ZnO-lattice parameters problem

[vaspman]

Hello
I am a new VASP user and pacticed all the Examples without any problem.This is my second time but I dont get any reply.
I am trying to find the lattice parametrs of Wurtzite ZnO using LDA with Gama centered K-points, but the reults are vbery bad. I tried to use differnet K-points and Ecut but every time the lattice parametrs are bad.
Here is my results for Gama cenetered k-points

Grid E(eV) a c c/a
111 6.40 3.65 5.69 1.56
228 10.21 3.55 5.28 1.49
443 -10.13 3.25 5.48 1.68
444 -7.54 3.28 5.51 1.68
446 -11.69 3.24 5.45 1.68
448 -9.36 3.29 5.35 1.63
888 -11.16 3.26 5.50 1.69
663 -11.61 3.26 5.48 1.68
554 -8.12 3.29 5.51 1.68

and here are the Experimental values.
a b c c/a
3.250 3.250 5.207 1.602
======================================================================================
My INCAR file is here
SYSTEM = ZnO
Starting parameters for this run:
NWRITE = 2 write-flag
ISTART = 0 job : 0-new, 1-cont, 2-samecut
ICHARG = 0 charge: 0-wave, 1-file, 2-atom, >10-const
INIWAV = 1 electr: 0-lowe 1-rand 2-diag
IBRION = 2
NSW = 50 number of steps for IOM
Electronic Relaxation:
PREC = high low | medium | high
ENCUT = 550 kinetic energy cutoff (eV)
ISPIN = 1

NELMDL = -7 number of delayed ELM steps
NELM = 101 number of ELM steps
EDIFF = 1E-03 energy stopping-criterion for ELM
EDIFFG = .1E-02 energy stopping-criterion for IOM
LREAL = .FALSE. real-space projection (.FALSE., .TRUE., On, Auto)
IALGO = 48 algorithm (8=CG for small, 48=RMM for big systems)

LCHARG = .T.
LWAVE = .FALSE.

ISIF = 3 (1:force=y stress=trace only ions=y shape=n volume=n)
ISYM = 1 (1-use symmetry, 0-no symmetry)

DOS related values:
ISMEAR = 0 (-5:tet_b,-4:tet,-3:scan,-1:Fermi,0:gaus,1:MP)
SIGMA = 0.1 broadening in eV
~
======================================================================================
And POSCAR is here

Wurtzite
3.24950
0.50000 -0.8660254 0.00000
0.5000 0.8660254 0.0000
0.0000 0.0000000 1.6024
2 2
d
0.00 0.000 0.00
0.33 0.666 0.50
0.00 0.000 0.345
0.33 0.666 0.845
======================================================================================
And KPOINTS file is here
K-points
0
G
8 8 8
0 0 0
~
I will be very thankful if some one let me know where I am doing mistake.
Thank you

[zhichengzhong]

My suggestion:
1: increase EDIFF EDIFFG
2: you can do the static lattice~energy scan. ( set SMEAR = -5)
3: I don't think your result is very "bad". Maybe, you should check the literature. LDA potential?