[Pw_forum] Na phonon issues

[Miguel Martínez Canales]

Dear folks,

I've been doing some phonon calculations on Na lately, and I've got an 
issue with the Na.pw91-n-mt.UPF pseudopotential. Well, it might be an issue 
with the input, so I also attach them.

When doing a phonon calculation at q=(0 -0.1 0.1) for bcc Na, which is the 
same as na.dyn2 in the enclosed input, I get the following frequencies:

$ tail na.dyn2
      q = (    0.000000000  -0.100000000   0.100000000 )

  **************************************************************************
      omega( 1) =      -0.102259 [THz] =      -3.411026 [cm-1]
  (  0.000000  0.000000 -0.707107  0.000000 -0.707107  0.000000 )
      omega( 2) =       0.766410 [THz] =      25.564857 [cm-1]
  (  1.000000  0.000000  0.000000  0.000000  0.000000  0.000000 )
      omega( 3) =       1.171147 [THz] =      39.065511 [cm-1]
  (  0.000000  0.000000 -0.707107  0.000000  0.707107  0.000000 )
  **************************************************************************

If I use the LDA pseudo Na.pz-n-vbc.UPF near equilibrium (a=7.738a_0) with 
ecut=30 Ry and 18 18 18 shifted M-P grid, I get the following omegas:
      omega( 1) =       0.273215 [THz] =       9.113532 [cm-1]
      omega( 2) =       0.835358 [THz] =      27.864728 [cm-1]
      omega( 3) =       1.264994 [THz] =      42.195940 [cm-1]

While the Na.pbe-sp-van_ak.UPF pseudo, with the same grid as the enclosed 
input (20 20 20 shifted M-P), ecut=35 and ecutrho=300 Ry, I get also 
sensible results (with 7.932 a_0 as lattice parametre):
      omega( 1) =       0.390200 [THz] =      13.015753 [cm-1]
      omega( 2) =       0.857166 [THz] =      28.592182 [cm-1]
      omega( 3) =       1.238921 [THz] =      41.326237 [cm-1]

So I have the following question: Is the mechanical instability I see a 
product of my input or a product of the pseudopotential?

Thanks a lot,

Miguel


============= input follows ==============
# self-consistent run
cat > na.scf.in << EOF
  &control
     calculation='scf'
     restart_mode='from_scratch',
     tstress = .true.
     tprnfor = .true.
     prefix='na',
     pseudo_dir = '$PSEUDO_DIR/',
     outdir='$TMP_DIR/'
  /
  &system
     ibrav=  3, celldm(1) = 7.8956 , nat=  1, ntyp= 1,
     ecutwfc = 35.0,
     occupations='smearing', smearing='m-p', degauss= 0.010
  /
  &electrons
     conv_thr =  1.0d-8
     mixing_beta = 0.7
  /
ATOMIC_SPECIES
  Na  22.989768  Na.pw91-n-mt.UPF
ATOMIC_POSITIONS
  Na 0.00 0.00 0.00
K_POINTS {automatic}
  20 20 20   1 1 1
EOF


# phonon calculation
cat > na.ph.in << EOF
phonons of Na
  &inputph
   tr2_ph=1.0d-16,
   prefix='na',
   ldisp=.true.,
   nq1=10, nq2=10, nq3=10
   amass(1)=22.989768,
   outdir='$TMP_DIR/',
   fildyn='na.dyn',
  /
EOF


-- 
----------------------------------------
Miguel Martínez Canales
Dto. Física de la Materia Condensada
UPV/EHU
Facultad de Ciencia y Tecnología
Apdo. 644
48080 Bilbao (Spain)
Fax:  +34 94 601 3500
Tlf:  +34 94 601 5437
----------------------------------------

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