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

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