
>      omega( 3) =      -1.003834 [THz] =     -33.484515
> [cm-1]
>      omega( 4) =       9.706117 [THz] =     323.763351
> [cm-1]
>      omega( 5) =       9.706117 [THz] =     323.763351
> [cm-1]
>      omega( 6) =      11.666010 [THz] =     389.138778
> [cm-1]
>      omega( 7) =      15.807885 [THz] =     527.297796
> [cm-1]
>      omega( 8) =      15.807885 [THz] =     527.297796
> [cm-1]
>      omega( 9) =      20.623897 [THz] =     687.943739
> [cm-1]
> 
> **************************************************************************
> 
> I had already relax the lattice.And below is my inputs for
> scf and phonon
> calculation.Any suggest and comments on how to avoid the
> minus values was
> appreciated.
> 
> scf.in
>  &control
>      calculation = 'scf'
>      restart_mode='from_scratch'
>      prefix='MgB2',
>      pseudo_dir =
> '//Quantum-Espresso/espresso-4.0.3/pseudo/',
>      outdir='/tmp/'
>  /
>  &system
>      ibrav=4,
>      celldm(1)=5.8398,
>      celldm(3)=1.1420,
>      nat=3,
>      ntyp=2,
>      ecutwfc = 45.0,
>      ecutrho = 450.0,
>      occupations= 'smearing',
>      degauss= 0.03,
>      smearing= 'cold',
>  /
>  &electrons
>      mixing_beta = 0.7
>      conv_thr =  1.0d-12
>  /
> ATOMIC_SPECIES
>  Mg 24.3 Mg.pw91-np-van.UPF
>  B  10.8 B.pw91-n-van.UPF
> ATOMIC_POSITIONS
> Mg    0    0       0
> B     0    0.57735 0.571
> B     0.5  0.28868 0.571
> K_POINTS {automatic}
>  32 32 32 0 0 0
> 
> ph.in
> 
>  MgB2
> &inputph
>   amass(1)= 24.3,
>   amass(2)= 10.8,
>   prefix='MgB2'
>   tr2_ph=1.0d-25,
>   alpha_mix(1)=0.5,
>   iverbosity=1,
>   outdir='//tmp/',
>  /
> 0 0 0
> 
> I also wonder how to apply 'asr' to my
> calculation.I could not find any
> option for ph.x.
> 
> Have a nice day!
> 
> Deng Pan
> Nanjing University
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum