[Pw_forum] Convergence or band-gap problem in orthorhombic CsPbBr3

[Christoph Wolf(신소재공학과)]

Dear all!

I am currently trying to simulate the electronic properties of CsPbBr3 in its low-temperature phase (orthorhombic; a=8.244, b=11.735, c=8.198 angstrom, according to Stoumpos; Cryst. Growth Des. 2013); My principle plan is to follow Radi et al. (arXiv:1405.1706v2), who have done this using Wien2k, muffin-tin potentials. Eventually I want to continue processing this with YAMBO (doesn’t take US PP), so I am using normcons PBE (from the standard repository).

However, the band-gap obtained is really tiny (without SOC, SOC is known to further reduce the gap in this material); I have done the following:


1)      Used the CIF file provided by stoumpos et al to create my unit cell and double checked with xcrysden -pwi if it turns out properly

2)      Tested convergence wrt. Total energy leading to a good compromise with ecutwfc =60.0 (tested between 30 and 180 Ry) and a unshifted automatic 4x3x4 k-grid (tested up to 16x16x16).

3)      Run Relax (gamma only) - scf - bands along G-X-S-R-G

The resulting band structure is in the pdf. The band gap at R is so tiny (0.x eV) that I am not sure if applying scissors in yambo is justified. I also tried smaller degauss and fixed occupations to not much success. If someone can spot an obvious error in my input or has any hint how I can improve my results I would be very grateful! Thank you in advance for your time and help!

Yours,
Chris

PS: I have also tried a VC-relax which clearly favors a shrinking cell (the cubic phase of CsPbBr3 for example is a=5.605 A, so the Pnma cell is probably far away from 0K - how to deal with this problem when trying to do a 300K phase…?)

My scf input:

&control
    calculation = 'scf'
    restart_mode='from_scratch',
    prefix='cspbbr3',
    pseudo_dir = '. /pseudo/',
    outdir='./'
    wf_collect=.true.
    verbosity='high'
/
&SYSTEM
  ibrav = 8
  celldm(1)= 15.5789
  celldm(2)=  1.4235
  celldm(3)=  0.9944
  nat = 5
  ntyp = 3
  ecutwfc =60.0
  occupations='smearing'
  smearing='mp'
  degauss=0.002
  force_symmorphic=.true.
  vdw_corr='grimme-d2'
/
&electrons
diago_full_acc=.true.,
conv_thr=1.0d-8
diagonalization='david'
electron_maxstep=300
/
ATOMIC_SPECIES
  Pb  207.20000  Pb.pbe-d-hgh.UPF
  Cs  132.90500  Cs.pbe-sp-hgh.UPF
  Br   79.90400  Br.pbe-hgh.UPF
ATOMIC_POSITIONS (crystal)
Pb       0.500000000   0.495569542   0.500000000
Cs       0.000000000  -0.000054173   0.000000000
Br       0.000000000   0.496389920   0.500000000
Br       0.500000000   0.002099521   0.500000000
Br       0.500000000   0.496980388   0.000000000
K_POINTS {automatic}
4 3 4 0 0 0


My bands input
nbnd=200 ! include plenty of empty bands for yambo

And

K_POINTS {crystal_b}
5
   0.0000000000     0.0000000000     0.0000000000     10 !G
   0.5000000000     0.0000000000     0.0000000000     10 !X
   0.5000000000     0.5000000000     0.0000000000     10 !S
   0.5000000000     0.5000000000     0.5000000000     10 !R
   0.0000000000     0.0000000000     0.0000000000     10 !G



Christoph Wolf

Department of Materials Science and Engineering, POSTECH
RIST #3, 3152, San 31 Hyoja-dong, Nam-gu, Pohang, Kyungbuk, 790-784, Republic of Korea





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