Dear QE community,
I've tried to calculate the phonon-dispersion of SrTiO3 (STO) and EuTiO3
(ETO) using the PHonon package of QE. SCF, DOS and Bandsctructure
calculation worked well on these materials, I've used a
Hubbard-correction for both. My scf file for STO looks as follows:
STO:
&CONTROL
calculation = 'scf'
outdir = './out/'
prefix = 'STO_gamma'
pseudo_dir = './nc-sr-04_pbe_stringent_upf/'
/
&SYSTEM
! a = 3.94513 # old value before vc-relax.x
A = 3.93795
degauss = 0.005
ecutwfc = 80
lda_plus_u=.true.
lda_plus_u_kind = 0
hubbard_u(1) = 0
hubbard_u(2) = 5.6
hubbard_u(3) = 4
ibrav = 0
nbnd = 33
nat = 5
nosym = .false.
nspin = 2
ntyp = 3
occupations = 'smearing'
starting_magnetization(1) = 1.0d-01
starting_magnetization(2) = 1.0d-01
starting_magnetization(3) = 4.16d-01
/
&ELECTRONS
conv_thr = 1.d-10
mixing_beta = 0.7
/
ATOMIC_SPECIES
Sr 87.62000 Sr.upf
Ti 47.86700 Ti.upf
O 15.99940 O.upf
CELL_PARAMETERS {alat}
0.998180509 0.000000000 0.000000000
0.000000000 0.998180509 0.000000000
0.000000000 0.000000000 0.998180509
ATOMIC_POSITIONS {crystal}
Sr 0.0000000000 0.0000000000 0.0000000000
Ti 0.5000000000 0.5000000000 0.5000000000
O 0.5000000000 0.0000000000 0.5000000000
O 0.5000000000 0.5000000000 0.0000000000
O 0.0000000000 0.5000000000 0.5000000000
K_POINTS {automatic}
16 16 16 0 0 0
The pesudopotentials are the norm conserving potentials from
pseudo-dojo.org. The bandstructure shows an energy gap of about 3.2 eV
with these settings.
The ph.x input looks like:
phonon calculation at Gamma point.
&inputph
outdir = './out/'
prefix = 'STO_gamma'
tr2_ph = 1.0d-3
nmix_ph = 10
epsil = .false.
amass(1) = 87.62000
amass(2) = 47.86700
amass(3) = 15.99940
fildyn = 'STO_phonon.dyn'
/
0 0 0
The output generated by dynmat.x after applying the acustic sum rule =
crystal looks like:
# mode [cm-1] [THz] IR
1 -6235.76 -186.9435 0.0000
2 -6235.76 -186.9435 0.0000
3 -6235.76 -186.9435 0.0000
4 -4352.85 -130.4953 0.0000
5 -4352.85 -130.4953 0.0000
6 -4352.85 -130.4953 0.0000
7 -4105.78 -123.0883 0.0000
8 -4105.78 -123.0883 0.0000
9 -4105.78 -123.0883 0.0000
10 -2138.73 -64.1174 0.0000
11 -2138.73 -64.1174 0.0000
12 -2138.73 -64.1174 0.0000
13 0.00 0.0000 0.0000
14 0.00 0.0000 0.0000
15 0.00 0.0000 0.0000
This calculation alone took over 3 hours only for the gamma point. The
input file for my scf calculation of ETO looks like
ETO:
&CONTROL
calculation = 'scf'
outdir = './out/'
prefix = 'EuTiO3'
pseudo_dir = './pseudo/'
/
&SYSTEM
a = 3.9611945300
degauss = 0.01
ecutrho = 600
ecutwfc = 75
lda_plus_u=.true.
lda_plus_u_kind = 0
hubbard_u(1) = 9
hubbard_u(2) = 0
hubbard_u(3) = 0
ibrav = 1
nat = 5
nosym = .false.
nspin = 2
ntyp = 3
occupations = 'smearing'
smearing = 'mv'
starting_magnetization(1) = 4.1176470588d-01
starting_magnetization(2) = 1.0000000000d-01
starting_magnetization(3) = 4.1666666667d-01
/
&ELECTRONS
conv_thr = 1.0000000000d-06
electron_maxstep = 1000
mixing_beta = 4.0000000000d-01
/
ATOMIC_SPECIES
Eu 151.964 Eu.GGA-PBE-paw-v1.0.UPF
O 15.9994 O.pbe-n-kjpaw_psl.0.1.UPF
Ti 47.867 ti_pbe_v1.4.uspp.F.UPF
ATOMIC_POSITIONS {crystal}
Eu 0.0000000000 0.0000000000 0.0000000000
Ti 0.5000000000 0.5000000000 0.5000000000
O 0.5000000000 0.5000000000 0.0000000000
O 0.5000000000 0.0000000000 0.5000000000
O 0.0000000000 0.5000000000 0.5000000000
K_POINTS {automatic}
6 6 6 0 0 0
Bandstructure shows a band gap of about 1.05 eV, the ph.x input is:
phonons of EuTiO3 at Gamma
&inputph
prefix = 'EuTiO3'
outdir = './out/'
tr2_ph = 1.0d-14
amass(1) = 151.964
amass(2) = 15.9994
amass(3) = 47.867
! epsil = .true.
! lraman .true.
fildyn='EuTiO3.dynG'
/
0 0 0
Which got me the following phonon frequencies (again asr = crystal):
# mode [cm-1] [THz] IR
1 -636.90 -19.0939 0.0000
2 -636.90 -19.0939 0.0000
3 -636.90 -19.0939 0.0000
4 -547.18 -16.4041 0.0000
5 -547.18 -16.4041 0.0000
6 -547.18 -16.4041 0.0000
7 -401.41 -12.0340 0.0000
8 -401.41 -12.0340 0.0000
9 -401.41 -12.0340 0.0000
10 -0.00 -0.0000 0.0000
11 -0.00 -0.0000 0.0000
12 0.00 0.0000 0.0000
13 64.61 1.9371 0.0000
14 64.61 1.9371 0.0000
15 64.61 1.9371 0.0000
I have some different questions about these results:
1) Why is it not possible to set epsil = .true., lraman .true. ?
They show semiconducter properties.
2) Is it normal that the ph.x code takes a lot of comupting time
compared to pw.x scf, nscf, bands calculations? (ETO phonon at gamma
took over 13 h)
3) Almost all phonon frequencies are negative, which indicates
instabilities in the system (?), I've run vc-relax caclulations and used
the new lattice parameters but got the same results, any idea what these
negative frequencies could tell me?
4) Is it possible obtain the dielectric function for these materials
in a different way?
I hope you are still around and thank you for reading :)
Have a nice day,
Simon Rombauer
Physics student
University of Augsburg
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