Thanks Andrei. 1. I need superecells because i will use slab calculations(surfaces) symmetric slabs at Gamma point. They are not going to be centrosymmetric(even if the slabs are symmetrically terminated) due to particular arrangement of the CH3NH3 in the supercell so i have to use Dipole Correction, although maybe a faster way is to add more vacuum. 2. 3x3x3 cell i just took it forgetting that bulk is the direct bandgap at R point, to see whether it will capture the bandgap. Tetragonal on the other hand is much easier to deal with(It is direct G-point at bulk initially). 1. It is disordered. What a mess.
Thank you for your help. With Best Regards, Andrei Buin. On Wed, Aug 21, 2013 at 8:18 AM, <[email protected]> wrote: > OK Andrei. > Now the chemical formula makes sense. > (Is the methyl ammonium rotating in its site? > Disordered in this sense? - Must be funny... - A nice task for Siesta) > > 1. The 2x2x2 supercell would nicely map X, M and R points > of the primitive cell onto its new Gamma. Hopefully it's easy > to trace "by hand", checking contributions from different atoms > to the wavefunction at these points in case of difficulty. > I don't understand why do you need a 3x3x3 supercell if you are > interested just in folding. Must be a long calculation... > > 2. Be careful with the supercell block; my suggestion is > to use it just for generating the supercell, then put the generated > coordinates into the input (or start from XV) and deactivate > the supercell block. Otherwise it is easy to make an error. > > Good luck > > Andrei Postnikov > > > > 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. > >> > > >> > >> > > > >
