[Pw_forum] different frequencies within the same q star

[W. YU]

Dear pw users, 

I calculated the phonon mode for a simple cubic system
Cu3N at X point, the frequencies are as follows for
different q within the same q star. The difference is
relatively small,less than 0.6cm-1 for different q in
the same q star. In this case, can I safely say the
three q point give equivalent results? If so, I still
have no idea why for different q in the same star,
some of the frequencies are quite close to each other,
but some are different? For example, the first and the
seventh.       

     Number of q in the star =    3
     List of q in the star:
          1   0.000000000   0.000000000   0.500000000
          2   0.500000000   0.000000000   0.000000000
          3   0.000000000   0.500000000   0.000000000

     Diagonalizing the dynamical matrix

     q = (    0.000000000   0.000000000   0.500000000
) 


************************************************************
     omega( 1) =       1.701326 [THz] =      56.750488
[cm-1]
     omega( 2) =       1.701326 [THz] =      56.750488
[cm-1]
     omega( 3) =       2.518668 [THz] =      84.014282
[cm-1]
     omega( 4) =       3.048279 [THz] =     101.680323
[cm-1]
     omega( 5) =       3.437369 [THz] =     114.659034
[cm-1]
     omega( 6) =       3.437369 [THz] =     114.659034
[cm-1]
     omega( 7) =       4.451716 [THz] =     148.494263
[cm-1]
     omega( 8) =       4.451716 [THz] =     148.494263
[cm-1]
     omega( 9) =       9.721727 [THz] =     324.284046
[cm-1]
     omega(10) =      19.953855 [THz] =     665.593379
[cm-1]
     omega(11) =      20.913434 [THz] =     697.601696
[cm-1]
     omega(12) =      20.913434 [THz] =     697.601696
[cm-1]

************************************************************

     Diagonalizing the dynamical matrix

     q = (    0.000000000   0.500000000   0.000000000
) 


************************************************************
     omega( 1) =       1.700992 [THz] =      56.739356
[cm-1]
     omega( 2) =       1.700992 [THz] =      56.739356
[cm-1]
     omega( 3) =       2.518671 [THz] =      84.014380
[cm-1]
     omega( 4) =       3.048254 [THz] =     101.679485
[cm-1]
     omega( 5) =       3.437367 [THz] =     114.658972
[cm-1]
     omega( 6) =       3.437367 [THz] =     114.658972
[cm-1]
     omega( 7) =       4.435882 [THz] =     147.966089
[cm-1]
     omega( 8) =       4.435882 [THz] =     147.966089
[cm-1]
     omega( 9) =       9.721788 [THz] =     324.286106
[cm-1]
     omega(10) =      19.954086 [THz] =     665.601081
[cm-1]
     omega(11) =      20.913595 [THz] =     697.607081
[cm-1]
     omega(12) =      20.913595 [THz] =     697.607081
[cm-1]

************************************************************
     Diagonalizing the dynamical matrix

     q = (    0.500000000   0.000000000   0.000000000
) 


************************************************************
     omega( 1) =       1.719289 [THz] =      57.349695
[cm-1]
     omega( 2) =       1.719289 [THz] =      57.349695
[cm-1]
     omega( 3) =       2.518665 [THz] =      84.014176
[cm-1]
     omega( 4) =       3.048277 [THz] =     101.680259
[cm-1]
     omega( 5) =       3.437370 [THz] =     114.659100
[cm-1]
     omega( 6) =       3.437370 [THz] =     114.659100
[cm-1]
     omega( 7) =       4.435697 [THz] =     147.959901
[cm-1]
     omega( 8) =       4.435697 [THz] =     147.959901
[cm-1]
     omega( 9) =       9.721777 [THz] =     324.285722
[cm-1]
     omega(10) =      19.953888 [THz] =     665.594483
[cm-1]
     omega(11) =      20.912939 [THz] =     697.585200
[cm-1]
     omega(12) =      20.912939 [THz] =     697.585200
[cm-1]

************************************************************

One of the input file is as follows:

# self-consistent calculation
cat > cu3n.scf.in << EOF
 &control
    calculation='scf'
    restart_mode='from_scratch',
    prefix='cu3n',
    pseudo_dir = '$PSEUDO_DIR/',
    outdir='$TMP_DIR/'
 /
&system    
    ibrav = 1, celldm(1) = 7.090, nat = 4, ntyp = 2,
    ecutwfc = 75.0 ,ecutrho=450.0, 
/
 &electrons
    conv_thr =  1.0d-8
    mixing_beta = 0.7
 /
ATOMIC_SPECIES
  N   14.00674   N.pz-rrkjus.UPF 
 Cu  63.54600  Cu.pz-d-rrkjus.UPF 
ATOMIC_POSITIONS alat
  N     0.000000000    0.000000000    0.000000000     
 Cu     0.500000000    0.000000000    0.000000000     
 Cu     0.000000000    0.500000000    0.000000000     
 Cu     0.000000000    0.000000000    0.500000000     
K_POINTS automatic
4 4 4 1 1 1
EOF
echo "  running the scf calculation...\c"
$PW_ROOT/bin/pw.x < cu3n.scf.in > cu3n.scf.out
echo " done"

# non self-consistent calculation 

cat > cu3n.nscf.in << EOF
 &control
    calculation='phonon'
    restart_mode='from_scratch',
    prefix='cu3n',
    pseudo_dir = '$PSEUDO_DIR/',
    outdir='$TMP_DIR/'
 /
 &system    
    ibrav = 1, celldm(1) = 7.090, nat = 4, ntyp = 2,
    ecutwfc = 75.0 ,ecutrho=450.0, 
 /
 &electrons
    conv_thr = 1.0d-8 ,
 /
 &phonon
    xqq(1) = 0.5, xqq(2) = 0.0, xqq(3) = 0.0
 /
ATOMIC_SPECIES
  N   14.00674   N.pz-rrkjus.UPF 
 Cu  63.54600  Cu.pz-d-rrkjus.UPF 
ATOMIC_POSITIONS alat
  N     0.000000000    0.000000000    0.000000000     
 Cu     0.500000000    0.000000000    0.000000000     
 Cu     0.000000000    0.500000000    0.000000000     
 Cu     0.000000000    0.000000000    0.500000000     
K_POINTS automatic
4 4 4 1 1 1
EOF

echo "  running the nscf calculation at
q=0.5,0.0,0.0...\c"
$PW_ROOT/bin/pw.x < cu3n.nscf.in >> cu3n.nscf.out
echo " done"

# phonon calculation
cat > cu3n.ph.in << EOF
phonons of cu3n at 0.5,0.0,0.0
 &inputph
  tr2_ph=1.0d-14,
  prefix='cu3n',
  amass(1)=14.00674, 
  amass(2)=63.546,
  outdir='$TMP_DIR/',
  fildyn='cu3n.dyn'
 /
0.5 0.0 0.0
EOF
echo "  running the phonon calculation at
q=0.5,0.0,0.0...\c"
$PW_ROOT/bin/ph.x < cu3n.ph.in >> cu3n.ph.out
echo " done"
 

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