Dear Siesta users, I'm having very bizarre problem. I'm simulating the primitive cubic unit cell(12 atoms) of the CH3N3SnI3 perovksite with GGA and PBE and gives me band gap of Eg=0.9 eV at R point with 8x8x8 Moshkrof automatic k sampling(i know it's wrong in DFT, but still) and then i use supercell approach built in with 3x3x3 unit cell(432 atoms) with Gamma point only and i get a bangap of 1.6 eV. I've checked against plane cutoff convergence seems ok. I thought maybe dispersion is too big so one should sample it more finely, but it turns out meff=0.13me, which should be fine. Any ideas ?
With Best Regards, Andrei Buin.
