Hello. I am trying to modify the band gap of (0001) surface of ZnO containing 180 atoms by using QuantumATK . I have almost used all of the functionals and reached at the following band gap:

GGA-PBE=0

MGGA-TB09=0

LDA-PW=0

Hybrid-(HSE06 & B3LYP)=0

DFT1/2-(BLYP)=0.9 eV

DFTB-extended Huckel=1.1 eV

DFTB-cp2k=1.3 eV

As you see, there is a wide gap between my calculations and experimental (3.3 eV).

I shall be thankful if somebody can help me.