[Pw_forum] Phonon simulation of Titanium

[Cameron Foss]

Hello all,

I am trying to simulate phonons in titanium with espresso-5.1. My results
so far seem to show the presence of soft phonons (negative frequencies) an
indicator of material instability and possibly the initiation of phase
transitions (EPL, 95 (2011) 18002) as well as some other irregularities in
the shape of the branches.

To my knowledge, different pseudopotentials can have different equilibrium
lattice constants for a given material. Hence I perform vc-relax
simulations and update the lattice constant until the variations seen in
the lattice vectors is below .05%. I then use the relaxed lattice vectors
and atomic positions in the scf (pw.x) simulation and proceed with a phonon
(ph.x) simulation.

I have generated my scf input file based off of the following *Phys. Scr.
83 (2011) 065603* and *Condensed Matter Physics, 2011, Vol. 14, No 2,
23601: 1–7.*

Below are the input files for the scf and phonon simulations. The vc-relax
simulation is excluded because its input parameters are the same as the scf
simulation (MP grid, conv_thr, ecutwfc, etc). Lastly, the bfgs scheme is
used in the vc-relax calculation.

%%%%%  SCF %%%%%
  1 &control
  2   calculation='scf',
  3   restart_mode='from_scratch',
  4   outdir='/oasis/scratch/cjfoss/temp_project/out_ti',
  5   pseudo_dir='/home/cjfoss/espresso-5.1/pseudo',
  6   prefix='TiPHv4'
  7 /
  8 &system
  9   ibrav=0,
 10   celldm(1)=5.507258084,
 11   celldm(3)=1.587457,
 12   nat=2, ntyp=1,
 13   ecutwfc=100.0
 14 /
 15 &electrons
 16   mixing_beta=0.7,
 17   conv_thr = 1.0d-12
 18 /
 19 ATOMIC_SPECIES
 20 Ti 47.867 Ti.pw91-nsp-van.UPF
 21 ATOMIC_POSITIONS {crystal}
 22 Ti 0.33333333 0.66666666 0.25
 23 Ti 0.66666666 0.33333333 0.75
 24 K_POINTS automatic
 25 7 7 7 1 1 1
 26 CELL_PARAMETERS (alat)
 27    1.000043369   0.000000000   0.000000000
 28   -0.500021685   0.866062962   0.000000000
 29    0.000000000   0.000000000   1.615221757
 30

%%%%%         PH        %%%%%
  1 PHONONS IN TITANIUM
  2 &inputph
  3   tr2_ph=1.0d-14,
  4   max_seconds=129600,
  5   prefix='TiPHv4',
  6   ! recover=.true.
  7   ldisp=.true.,
  8   nq1=7, nq2=7, nq3=7,
  9   amass(1)=47.867,
 10   outdir='/oasis/scratch/cjfoss/temp_project/out_ti',
 11   fildyn='ti.dyn',
 12 /

My BZ traversal moves along the path G-K-M-G-A, where G stands for the
Gamma zone-center point. The symmetry points used are K=(2/3,0,0) ;
M=(0.5,-1/(2*sqrt(3)),0) ; A=(0,0,a/(2c)) ; and G=(0,0,0). I am confident
in these points since I have used them to get a proper dispersion of AlN
but include them here for completeness.

It is unclear to me whether the soft phonon modes are a result of my
celldm(3) being different than the v3 z-component in CELL_PARAMETERS or if
I am missing a parameter that is pertinent to simulating phonons in metals?
Any help is appreciated.

Best Regards,
Cameron
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