Follow the science, not the DFT. 1) Unless you have a large grain size and synchotron data I really wonder about the logic of removing 1 atom in a 2x1x1 supercell. Particularly if this is an oxide, have you made it a metal whereas it should be an insulator? Does the fit pass significance tests etc? Do you have other data that implies vacancies? 2) In terms of the DFT, I expect that you have made it a metal which is harder to converge than an insulator. Check in case.output2 in both cases (a quick, pseudo DOOS). If it really does have vacancies, you might need more than 7 kpts, and I suggest using TEMPS.
2011/9/5 shamik chakrabarti <shamikphy at gmail.com>: > Dear wien2k users, > ?? ? ? ?I have performed a 56 atom / unit cell calculation for a > ferrimagnetic material. It was well converged both in energy and charge by > 60 iterations. 14 K points were used for this calculation. Then I have > generated a 2x1x1 supercell having 112 atoms / supercell. This was done to > achieve better experimental model as obtained through Rietveld analysis. > From Rietveld analysis it was also seen that for 112 atoms we can consider a > cation (Fe) vacancy and we have generated it by deleting one Fe atom from > the supercell. Final structure, hence, contains 111 atoms and all other > occupancies at different lattice sites are well matched with Rietveld > analysis of experimental XRD. I am using 2 k points for this 111 atom > supercell. But even after 60 iterations the convergence achieved are as > follows: > ?? cc: 0.234... ? ?cc was set to 0.001 > ?? ec: 0.063... ? ?ec was set to 0.0001 > Sir, my question are: > 1) As the supercell is merely an extension of the 56 atoms unit cell with > more accurate values of atomic occupancies as in the experimental structure, > why it should at all taking so much time to converge? > 2) We have created one defect by creating a Fe vacancy...is it that the > structures having defect take more time to converge?...but as far as our > analysis until now..the structure should have one Fe vacancy per 112 > atoms... > 3) As we have obtained converged solutions for 56 atom cell by using 14 k > points..is it that we should use 7K points for 111 atom supercell > calculation?..is 2 K points are actually small in order to achieve > convergence of this structure? > Any response in this regard will be very helpful for us. Thanks in advance. > with regards, > > -- > Shamik Chakrabarti > Senior Research Fellow > Dept. of Physics & Meteorology > Material Processing & Solid State Ionics Lab > IIT Kharagpur > Kharagpur 721302 > INDIA > > _______________________________________________ > Wien mailing list > Wien at zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien > > -- Laurence Marks Department of Materials Science and Engineering MSE Rm 2036 Cook Hall 2220 N Campus Drive Northwestern University Evanston, IL 60208, USA Tel: (847) 491-3996 Fax: (847) 491-7820 email: L-marks at northwestern dot edu Web: www.numis.northwestern.edu Research is to see what everybody else has seen, and to think what nobody else has thought Albert Szent-Gyorgi