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[ESPResSo-devel] harmonic potential

From: Axel Arnold
Subject: [ESPResSo-devel] harmonic potential
Date: Tue, 21 Oct 2008 10:04:44 +0200
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Hi all,

currently, there is a problem with the harmonic potential on multi-processor 
systems: to calculate any bonded potential, both bonding partners need to be 
on the same processor, at least as a ghost. To ensure this, the maximal 
cutoff needs to be larger than the distance of the two bonding partners. For 
the harmonic potential, this distance can in general be infinite; but 
currently, the maximal cutoff is only set to the _minimum_ of the harmonic 

There are some solutions:
- use the skin to increase the max_cutoff: Bad idea, since Espresso will then 
create huge Verlet lists, and on the other hand, particles can move at most 
skin/2 before the ghost shells are recalculated, leaving essentially the same 
- give the harmonic potential also a breaking distance, not only a minimum: 
that is, we specify a second length, after which the potential breaks. That 
will however change the syntax of the harmonic potential, although we could 
provide a default (e.g. 2*r). In addition, we could also provide a switch to 
turn off breaking. At least on single processors, a harmonic bond would never 

Any votes? Suggestions?


Dr. Axel Arnold 
Fraunhofer SCAI
Schloss Birlinghoven, 53754 Sankt Augustin, Germany
Tel: +49 2241 14 2575

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