Thank you Andrei. 1. I do apologize for the typo. System is pervoskite (CH3NH3)SnI3. It has 12 atoms in cubic cell. 2. Going to supercell should not increase the bandgap. I agree on this. 3. I did tetragonal cell of the same system of 48 atoms , and it looks ok if i use superecell but it has bandgap at G-point. 4. I looked structures before, basically 12 atoms at 8x8x8 k-mesh gives bandgap of 0.95 eV, whereas with %blocksupercell 3 3 3 %endblcoksupercell with only G point gives 1.6 eV. 5. And you are right, i also started to suspect that zone folding is the problem(i mean, my whole point of doing supercell calculations is to map R-point into G-point). 6. Visually it looks ok. It contains methylammonium. The only thing it might have is the dipole moment.
Witll try to use 2x2x2,4x4x4 (It should correctly map R point onto G point, correct me if i'm wrong ) supercell to see whether problem disappears. With Best Regards, Andrei Buin. On Wed, Aug 21, 2013 at 6:29 AM, <[email protected]> wrote: > Dear Andrei: > I don't see how your system can be a perovskite, > nor how CH3N3SnI3 sums up to 12 atoms. I'll appreciate if you > give a hint, just for my education. > However, this is beyond the point. > In principle, a mere passing to a supercell > should not result in an increase of the band gap. > The mapping of k-points may be not exact, i.e., > Gamma of the 3*3*3 supercell maps onto 0, 1/3, 2/3 [*(2*pi/a)] > divisions along axes of the reciprocal cell, > none of which points includes R=(1/2 1/2 1/2). You can check > what the band gap (in the primitive cell) is at (1/3 1/3 1/3), > or at (3/8 3/8 3/8), the closest of your k-points at the 8x8x8 mesh. > [However, you say that you have an automatic k sampling, then > the chances are that your mesh is shifted... and does not contain > the R point..?] > Apart from these tests, I think that some simple error in structure > (as simple as a wrong lattice constant in a supercell, sorry) > is a very good candidate for this kind of problem. > To check it, it might be a good idea to have a view at both structures > (the primitive one and the supercell) as stored in corresponding XV files, > and to compare their corresponding DOS. > > Again apart from all this, if your system contains methyl azide, > I think it could be tricky to initialize the different charge states > at different N atoms correctly... If you just let it go as it wish > the calculation may converge to something wrong... Even to differently > wrong in two different supercells. > > Sorry for too many suggestions. > > Best regards > > Andrei Postnikov > > > > 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. > > > >
