i am trying to find the band gap of unitcell & supercell of TbMnO3 by using CASTEP. But there is a difference in bandgaps like the the bandgap of unitcell is 0.63eV and band gap of Supercell (2*2*1) is 0.58eV. as far as i know the bandgap should not be change.
there are some details of geometry optimization
cutoff energy= 437.4eV
Pseudopotential=OTFG ultrasoft
Quality = fine
function = GGA-PBE
spin polarized calculation with DFT+U