[Wien] re : Supercells - what is the right procedure to create an acceptable one?

[L. D. Marks]

Three comments:

1) An hour is not long - expect the minimization to take days.
2) It looks like you are using the old NEWT method rather than the
suggested Quasi-Newton method PORT. The latter traps this, although see
below.
3) You need to use a smaller RMT when doing a minimization, something like
10-20% smaller than a reasonable estimate of how close the atoms will
come. To explain why, during the minimization the atom positions are
adjusted and the algorithm might explore configurations where the atoms
are very close. (It's only an algorithm, it does not know about bond
lengths.) The PORT algorithm will test for overlapping spheres and reduce
the distance it moves the atoms (issuing a warning) whereas NEWT will
terminate with an error. If this only happens once or twice it is not a
problem, but if it keeps on happening the minimization will get very
inefficient and take a long long time -- and may not even work. (Think
about trying to minimize the positions of hard-wall spheres which cannot
overlap; you can get trapped into local minima.) By reducing the RMT's
(accepting the longer time per iteration) you prevent this happening and
the minimization will work better.

On Tue, 15 Jun 2004, George Amolo wrote:

> Dear Wien2k users,
>
> I am a fairly new user of the Wien2k code trying out some calculations of
> dopant impurities in a diamond lattice supercell. The users forum has been
> of immense help to me. I have followed with interest suggestions of Steven
> Homolya (21st - 23rd October 2004) on how to fix hand-made struct files
> and hence modify them to supercells. The last email, 23rd October, has a
> caution from Jorrisen Kevin about the CXZ-bug if one accepts all the
> output from sgroup. Would someone out there kindly outline in simple terms
> what should be done.
>
> _______________________________________________
> Wien mailing list
> Wien at zeus.theochem.tuwien.ac.at
> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
>

-----------------------------------------------
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2225 N Campus Drive
Northwestern University
Evanston, IL 60201, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
mailto:L-marks at northwestern.edu
http://www.numis.northwestern.edu
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