xred and kbounds

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Sun Jul 15, 2012 12:52 am

I am new in crystallography, my major problem now is how to determine xred and kbounds for various systems. Someone can pls explain with this two systems FeO(wustite) and CaB6. Thanks alot in anticipation of your assistance.

jzwanzig · Post by **jzwanzig** » Sun Jul 15, 2012 2:11 pm

For kptbounds you need to see the special symmetry points and paths in the Brillouin zone for the system of interest, see for example the book by Bradley and Cracknell, The Mathematical Theory of Symmetry in Solids, and also the web site http://www.cryst.ehu.es/, especially the kvec module. For xred you need the crystal structure for your system, usually obtained either from a publication or a .cif file (a good source of which is the non-free database http://icsd.fiz-karlsruhe.de/. Such files give the lattice parameters, space group, and atomic coordinates in xred-type units of the atoms in the asymmetric unit, the rest are computed by symmetry (abinit can do this for you too).

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Mon Jul 16, 2012 11:57 am

I got to the site http://www.cryst.ehu.es/and kpoints for space group 136 which TiO2 space group are listed as follows,
G 0 0 0
Z 0 0 1/2
M 1/2 1/2 0
A 1/2 1/2 1/2
R 0 1/2 1/2
X 0 1/2 0
Now from ainit example t14.in, kpoints are arrange as follows
kptbounds2 0.5 0.5 0.5 #A
0.0 0.0 0.0 #Gamma
0.5 0.5 0.0 #M
0.5 0.0 0.0 #X
0.0 0.0 0.0 #Gamma
0.0 0.0 0.5 #Z
0.5 0.0 0.5 #R
there is different between the two and i dont know the order i have to follow in arraging my kpoints. Moreover the X points from the website is different from the X in the example. Kindly put me through and bear with me the disturbance am given you. Thanks

jzwanzig · Post by **jzwanzig** » Mon Jul 16, 2012 12:50 pm

You can use whatever order you like--but the general point is to have the path in k-space be continuously trace-able. The abinit path does that--look at the figure of the Brillouin zone, you can trace a line without lifting your pencil, following the abinit ordering; the web site merely lists the points, without reference to a path.

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Mon Jul 16, 2012 3:46 pm

Prof, thanks alot. You said path should be cotinuously trace-able in k-space, please shed more light or do mean, take for example for space group 136, i can have path like,
G 0 0 0
Z 0 0 1/2
R 0 1/2 1/2
A 1/2 1/2 1/2
M 1/2 1/2 1/2
X 1/2 0 0
G 0 0 0
Is this order continuously trace-able.
Thanks alot.
-BAMGBOSE, M. K.

jzwanzig · Post by **jzwanzig** » Mon Jul 16, 2012 5:05 pm

Just look at the picture of the Brillouin zone on the web site I sent you, and follow along from point to point.

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Mon Jul 16, 2012 9:47 pm

Prof, thank alot for usual understanding. I will feed you back with my band structure result as soon as possible.

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Tue Jul 17, 2012 8:52 am

Igol can u please comment on my input file and the attached band structure plot.
############################################################
# GS + Band Structure + Fermi Surface calculation for MgB2
############################################################

ndtset 3

#Dataset 1 : GS calculation
kptopt1 1 # Use symmetry and treat only inequivalent points
ngkpt1 6 6 6
nshiftk1 1
shiftk1 0.0 0.0 0.5 # This choice preserves the hexagonal symmetry of the grid,
occopt1 1
nband1 26
prtden1 1
tolvrs1 1.0d-18

#Dataset 2 : Band structure calculation
iscf2 -2
getden2 -1
kptopt2 -5
tolwfr2 1.0d-12
nband2 40
ndivk2 60 60 60 60 60 60 #
kptbounds2 0.0 0.0 0.0 #Gamma
0.0 0.0 0.5 #A
0.3 0.3 0.5 #H
0.3 0.3 0.0 #K
0.0 0.0 0.0 #Gamma
0.5 0.0 0.0 #M
0.3 0.3 0.0 #K

enunit2 1

#Dataset 3 : Output of the Fermi surface
prtfsurf3 1 # To output the FS
getwfk3 -1
iscf3 -3 # NSCF calculation, occupancies and Fermi level are calculated once convergence is achieved.
# It should give the same FS as a GS calculation provided that the two
# k-meshes are sufficiently dense.
getden3 1
occopt3 1
nband3 40
kptopt3 1
ngkpt3 8 8 8 # too coarse. 24 24 24 for a reasonable FS
shiftk3 0.0 0.0 0.0 # Shifted grids are not supported by Xcrysden.
tolwfr3 1.0d-12

