[Pw_forum] Error in vc-relax

[Axel Kohlmeyer]

On Mon, 17 Sep 2007, Paolo Giannozzi wrote:

hi paolo,

PG> Since complaints about the variable-cell optimization
PG> show up more often than not: can anybody provide a
PG> complete test job (i.e. one that can be rerun), with

my impression is more often, that this is a manifestation
of vc-relax being a less straightforward to use method and
that people don't always understand all the involved subtleties.
as a consequence of that, one is tempted to use vc-relax
in a way that is not working well, or for calculations 
(lattice constant determination) where there may be other
more efficient and reliable ways to obtain this information.

i can sympathize very much with these ideas, since i had
them all myself and it was a (sometimes painful) learning
process to get a better understanding of where i was 
going wrong.

PG> complete output, showing a problem in a reproducible
PG> way, in a reasonable amount of computer time, in a
PG> recent versions of Q-E?

to some level, we'll just have to get used to people trying
things with vc-relax that cannot work (you have to get
burned to stay away from the fire!) or do not work in the
way they hoped for. other than that a few more extensive 
explanations, pointers to (more) commented tutorial examples, 
also in the INPUT_PW file and perhaps an adjusted default for 
the cell mass (as cesar has pointed out a couple of times), 
could handle the majority of the cases.

cheers,
   axel.

p.s.: i agree with your assessment, that when somebody posts
an incomplete input, that the chance of somebody completing it
and using it for testings is (almost) zero. posting only an
input fragement, should be ok (and saves a lot of bandwidth and
storage), when people are not certain about their input flags
altogether, but if a problem persists, there is not alternative
to making the whole input available, so that somebody can (try to) 
re-run the calculation elsewhere.

PG> 
PG> Paolo
PG> ---
PG> Paolo Giannozzi, Dept of Physics, University of Udine
PG> via delle Scienze 208, 33100 Udine, Italy
PG> Phone +39-0432-558216, fax +39-0432-558222
PG> 
PG> 
PG> 
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PG> 

-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
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If you make something idiot-proof, the universe creates a better idiot.