"Very low..." in respect to what?

With respect to experimental value. Its experimental value is ~1.55-1.66. even i have tried vander wall correction but there is no improvement. my incar file is :

ISMEAR = 0

ICHARG = 2

#NSW = 100

#PREC = Accurate

IBRION = 1

#POTIM = 0.8

ENCUT= 520

#ISIF = 3

LREAL = Auto

#NPAR = 8

EDIFF = 1E-7

EDIFFG = -0.001

ADDGRID = .TRUE.

GGA = ML

LUSE_VDW = .TRUE.

Zab_vdW = -1.8867

AGGAC = 0.000

I measure it by two ways i) by DOSCAR file and ii) by Henklemanl's group script (bandgap.pl )

Wait: What equation do you solve by your calculation? In other words: What kind of calculation do you perform: standard DFT (Kohn--Sham), time-dependent DFT, any kind of GW, or ...?