[Wien] structure relaxation requirement for input atom positions

[Laurence Marks]

I don't think you have posed the problem well. There are really two problems:
a) How to find a local minimum from some initial starting point.
b) How to find the global minimum, i.e. the lowest of all possible minima.

A good refinement code will always solve a) and should never crash. It
should always find the "right" positions in the sense that they are
the local minimum in some basin. The minimizer in Wien2k does this
fairly well, with the important caveat that if the DFT problem is
poorly posed/conditioned (e.g. bad RMT choices) it may not be too
accurate.

Doing b) is harder mainly because it takes orders of magnitude more
computer time, in most cases more computer time than is available
except for very simple systems. Simulated annealing or genetic
algorithms coupled with a local search are the two standard
techniques. In some fashion one has to search over starting points to
find the lowest possible energy among all the different local minima.
This is not implimented in WIen2k in any automated fashion.

Cases where DFT does not find the "right" positions are either because
the initial starting point was close to some local minimum, but not
the global minimum or because of the fact that DFT energies are far
from perfect. How big the basin is around each local minimum varies
from structure to structure, so it is almost impossible to generalize.
Sorting this out is not something that (in most cases) a computer can
do for you, you have to use the most important scientific tool above
your eyes.

On 1/17/07, qiang <q.xu at tnw.tudelft.nl> wrote:
> Dear all:
>
> As we know that given a starting structure model, molecular Dynamics can be
> used to refine atom to their energy (force) equilibrium position.
> Obviously, if the starting structure model is too far away from the correct
> one,
> the refinement may crash or may not lead to the right positions.
> For a successful structure relaxation, does anyone know the general
> requirement for a starting structure model?  How close to the correct should
> a starting structure model be? (e.g. the starting atom positions should be
> within 0.5 A away from the correct positions.)  Is there any ref. about this
> problem?
>
> Best regards
>
> Qiang
>
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>


-- 
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60208, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L-marks at northwestern dot edu
http://www.numis.northwestern.edu