ABINIT Discussion Forums

Dear colleagues,

we have calculated the phonon dispersions (PD) in BaO. We would like to compare them with experiments. The only exp. data we found are from inelastic thermal-neutron scattering experiments ({\it Springer Handbook of Condensed Matter and Materials Data}, edited by W.~Martienssen, and H.~Warliment, (Springer, 2005)). Here's the comparison

mode point exp we
------------------------------------------------------------------------------
$\overline{\nu}_{\textrm{TO}}$ & $\Gamma$ & 144.1 & 101.0
$\overline{\nu}_{\textrm{LO}}$ & $\Gamma$ & 434.3 & 402.2
$\overline{\nu}_{\textrm{TA}}$ & $L$ & 66.4 & 64.6
$\overline{\nu}_{\textrm{LA}}$ & $L$ & 162.8 & 163.8
$\overline{\nu}_{\textrm{TO}}$ & $L$ & 219.5 & 236.3

There's a pretty large discrepancy for the Gamma modes (although the LO-TO splitting is very close), but we cannot find the source of it. Phonon freqs are converged wrt ecut and ngkpt under 1 cm-1. Pseudos used are Teters "extended norm-conserving" pseudopotentials with O($2s ,2p$) and Ba($5s ,5p ,6s$) levels treated as valence states. O.~Sch\"{u}tt, P.~Pavone, W.~Windl, K.~Karch, and D.~Strauch, Phys. Rev. B {\bf 50}, 3746 (1994) checked these, in the case of SrO, to include enough valence electrons. But an analogy can be drawn between BaO and SrO. Likewise, we presume that if we hadn't enough valence electrons, then all phonon modes would differ in freqs significantly from the experiments.

There is another issue which seemed to us like the reason of this discrepancy. When we do a d/dk perturbation, our calcs do not converge

scprqt: WARNING -
nstep= 300 was not enough SCF cycles to converge;
maximum residual= 1.437E+00 exceeds tolwfr= 1.000E-22

There are no other warnings and the calcs finish.
Could this be it?
We think not, because we did the same type of calcs for SrO (same type of pseudos and the level of convergence). Here we had the same problem with d/dk pert., but all the phonon modes were in a very good agreement with exp. data.

Does someone have an idea why do we get so different Gamma freqs? It would be of a great help to us. Input file is bellow.

Thank you in advance!

Igor Lukacevic


# Crystalline BaO - rocksalt
# computation of the response to homogeneous
# electric field and atomic displacements, at q=0
# and phonon dispersions

ndtset 18

#Set 1 : ground state self-consistency

getwfk1 0 # Cancel default
kptopt1 1 # Automatic generation of k points, taking
# into account the symmetry
nqpt1 0 # Cancel default
tolvrs1 1.0d-18 # SCF stopping criterion (modify default)
rfphon1 0 # Cancel default

#Q vectors for all datasets

#Complete set of symmetry-inequivalent qpt chosen to be commensurate
# with kpt mesh so that only one set of GS wave functions is needed.
#Generated automatically by running GS calculation with kptopt=1,
# nshift=0, shiftk=0 0 0 (to include gamma) and taking output kpt set
# file as qpt set. Set nstep=1 so only one iteration runs.

nqpt 1 # One qpt for each dataset (only 0 or 1 allowed)
# This is the default for all datasets and must
# be explicitly turned off for dataset 1.

qpt2 0.00000000E+00 0.00000000E+00 0.00000000E+00
qpt3 0.00000000E+00 0.00000000E+00 0.00000000E+00
qpt4 1.66666667E-01 0.00000000E+00 0.00000000E+00
qpt5 3.33333333E-01 0.00000000E+00 0.00000000E+00
qpt6 5.00000000E-01 0.00000000E+00 0.00000000E+00
qpt7 1.66666667E-01 1.66666667E-01 0.00000000E+00
qpt8 3.33333333E-01 1.66666667E-01 0.00000000E+00
qpt9 5.00000000E-01 1.66666667E-01 0.00000000E+00
qpt10 -3.33333333E-01 1.66666667E-01 0.00000000E+00
qpt11 -1.66666667E-01 1.66666667E-01 0.00000000E+00
qpt12 3.33333333E-01 3.33333333E-01 0.00000000E+00
qpt13 5.00000000E-01 3.33333333E-01 0.00000000E+00
qpt14 -3.33333333E-01 3.33333333E-01 0.00000000E+00
qpt15 5.00000000E-01 5.00000000E-01 0.00000000E+00
qpt16 5.00000000E-01 3.33333333E-01 1.66666667E-01
qpt17 -3.33333333E-01 3.33333333E-01 1.66666667E-01
qpt18 -3.33333333E-01 5.00000000E-01 1.66666667E-01

iscf2 -3 # Need this non-self-consistent option for d/dk
kptopt2 2 # Modify default to use time-reversal symmetry
rfphon2 0 # Cancel default
rfelfd2 2 # Calculate d/dk wave function only
tolvrs2 0.0 # Cancel default for d/dk
tolwfr2 1.0d-22 # Use wave function residual criterion instead

