i try to calculate the bandgap of Unitcell and Supercell(2*2*1) of TbMnO3 in CASTEP but there is a difference in the band gap. like the bandgap of unitcell is 0.63 and bandgap of supercell was 0.58 but as far as i know the bandgap should not be change
there are some details.
Function = GGA-PBE
Pseodopotential = OTFG ultrasoft
cutoff energy = 437.4 eV
Spin polarized with DFT+U