[Wien] problems with supercells

[Peter Blaha]

> I'm studying the band structure of the Si crystal with Al impurities qith
> Wien 98.7 package and I have tree questions about it:
>
> The last week i send a mail asking for a problem that I have in the SCF
> cycle. It stoped after a few iterations. The solution that you gave me was
> to remove the broyd and scf files, change the broyden factor from 0.4 to 0.1
> and run x dstart and run_lapw again. The Scf cycle then stoped after just
> one iteration and the convergence wasn't reached yet. Again no error
> messages were displayed in any of the error and ouput files. So, what can i
> do?

Restart in a new directory. If this does not work either, we need more info.

> My other question is conserning the physical problem: In the structure file,
> How do I set the RMT fpr the Si and the Al?. I heard that there is a problem
> with the difference of the atomic radii when one works the impurity problem
> in a LAPW based program.

First guess: use equal radii. But read the faq page at www.wien2k.at

> My last question is about performance of the calculation. I work with a
> supercell of 64 atoms and it takes 3 days for just one iteration (even when
> I work in a good platform: Intel Xeon 1.5GHz with 2GB RAM). I compiled the
> program with the -O3 option for the f77 compiler and force an iterative
> diagonalization with the -it flag in the run_lapw. Should I reduce the size
> of the matrices in the Global parameters?. You also advised in the manual to
> compile with and option depending of the particular arquitecture. What is
> this option for the platform that i just mentioned.

You probably used too many k-points ? At least at the beginning of an scf-cycle
one !!! k-point should be enough for a 64 atom Si supercell. Lateron you may
check k-point convergence.

Also upgrading to WIEN2k may speed up things dramatically since this allows
for a smaller RKMAX and thus reduces the necessary matrix size.

Regards
                                      P.Blaha
--------------------------------------------------------------------------
Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
--------------------------------------------------------------------------