[Pw_forum] Phonon simulation of Titanium

Krishnamohan tkrishnamohan91 at gmail.com
Tue Jul 19 05:39:35 CEST 2016


Dear Cameron,

I have a lot of experience with Titanium. I noticed a couple of mistakes in
your input file.

1) You need to use smearing for metals (see this
http://caribeictp.uis.edu.co/lectures/R.Gebauer/Gebauer_Course2_KpointsMetals.pdf
)
2) You are using an ultrasoft pseudopotential, so you also need to specify
'ecutrho', which is 8*ecutwfc
3) It is better to use gamma centered k-mesh for hexagonal systems

I also recommend you do convergence studies w.r.t energy cutoffs, smearing
and k-mesh

%%%%%  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, -------------------> This flag will be ignored
for ibrav=0
 12   nat=2, ntyp=1,
 13   ecutwfc=100.0 -------------> This cutoff is very high for this
pseudopotential (I use 40 Ry for thsi pseudopotential). If you are using an
ultrasoft pseudopotential, you also need to mention ecutrho = 8*ecutwfc
 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 -----------> Ultrasoft pseudopotential
 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 --------------------------------> I use 18 18 12 0 0 0
here, but i recommend you do your own convergence studies.
 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

Hope this helps

Krishnamohan




On Tue, Jul 19, 2016 at 6:06 AM, Cameron Foss <cjfoss at umass.edu> wrote:

> 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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>



-- 
*Krishnamohan Thekkepat*
*Research Associate*
*Indo-Korea Science and Technology Center, Bangalore, India*

“We shall not cease from exploration, and the end of all our exploring will
be to arrive where we started and know the place for the first time.”- T.S
Eliot
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