Re: [Discus-announce] DISCUS mmc module

[<rei...@fa...>]

Hi Charles,

Your request is unfortunately not matched by any command within mmc...
The limited range refers to a range of atom numbers. As these atom
number  will usually not  be sorted by distance from the center the
limitation does not help. In  addition this is a hard limitation,
such that atoms that are outside the  range are not taken into
account at all.

Now, DISCUS has grown in part due to these kind of user inquiries.
Thus, lets  specify a bit more in detail the mmc simulation you
have in mind.

Would a mode suite you in which the target value changes linearly(sloped)
between two limiting values over a range of distances from a
user specified point in the crystal? See the graph for an example
I have in mind.
For a linear slope one would have to specify:
   value at center
   value at "infinity"
   distance of first corner to the center
   distance of second corner to the center
similarly, a sloped (tanh(distance)) function would need
   value at center
   value at "infinity"
   distance of inflection point from the center
   distance of a "quarter change" point from the center

The tanh nicely does a S-shaped slope, does not have the sudden changes
as a linear model would be, BUT as the two curves blue and black show,
with a smaller slope, i.e. short distance of the "quarter" point to the
center, the actual value at the center will be higher than  the specified
value unless I do a lot of adjustments.

Best
Reinhard Neder





> Hi,
> We are interested in doing a Monte Carlo displacement to a target value that depends
on the radial distance from the center of a spherical crystal. It appears that the
'mmc set limited' command might be appropriate for restricting the relaxation to a
limited region about a specific lattice site in our sample. However, it is not clear
to us what the interval [<nmin>:<nmax>] refers to in the help menu (distance, atom
index, etc?). Any help would be appreciated. Thanks for any help.
> Charles
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-- 
Prof. Dr. Reinhard Neder
Kristallographie und Strukturphysik
Universität Erlangen
Staudtststr. 3; 91058 Erlangen
tel. +49-9131-8525191
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