[Pw_forum] Problem in carrying out Phonon calculations

[saswata halder]

Dear All,

I am a beginner in quantum espresso. I am trying to perform phonon
calculation for SrTiO3. I am able to perform the calculations but I am
not able to reproduce the results for SrTiO3 which are already
published in PHYSICAL REVIEW B 83, 134108 (2011).

http://journals.aps.org/prb/pdf/10.1103/PhysRevB.83.134108

The input file for relax is given below.

 &CONTROL
    calculation='relax',
    restart_mode='restart',
    prefix='ST'
    pseudo_dir = '/home/quantum/quantum/
potentials/',
    outdir='./'
    tstress=.true.
    tprnfor=.true.
    wf_collect=.true.
    nstep=1000
    etot_conv_thr=1.0D-8
    forc_conv_thr=1.0D-6
    disk_io='high'
 /
 &SYSTEM
    ibrav = 1, celldm(1) =7.38, nat = 5, ntyp= 3,
    ecutwfc = 40.0, ecutrho=600, occupations='fixed',
    smearing='gaussian', degauss=0.01
 /&ELECTRONS
    diagonalization='cg',
    mixing_mode='plain',
    mixing_beta = 0.7D0
    conv_thr =  1.0D-9
    electron_maxstep=1000
 /
&ions
/
ATOMIC_SPECIES
 Sr  87.620  Sr.pbe-mt_fhi.UPF
 Ti  47.867  Ti.pbe-mt_fhi.UPF
 O   15.999  O.pbe-mt_fhi.UPF
ATOMIC_POSITIONS crystal
 Sr 0.00 0.00 0.00 0 0 0
 Ti 0.50 0.50 0.50 0 0 0
 O  0.50 0.50 0.00 0 0 0
 O  0.00 0.50 0.50 0 0 0
 O  0.50 0.00 0.50 0 0 0
K_POINTS automatic
  6 6 6   0 0 0

The input file for phonon is given below:

phonons of ST at Gamma
 &inputph
  tr2_ph=1.0d-14,
  prefix='ST',
  epsil=.true.,
  amass(1)=87.620,
  amass(2)=47.867,
  amass(3)=15.999,
  outdir='./'
  fildyn='ST.dynG',
 /
0.0 0.0 0.0
The frequencies obtained are given below:

     omega(  1 -  3) =         35.5  [cm-1]   -->  T_1u G_15  G_4- I
     omega(  4 -  6) =        151.8  [cm-1]   --> T_1u G_15  G_4- I
     omega(  7 -  9) =        187.0  [cm-1]   --> T_1u G_15  G_4- I
     omega( 10 - 12) =        213.6  [cm-1]   --> T_2u G_25  G_5-
     omega( 13 - 15) =        600.7  [cm-1]   --> T_1u G_15  G_4- I

I also carried out phonon calculations using the volume cell
relaxation. the input files for both vc-relax and phonon are:

 &CONTROL
    calculation='vc-relax',
    restart_mode='restart',
    prefix='ST'
    pseudo_dir = '/home/quantum/quantum/
potentials/',
    outdir='./'
    tstress=.true.
    tprnfor=.true.
    verbosity='high'
    wf_collect=.true.
    nstep=1000
    etot_conv_thr=1.0D-8
    forc_conv_thr=1.0D-6
    disk_io='high'
 /
 &SYSTEM
ecutwfc = 40.0, ecutrho=500, occupations='fixed',
    smearing='gaussian', degauss=0.01
 /
 &ELECTRONS
    diagonalization='cg',
    mixing_mode='plain',
    mixing_beta = 0.7D0
    conv_thr =  1.0D-9
    electron_maxstep=1000
 /
&IONS
    ion_dynamics='bfgs'
/
&CELL
    cell_dynamics='bfgs',
    press=2.0,
    press_conv_thr=0.5,
/
ATOMIC_SPECIES
 Sr  87.620  Sr.pbe-mt_fhi.UPF
 Ti  47.867  Ti.pbe-mt_fhi.UPF
 O   15.999  O.pbe-mt_fhi.UPF
ATOMIC_POSITIONS crystal
 Sr 0.00 0.00 0.00 0 0 0
 Ti 0.50 0.50 0.50 0 0 0
 O  0.50 0.50 0.00 0 0 0
 O  0.00 0.50 0.50 0 0 0
 O  0.50 0.00 0.50 0 0 0
K_POINTS automatic
  6 6 6   1 1 1

phonons of ST at Gamma by anup
 &inputph
  tr2_ph=1.0d-14,
  prefix='ST',
  epsil=.true.,
  amass(1)=87.620,
  amass(2)=47.867,
  amass(3)=15.999,
  outdir='./'
  fildyn='ST.dynG',
 /
0.0 0.0 0.0

the output for phonon frequencies as obtained is given below:


     omega(  1 -  3) =       -628.3  [cm-1]   --> T_2u G_25  G_5-
     omega(  4 -  6) =       -615.5  [cm-1]   --> T_1u G_15  G_4- I
     omega(  7 -  9) =       -375.6  [cm-1]   --> T_1u G_15  G_4- I
     omega( 10 - 12) =        190.2  [cm-1]   --> T_1u G_15  G_4- I
     omega( 13 - 15) =        811.8  [cm-1]   --> T_1u G_15  G_4- I

I have carried out both the relax and volume cell relaxations and have
obtained two different results in the phonon mode calculations. Both
the results are not matching with reported results for SrTiO3.
Could you please suggest as to which of the methods is applicable to
SrTiO3 system so as to provide me with more accurate results.

Thanking you,

With Regards.

Saswata Halder.