Mehmet, Please do share the knowledge. It is alwayfs useful, especislly because the calculation of phonons is always of interest. I for one would be very much interested in knowing how to use Dario's code in conjunction with Siesta.
Cheers, Marcos On Wed, Jul 28, 2010 at 1:43 PM, Mehmet Topsakal < topsa...@unam.bilkent.edu.tr> wrote: > Dear Jusar, > > I have obtained the attached phonon spectrum in 2 hours by using > 8-cpu-core. The supercell is 5x5x1 and the MeshCutoff is 300 Ry. I have used > Dario Alfè's phon program. Because I'm not familiar with siesta utilities > such as FCBUILD and VIBRA. Phon only needs forces which can be calculated by > other programs such as VASP and PWSCF. > > I can prepare a nice and detaied phon tutorial where the forces are > calculated by SIESTA if you (or others) want. > > Mehmet. > > > > On Wed, Jul 28, 2010 at 11:46 AM, Juzar Thingna <juza...@gmail.com> wrote: > >> Thank you Prof. Postnikov and Mehmet for your quick responses. Here are a >> few observations. >> >> After Mehmets comment on increasing the Mesh cut-off I set it to 800 Ry I >> still observed negative part but the magnitude has reduced. Sorry I didn't >> know one had to go to such high cut-offs to observe a change, earlier I was >> trying cut-offs from 200-500 and there were no changes (or maybe changes >> that were not very distinguishable). >> >> I'm currently trying Prof. Postnikov solution of increasing the super-cell >> size. Now I have set SuperCell_1 = 5; SuperCell_1 = 5; SuperCell_1 = 1. >> Increasing the supercell size has drastically increased the computation time >> (as expected) so I guess it will take a long time to get over. >> >> Is there a way to increase the SuperCell size but not affect the >> computation time drastically? >> >> >> Regards, >> >> Juzar Thingna >> Department of Physics, >> Center for Computation Science and Engineering, >> National University of Singapore. >> >> >> On Tue, Jul 27, 2010 at 5:42 PM, <apost...@uni-osnabrueck.de> wrote: >> >>> > Hi, >>> > Thank you Prof. Postnikov and Mehmet for your quick replies. >>> > >>> > I do get 3 acoustic modes and 3 optical modes as I should since I just >>> > have >>> > a 2 point basis. >>> > The problem is that one of the acoustic mode starts from zero and goes >>> > negative immediately (Graph attached). I have tried increasing the mesh >>> > cut-off as well as optimized the structure using CG. But still I get >>> the >>> > same behavior. >>> >>> Dear Juzar, >>> then it is clear. (You should have described your problem >>> more precisely in the first place, to save some guess work). >>> The trouble is due to >>> insufficient supercell size in your VIBRA calculation. >>> In other words, the force constants >>> are not sufficiently converged in real space. You need to construct >>> a larger supercell, using FCBUILD, then rerun VIBRA. >>> >>> > @ Prof. Postnikov: Sir you mentioned "Go search for a better >>> equilibrium. >>> > Use eigenvectors of instable modes as a guide." Please could you >>> elaborate >>> > on how to use the eigen vectors of the instable mode as a guide? >>> >>> Seemingly, this is not your problem here. >>> However, you may have a look in the following talk >>> >>> http://www.home.uni-osnabrueck.de/apostnik/Lectures/APostnikov-Phonons.pdf >>> - your problem is addressed in pp.18-20. >>> >>> Good luck >>> >>> Andrei Postnikov >>> >>> >>> >> > > > -- > > Mehmet Topsakal (Ph.D. Student) > UNAM-Institute of Materials Science and Nanotechnology. > Bilkent University. 06800 Bilkent, Ankara/Türkiye > Tel: 0090 312 290 3527 ; Fax: 0090 312 266 4365 > UNAM-web : www.nano.org.tr > >
