ABINIT Discussion Forums

Dear All,
I have carried out Band Structure Calculation on supercell having 58 atoms (32 Te atoms, 13 Germanium atoms, 13 Antimony atoms & 6 vacancies) of Ge2Sb2Te5 material.I found that the valence band and conduction band are overlapping with each other and which is wrong because The expected band gap energies of this material in fcc phase is 0.53-1.5eV.
I have tried modifying my input file but unfortunately didn't able to correct it properly to get the desired results.
Band structure plot and output file is also attached with this post.
my input file is as follows:

# Crystalline gst225:DOS & B.S.
ndtset 2
# Definition of the unit cell: fcc
acell 2.1185145826E+01 2.0997606615E+01 2.1014447803E+01

rprim 9.9999997318E-01 1.3411628225E-07 -2.3160474578E-04
1.3388475944E-07 1.0000000000E+00 2.8873278611E-07
-2.3357712918E-04 2.8853516331E-07 9.9999997272E-01
natom 58
chkprim 0
typat 3 3 3 3 3 3 3 3 1 1 1 1 1 1 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2
ntypat 3
znucl 32 51 52
xcart
8.9121776340E-02 -3.2586490793E-06 -4.8177436993E-02
1.0545462551E+01 -6.7444199508E-06 -2.6668206337E-02
2.5776022708E-01 1.0498799923E+01 -2.9611038270E-01
1.0476163739E+01 1.0498800656E+01 -4.5090354606E-02
3.2537169222E-02 1.4777275630E-06 1.0331188928E+01
1.0595495671E+01 -2.9125731164E-05 1.0471012940E+01
-4.0470042332E-02 1.0498808480E+01 1.0287332513E+01
1.0057943334E+01 1.0498803015E+01 1.0494157798E+01
1.5582902790E+01 5.3332881020E+00 5.2794873597E+00
1.5582907314E+01 1.5664327244E+01 5.2794917603E+00
5.4139407611E+00 5.3391248204E+00 1.5314837960E+01
1.5776205040E+01 5.2811507456E+00 1.5591182405E+01
5.4139429860E+00 1.5658503097E+01 1.5314839732E+01
1.5776207176E+01 1.5716468896E+01 1.5591184876E+01
5.4255158707E+00 5.3989808824E+00 -2.1231548685E-01
1.5913063442E+01 5.2418325472E+00 2.3455370622E-02
5.4255203091E+00 1.5598589366E+01 -2.1229926993E-01
1.5913068147E+01 1.5755772685E+01 2.3460207045E-02
5.7706969668E+00 5.4334260190E+00 1.0247587511E+01
1.6049196527E+01 5.3119940887E+00 1.0415542672E+01
5.7706953809E+00 1.5564221149E+01 1.0247600534E+01
1.6049201202E+01 1.5685645171E+01 1.0415545688E+01
5.2037859711E+00 2.1610123517E-06 5.1249266624E+00
1.5860592197E+01 -6.7818288369E-07 5.1377204824E+00
4.9109033864E+00 1.0498858163E+01 6.8853689678E+00
1.5898095000E+01 1.0498805900E+01 5.2309414377E+00
5.3393287766E+00 7.3015889628E-06 1.5644545661E+01
1.5936645602E+01 2.3090777815E-06 1.5649301418E+01
5.2949913618E+00 1.0498810993E+01 1.5688602030E+01
1.5996760795E+01 1.0498808093E+01 1.5607800719E+01
-1.1621511233E-01 5.2772055052E+00 5.3009265231E+00
1.0370828752E+01 5.2956750290E+00 5.5104868170E+00
-1.1621158392E-01 1.5720402871E+01 5.3009386130E+00
1.0370835559E+01 1.5701934782E+01 5.5104899152E+00
9.3941300362E-02 5.3105415384E+00 1.5705513446E+01
1.0575385430E+01 5.2583496282E+00 1.5684356247E+01
9.3943521812E-02 1.5687070544E+01 1.5705524262E+01
1.0575387025E+01 1.5739262314E+01 1.5684366353E+01
3.0022242947E-02 9.6778565330E-07 5.1557108139E+00
1.0343613771E+01 3.5841800547E-06 5.3555471087E+00
9.9756129620E+00 1.0498808026E+01 5.4081057363E+00
7.6624424716E-02 3.0541384102E-06 1.5650641717E+01
1.0530798627E+01 6.0124052357E-06 1.5605224063E+01
1.0121924749E-01 1.0498807152E+01 1.5341069458E+01
1.0458604864E+01 1.0498808789E+01 1.5580948734E+01
2.6488344515E-01 5.3516802412E+00 2.6309510665E-01
1.0571485578E+01 5.2412099801E+00 3.4826893425E-01
2.6489046750E-01 1.5645917428E+01 2.6310797971E-01
1.0571490453E+01 1.5756388525E+01 3.4827433786E-01
6.8437712031E-01 5.4084662972E+00 1.0374395381E+01
6.8438311428E-01 1.5589151906E+01 1.0374407927E+01
5.4021615210E+00 -2.3971553913E-05 5.6414537772E-03
1.5812457067E+01 1.4071265203E-06 -1.4226040869E-01
5.2204443307E+00 1.0498771775E+01 2.1266166663E+00
1.5961514788E+01 1.0498803455E+01 -4.4396044535E-02
5.3875412870E+00 4.0123029128E-05 1.0328716845E+01
1.5681592548E+01 2.9679127772E-05 1.0375467995E+01
1.5072987096E+01 1.0498823881E+01 1.0377542663E+01

# output the density of states
prtdos1 2
# output the charge density
prtden1 1

# Definition of the planewave basis set
ecut 30

ixc 1

# Definition of the SCF procedure
nstep 200
#toldfe1 1.0d-7
toldff1 5.00000000E-06
diemac 35

# Calculation of the band structure
iscf2 -2
getden2 -1
kptopt2 -7
ndivk2 5 20 17 8 18 11 21
kptbounds2 # limits of the segments
0.25 0.625 0.625 # U
0.25 0.5 0.75 # W
0 0 0 # Gamma
0 0.5 0.5 # X
0.25 0.5 0.75 # W
0 0.5 0 # L
0 0 0 # Gamma
0.375 0.375 0.75 # K

tolwfr2 5.00000000E-06
nband2 30
enunit2 1
prtvol2 2

I would appreciate any suggestions to correct it.
Waiting for your response
Best Regards
Radhika

well, it is standard for DFT (especially LDA, but GGA too) to underestimate band gaps, often by 1/2 eV. A well-known example is bulk Ge, which is predicted by LDA to be a metal but of course it's not. You may be seeing this behavior in your system. GW corrects this error fairly well but the calculations are large even in simple systems. Are you certain you can reproduce standard results in Te or Ge with your pseudopotentials, before moving on to complicated systems with vacancies? If so, your problem may be fixed by using some exact exchange ("PBE0"), which can be done locally in the PAW spheres using the PAW method in abinit.