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Hi ,

I have been trying to do relaxation for ZnO bulk and am not able to obtain unit cell value as obtained through experiments. I have used gamma centered grids for hexagonal symmetry as well as tried to increase the k point grid but the result is the same.Could someone please look at my INCAR and KPOINTS file and tell the error which I am making.


SYSTEM = Zinc oxide (ICSD #94004)_1 (VASP)
PREC = Normal
ENCUT = 520.000
IBRION = 2
NSW = 100
ISIF = 3
ALGO = Normal (blocked Davidson)
NELM = 60
NELMIN = 2
POTIM = 0.5
EDIFF = 1.0e-05
EDIFFG = -0.02
VOSKOWN = 1
NBLOCK = 1
ISPIN = 1
INIWAV = 1
ISTART = 0
ICHARG = 2
LWAVE = .FALSE.
LCHARG = .TRUE.
ADDGRID = .TRUE.
ISMEAR = -5
SIGMA = 0.02
LREAL = .FALSE.
RWIGS = 1.25 0.73
LORBIT = 11


and KPOINTS

Automatic mesh
0
Gamma
19 19 13
0. 0. 0.

what's the difference between calculation and experiment?

I don't think k-point grid is meaningful for this particular type of calculation.

The best way for getting the optimized cell volume is to perform a series of fixed-volume calculations (ISIF=2) at volumes around the experimental value and to make a fit of the E/V curve. LDA gives slightly smaller and GGA slightly larger volume than the experimental one.

I second the opinion of doing a series of fixed-volume calculations, but why not with ISIF=4? This should allow shape relaxations which is essential in non-cubic cells.