#Set 3 : Response function calculation of Q=0 phonons and electric field pert.

getddk3 2 # d/dk wave functions from last dataset
kptopt3 2 # Modify default to use time-reversal symmetry
rfelfd3 3 # Electric-field perturbation response only

#Sets 4-18 : Finite-wave-vector phonon calculations (defaults for all datasets)

getwfk 1 # Use GS wave functions from dataset1
kptopt 3 # Need full k-point set for finite-Q response
rfphon 1 # Do phonon response
rfatpol 1 2 # Treat displacements of all atoms
rfdir 1 1 1 # Do all directions (symmetry will be used)
tolvrs 1.0d-8 # This default is active for sets 3-10

#######################################################################
#Common input variables

#Definition of the unit cell - p = 0 GPa
acell 3*10.323077978
rprim 0.0 0.5 0.5
0.5 0.0 0.5
0.5 0.5 0.0

#Definition of the atom types
ntypat 2
znucl 56 8

#Definition of the atoms
natom 2
typat 1 2
xred 0.00 0.00 0.00
0.50 0.50 0.50

#Gives the number of band, explicitely (do not take the default)
nband 10

#Exchange-correlation functional
ixc 3

#Definition of the planewave basis set
ecut 70
ecutsm 0.5
dilatmx 1.2

#Definition of the k-point grid
ngkpt 3*6
nshiftk 4
shiftk 0.5 0.5 0.5
0.5 0.0 0.0
0.0 0.5 0.0
0.0 0.0 0.5

#Definition of the SCF procedure
nnsclo 10
nline 8
npulayit 12
nstep 300
diemac 4

timopt 2

I have also checked the level of convergence of the wavefunctions in the 1st dataset. But I think that htey are converged quite nicely:

iter Etot(hartree) deltaE(h) residm vres2 diffor maxfor
ETOT 1 -41.442197950961 -4.144E+01 1.012E-06 6.850E+03 0.000E+00 0.000E+00
ETOT 2 -40.597111353778 8.451E-01 2.900E-06 2.493E+04 0.000E+00 0.000E+00
ETOT 3 -41.576363673921 -9.793E-01 4.577E-04 1.252E+03 0.000E+00 0.000E+00
ETOT 4 -41.622226291886 -4.586E-02 2.666E-09 7.709E+01 0.000E+00 0.000E+00
ETOT 5 -41.623720185865 -1.494E-03 4.749E-12 2.492E+01 0.000E+00 0.000E+00
ETOT 6 -41.624548992194 -8.288E-04 2.917E-14 1.751E-02 0.000E+00 0.000E+00
ETOT 7 -41.624549227243 -2.350E-07 1.747E-16 2.265E-03 0.000E+00 0.000E+00
ETOT 8 -41.624549283193 -5.595E-08 1.827E-20 2.888E-05 0.000E+00 0.000E+00
ETOT 9 -41.624549284158 -9.649E-10 3.406E-21 5.613E-08 0.000E+00 0.000E+00
ETOT 10 -41.624549284161 -2.963E-12 8.018E-22 5.495E-11 0.000E+00 0.000E+00
ETOT 11 -41.624549284161 2.345E-13 1.160E-21 6.695E-13 0.000E+00 0.000E+00
ETOT 12 -41.624549284163 -1.911E-12 2.514E-21 4.428E-15 0.000E+00 0.000E+00
ETOT 13 -41.624549284162 8.313E-13 5.103E-22 3.456E-18 0.000E+00 0.000E+00
ETOT 14 -41.624549284161 5.400E-13 1.071E-21 1.001E-22 0.000E+00 0.000E+00

At SCF step 14 vres2 = 1.00E-22 < tolvrs= 1.00E-18 =>converged.



I presume that this is not the source of the error? I would appreciate any comments.

Thank you!

Igor Lukacevic

Well.. what works for me is to use occopt and tsmear. Did you check that all eigenvalues are correctly converging?, at least for the cases I have encountered, usually
the error source comes from a lousy convergence of one of the eigenvalues... the occopt and tsmear really helps!

regards

-aldo.

Dear Aldo,

thanks for answering. BaO in rocksalt phase is an insulator, so there no sense in using tsmear. And yes, all the phonon frequencies are converged wrt ecut under 1 cm^-1.

Do you have any other suggestions?

thnx again

Igor

For the ddk convergence, see other posts on the subject. Usually the perturbation is metallizing the system and you could try with metallic occupations and a small temperatures

For BaO, we have also had many problems reproducing frequencies. Pseudopotentials and XC are the main issue. Using the ELK code and frozen phonons I got ok results, and some pseudopotentials worked in abinit too - I no longer have the reference files unfortunately

Matthieu

mverstra wrote:For the ddk convergence, see other posts on the subject. Usually the perturbation is metallizing the system and you could try with metallic occupations and a small temperatures

Very well..thanks! I'll keep this in mind.

Well, Philippes' e-mail was quite discouraging. I have an idea, but I have to check some things before blurting myself out on the forum. I'll be in touch.

Cheers!

Igor