[Wien] Slab convergence

[Laurence Marks]

N.B., of course when you compare to the number of k-points for bulk Au
remember to use the primitive cell volume.

On Sun, Sep 12, 2010 at 8:21 AM, Laurence Marks
<L-marks at northwestern.edu> wrote:
> 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.
>



-- 
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.