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Hi all,

I wish to determine the band gap of the wurtzite ZnO bulk.
I have never optimized a bulk structure in a hexagonal cell before, hence I need some advices.
I know that I need to geometrically optimize the bulk before I can run a single point calculation to obtain the DOS for band gap determination.
Can anyone show me the correct way of doing so?

Here is the POSCAR setting:
NPAR = 8
ISIF = 3
IBRION = 1
IALGO = 48
NELM = 400
NSW = 200
POTIM = 0.5 //For POTIM = 0.1, the POSCAR scale vector changed after optimization as well//
ISPIN = 2
PREC = ACCURATE
ENCUT = 570 //1.3*ENMAX//
ISMEAR = -5 //-5 tetrahedron for accurate energy calc//
SIGMA = 0.1
EDIFFG = -0.01



And my KPOINTS setting:
Automatic mesh
0
Gamma
8 8 6
0. 0. 0.


Here is the POSCAR and CONTCAR for a gamma point calculation:

POSCAR:

ZnO_bulk surface ,
1.00000000000000
2.7993513905862777 -1.2276248829450760 0.0002034272449117
0.3365686001871410 3.0382350626245445 0.0000135176664305
0.0004755822944742 0.0000732448509090 4.6298836810299893
Zn O
1 1
Selective dynamics
Direct
0.2204282714696630 0.1905690695630249 0.4509427523120246 T T T
0.7214219228275567 0.6895018142532939 0.4537315283309403 T T T

0.00000000E+00 0.00000000E+00 0.00000000E+00
0.00000000E+00 0.00000000E+00 0.00000000E+00


CONTCAR:

ZnO_bulk surface ,
1.00000000000000
2.7476064165021543 -0.7526958968191307 -0.0024482300129637
0.7213152191322992 2.7538066077930514 -0.0007717567962115
-0.0039700872928800 -0.0000281483169874 4.5271120055055549
Zn O
1 1
Selective dynamics
Direct
0.2209190816950810 0.1900270093926982 0.4523442591843658 T T T
0.7209311126021435 0.6900438744236176 0.4523300214585991 T T T

0.00000000E+00 0.00000000E+00 0.00000000E+00
0.00000000E+00 0.00000000E+00 0.00000000E+00


My question is, why does the scale vector for the CONTCAR changes after optimization?

This puzzles me as I could not visualize this on Material Studio.


<span class='smallblacktext'>[ Edited ]</span>

Well, first of all your scale vector is 1.0000 and it does not change from POSCAR to CONTCAR.
Second, why are you using selective dynamics if you do not have any geometric constrain (all you atoms can move along x-y-z)?
From your file I can see you are actually modelling not the hexagonal ZnO, but a layer of flat hexagonal ZnO, I hope this is what you meant to calculate.
Anyway, VASP is realxing both ionic positions and cell vectors. You started from a hexagonal cell and as a result you got a orthorombic one.

PS, I suggest you to use a better visualization tool such as vesta...