#COMMON INPUT DATA###########################################################
#
ecut 60
ecutsm 0.5
nstep 100

# MgB2 lattice structure
ntypat 2
znucl 12 5
natom 3
typat 1 2 2

#Definition of the unit cell
acell 2*5.8421069568E+00 6.5137931664E+00

#rprim 1.0000000000E+00 0.0000000000E+00 0.0000000000E+00
# -5.0000000000E-01 8.6602540378E-01 0.0000000000E+00
# 0.0000000000E+00 0.0000000000E+00 1.0000000000E+00
angdeg 90 90 120
spgroup 191
brvltt -1
# Atomic positions
xred 0.00 0.00 0.00 #Mg1
1/3 2/3 1/2 #B2
2/3 1/3 1/2 #B2

ixc 11
timopt 2

Thanks for your usuall assistance

ilukacevic · Post by **ilukacevic** » Wed Jul 18, 2012 8:30 am

I can only say that it would be the best for you to compare it to MgB2 band structures other people got using DFT and pseudopotentials. That's how you'll check the correctness of your calculation.

Igor L.

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Wed Jul 18, 2012 11:30 am

Igol,
Thank alot, but some of their bands crosses Fermi level. I think there should be a correction to this input file from abint guru like u. thank you.

ilukacevic · Post by **ilukacevic** » Wed Jul 18, 2012 12:24 pm

You have a lot of things to think about here. Did you use the same method as they (DFT + pseudopotentials)? If yes, did you use the same type of pseudos and with the same parameters? Did you use the same values of all relevant input variables? These all could be the reason for the difference.

Igor L.

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Wed Jul 18, 2012 12:56 pm

Igol,
They used abinit, same peudopotential, they use experimeter lattices parameter. the two plot look alike except that my valency band refuse to cross Fermi level. Thanks

ilukacevic · Post by **ilukacevic** » Wed Jul 18, 2012 1:26 pm

Try to reproduce their results by doing everything the same as they. If you succeed, then you know that the BS difference comes from the differences in the input files. If not, then something else you missed is different.

Igor L.

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Wed Jul 18, 2012 3:41 pm

Igol thanks, i will get back to you whenever i am through with the calculation

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Tue Feb 19, 2013 7:43 pm

Dear Igor,
if i dope a system say CaB6, what will be the xred of the doped system. Also if the system is not cubic system how would i determine lattice parameters. CaB6 is cubic if dope it what will be the crystal structure of the doped system.

ilukacevic · Post by **ilukacevic** » Wed Feb 20, 2013 8:17 am

BAMGBOSE, M. K. wrote: if i dope a system say CaB6, what will be the xred of the doped system.

There are several posts on this subject on this very forum. Please, search it. Essentially, you have to build a supercell. If you draw it on the paper, I think you'll find it easy to calculate the xreds.

BAMGBOSE, M. K. wrote:Also if the system is not cubic system how would i determine lattice parameters.

Let abinit do it for you. Just turn on the optimization algorithm, and relax. And yes, don't forget to turn off the spgroup variable.

BAMGBOSE, M. K. wrote:CaB6 is cubic if dope it what will be the crystal structure of the doped system.

Who knows? Again let abinit do the job for you. Turn off your space group info and let abinit symmetrizer do the job.

Igor

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Wed Feb 20, 2013 6:40 pm

Dear Igor,
Thanks for ur response, but what are the basic input variable to turn on optimization algorithm.
Thanks.

ilukacevic · Post by **ilukacevic** » Thu Feb 21, 2013 7:30 am

Well, you have to go through the tutorial first:

http://www.abinit.org/documentation/helpfiles/for-v7.0/tutorial/lesson_base3.html#34

Igor

BAMGBOSE, M. K. · Post by **BAMGBOSE, M. K.** » Thu Feb 21, 2013 6:48 pm

Dear Igor,
I hereby attached input file of barium doped CaB6, i have done correction to optimization of lattice parameter according to lesson 3. I still need the input that will determine the crystal structure of doped system CaBaB6. Kindly help me on this and other necessary correction to my input file.
Thanks.
-Bamgbose

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