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 > > >
