[Wien] Slab convergence

[Laurence Marks]

Some comments:

1. 100 k-points for a surface is a lot. What I suggest you do is
determine how many k-points you need per reciprocal nm^3 (i.e. the
multiple of the 3 numbers after "div:" in line 1 of case.klist and the
cell volume) for a bulk calculation then use the same density for a
surface.

2. Are you using TETRA? I recommend TEMPS for surfaces with a
temperature factor of 0.0018 (room temperature).

3. DO NOT REDUCE THE MIXING FACTOR (better called MIXING GREED) unless
you understand what you are doing. For old PRATT and BROYD methods
this was correct, for MSEC1 it is fundamentally wrong. Too large a
mixing greed (say 0.5) is being too greedy, but the algorithm in fact
prevents this from happening. To small a greed and the algorithm will
starve to death.

4. In 98% of cases where the calculation does not converge this is
because something is wrong in the physics of the model, i.e. bad
functional or incorrect structure. Possibly the Fe atom is too far
from the surface -- have you set FOR in case.in2 and looked at how big
these are? With care, you can run a minimization with something like
-fc 4 -ec 0.001 at first, then improve these later.

5. When you say it is not converging what do you really mean? The
default -ec 0.0001 is very strict for a surface (with incorrect
positions), realise that the energy convergence should scale as
something like the number of atoms (or the square root of this).

6. Are you using 10.1 and iterative mode? 10.1 is noticeably better
and I prefer to use -noHinv.

On Sun, Sep 12, 2010 at 7:26 AM, Lukasz Plucinski
<pluto at physics.ucdavis.edu> wrote:
>  Hello,
>
> I am trying to calculate 1ML of Fe on top of Au(001).
>
> It was no problem to calculate 20ML slab of Au(001), it converged after 37
> iterations with mixing 0.1, 100k-points and all other standard settings,
> also using "spin-polarized" calculation mode. I use 50 A of the unit cell
> dimension, to have appropriate amount of vacuum.
>
> However, when I put 1 Fe atom on top of one side of the slab the calculation
> didn't converge after 100 iterations (I did couple of trials). I also tried
> to increase the cutoff to -8 Ry, and the calculation is running now,
> however, no convergence indications after 35 iterations.
>
> My slab is not relaxed, and the distance between Fe and Au (3.295 au) is
> taken from the old publication of  Freeman JMMM 75, 201 (1988).
>
> Automatic RMT distance procedure has put all RTM to 2.5, however, this way
> there is a lot of space between Au atoms. I think its better to use 2.72 for
> Au atoms and 2.34 for Fe atom -- is there any problem with this ? Neither
> setting helps the convergence...
>
> Maybe I could decrease the amount of k-points to have faster iterations with
> even lower mixing parameter ?
>
> Regards,
> Lukasz
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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
Web: www.numis.northwestern.edu
Chair, Commission on Electron Crystallography of IUCR
www.numis.northwestern.edu/
Electron crystallography is the branch of science that uses electron
scattering and imaging to study the structure of matter.