[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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