I am calculating the frequency dependent dielectric function of $\ce{SiC}$ using VASP. After a geometry optimization, the INCAR
file below is used for the calculation. The tags are mostly based on the JARVIS dielectric function database.
AGGAC = 0.0
EDIFF = 1e-07
ENCUT = 520
IBRION = 1
ISIF = 2
ISMEAR = 0
LCHARG = False
LOPTICS = True
LORBIT = 11
LWAVE = False
NBANDS = 600
NEDOS = 5000
NELM = 400
NPAR = 8
PARAM1 = 0.1833333333
PARAM2 = 0.22
PREC = Accurate
LREAL = AUTO
LscaAWARE=.False.
After some post-processing with p4vasp
, I can obtain the dielectric function of $\ce{SiC}$. Based on my calculations, the real part of the dielectric constant crosses zero around 7.7 eV (that is around 161 nm - I have converted photon energy to wavelength for comparison). This is very similar to $\ce{SiC}$ dielectric function available in JARVIS database, so I believe the calculation is consistent.
The problem is that there is a large mismatch between the DFT calculation and the experimental measurement. The experimental data is provided in kim et al. Optica 3, 339-346 (2016). Following is the comparison (first image from my calculation). According the their results the crossover wavelength is in the order of 10 $\mu$m. Also note the difference in y-axis magnitudes.
Why is there such a huge difference? Am I doing anything wrong?