hi, can you a calculation of dos for all the cases. this needs many k points. check if the location of the band gap changes and the band gap value as obtained from dos. s.auluck
> Dear Andrei, > > I did 2x2x2 supecell block and i get an increased bandgap(1.9 eV) almost > twice to bulk(0.9 eV). I did 4x4x4 , results are much better although > still > a bit off. > Although i did not put the supercellblock back into siesta with twice the > parameters(will do it). Now , i really start scratching my head. I > checked > manually bulk cell and it is R point for sure (1/2,1/2,1/2). > Any ideas ? > > Thank you. > > > > > > > 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. >> >> > >> >> >> >> >> > >> >> > ....................................................................... Prof. Sushil Auluck Phone:+91-512-6797092/6148 Department of Physics +91-512-6798177(Home) Indian Institute of Technology Cell :+91-9305548667 Kanpur 208016 (UP) Fax :+91-512-6790914 India E-mail:[email protected] ...............................................:[email protected] http://www.iitk.ac.in/phy/People/phy_facvis.html http://www.iitk.ac.in/phy/New01/profile_SA.html ....................................................................... ~
