[Pw_forum] Strange behavior of phonon calculation

[Srijan Kumar Saha]

   Dear pwscf users,

   My  calculated gamma phonon frequency for Si (spin-less)  is
    = 510 cm-1    When  I switch on the spin  tag to specify the spin
polarized
   calculation for   the same sample, then I got   corresponding
   gamma phonon frequency = 214 cm-1
   while all the other input parameters are exactly same.

   Paolo wrote:
   please provide a job demostrating such behaviour

   Dear Paolo,
   I'm also sending the following input file what is demostrating such
behaviour
   by attachment.

 **************************************************************************************
   #!/bin/sh
 executable_dir=./
 tmp_dir=./tmp
 pseudo_dir=./pseudo
cat > si.scf.in << EOF
 &control
    calculation='scf',
    restart_mode='from_scratch',
    prefix='si'
    pseudo_dir = '$pseudo_dir/',
    outdir='$tmp_dir/'
 /
 &system
    ibrav = 2, celldm(1) =10.20, nat=  2, ntyp= 1,
    ecutwfc = 18.0, nelec  =   8.00, nbnd =10
    nspin=2, tot_magnetization=0,
    occupations='smearing', smearing='fd', degauss=0.005,
/
 &electrons
    mixing_beta = 0.7
    conv_thr =  1.0d-8
 /
ATOMIC_SPECIES
 Si  28.086  Si.vbc.UPF
ATOMIC_POSITIONS
 Si 0.00 0.00 0.00
 Si 0.25 0.25 0.25
K_POINTS
  10
   0.1250000  0.1250000  0.1250000   1.00
   0.1250000  0.1250000  0.3750000   3.00
   0.1250000  0.1250000  0.6250000   3.00
   0.1250000  0.1250000  0.8750000   3.00
   0.1250000  0.3750000  0.3750000   3.00
   0.1250000  0.3750000  0.6250000   6.00
   0.1250000  0.3750000  0.8750000   6.00
   0.1250000  0.6250000  0.6250000   3.00
   0.3750000  0.3750000  0.3750000   1.00
   0.3750000  0.3750000  0.6250000   3.00
EOF
$ECHO "  running the scf calculation...\c"
$executable_dir/pw.x  <  si.scf.in > si.scf.out
$ECHO " done"

# phonon calculation at Gamma
cat > si.phG.in << EOF
phonons of Si at Gamma
 &inputph
  tr2_ph=1.0d-14,
  prefix='si',
  amass(1)=28.08,
  outdir='$tmp_dir/',
  fildyn='si.dynG',
 /
0.0 0.0 0.0
EOF
$ECHO "  running the phonon calculation at Gamma...\c"
$executable_dir/ph.x < si.phG.in > si.phG.out
$ECHO " done"

********************************************************************************
  To run the spin less calculation I just remove tot_magnetization tag
  and reset nspin=1 and I get correct Gamma point
  phonon frequency (510 cm-1) whereas above input file for spin calculation
  will give us a completely wrong result (214 cm-1).

  Looking forward to your valuable suggestions.

  With best regards,
  Srijan
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