Bipul, Definitely one of the reasons can be the use of the XC functional. When I was working with BaO in different phases, LDA would give me the wrong sequence of energetics at zero pressure, while GGA would give a correct prediction (paper is listed in the publications section of the siesta homepage). You will probably get the correct qualitative behavior for energetics with GGA.
If you want to have trustworthy results, better use highly converged results for cutoff and k-points: usually the energy difference between two phases as predicted by DFT is small, of the order of a (very!) few tens of meV. So, I would check that the difference of energies with respect to both mesh cutoff and k-points would be less than the smallest energy difference between two phases. For metals, I guess the main points to be careful about are: 1) k-points, when using a MP grid, should be extremely well-converged. Usually you will have to go to a pretty dense MP grid (larger than 6x6x6, perhaps). Unfortunately siesta doesn't allow one to specify arbitrary k-points, in which case one could perhaps use the Baldereschi points. That is, if they work for metals as well as for insulators, which I don't know exactly. 2) The electronic smearing should be as low as possible. Marcos >> but somehow i get beta phase stable than alpha phase.... >> while in literature it is reverse.... > > I'm not sure of this is the case for Sn, but for Fe, for example, the > reason is the use of LDA instead of GGA. > Also, check the convergence vs. energy cutoff and k-points and try the > optimized basis for Sn. > -- Dr. Marcos Verissimo Alves Post-Doctoral Fellow Condensed Matter and Statistical Physics Sector International Centre for Theoretical Physics Trieste, Italy -------- I have become so addicted to vi that I try to exit OpenOffice by typing :wq!
