On Wed, 23 Mar 2011 17:33:02 +0100, Eric Germaneau <germaneau at gucas.ac.cn> wrote: > I'm trying to optimize unit cell parameter of graphite. > The problem is the c axis gets too long. > Starting from published data my initial parameters are a=4.59203 Bohr > and c=12.513 Bohr. > The final values are 4.664 and 15.829 respectively. > I need someone to explain to me what I did wrong, I've attached my input > file.
Dear Eric, your input atomic positions are only specified to 4 significant digits: 0.3333 is not really 1/3. Hence you are loosing 18 out of 24 symmetry operation: I would suggest increasing the precision to 6 or 7 digits to recover them. With more symmetry you will be able to afford more kpoints, as graphite can be tricky to converge a 4x4x4 grid can not insufficient. I also doubt that you need 80Ry to converge an ultrasoft pseudopotential, but I assume you did your tests. Finally, your results seem to be in accord with other DFT calculation; considered you're using GGA they are actually quite good. In LDA the usual overbinding partially compensates for the lack of VdW interaction, while this effect does not occur for gradient-corrected functionals. best regards -- Lorenzo Paulatto (IdR) IMPMC - CNRS UMR 7590 & Universit? P&M Curie T23-C13/23-5e27 - 4 place Jussieu - 75252 Paris Cedex5 phone: +33 (0)144 27 5211 www: http://www-int.impmc.upmc.fr/~paulatto/
