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Re: [ESPResSo] constraints

From: Olaf Lenz
Subject: Re: [ESPResSo] constraints
Date: Mon, 03 Sep 2007 19:38:46 +0200
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Nils Binz wrote:
> Ich wollte mal nachfragen, aus welchem Grund du in der "constraint_collision" 
> in polymer.c die folded-positions der Teilchen ueberpruefst, wir hatten hier 
> naemlich folgendes Problem:
> Eine Wand (constraint) wird ganz an den Rand der Box gesetzt und nun wird ein 
> Polymer aufgesetzt. Wenn das Polymer die Wand ueberschreiten wuerde, 
> ueberschreitet es gleichzeitig auch die Box, die folded-positions der beiden 
> kritischen Teilchen sind also auf der gleichen Seite der wall obwohl sie die 
> constraint verletzen. 

Probably it is useful to rehearse the discussion on the developers list
on "Nsquare cellsystem and folded positions": The problem is, that the
constraints so far used the "internally folded" coordinates to check
whether a constraint was violated or not.  The internally folded
coordinates depend on the used cellsystem.

Consequently, the different constraints worked differently for different
cellsystem, which definitely can not be a good solution.
In your case of a wall on the border of the system, this had the effect
that the constraints worked in the way you described: when a particle
walks over the wall, the constraint was violated. This was, however,
more or less a bug. The system should behave periodically: when a
particle is moved over the limits of the box, it is folded, and the
coordinates have to be evaluated according to the folded position, and
not according to the absolute position - and even less according to the
internally folded position.

Also, think of the strange behaviour of your system: when you put a wall
directly on the border of the system, particles can get pretty close to
the border, which means that they effectively interact with the
particles close to the border on the other side, i.e. you get periodic

I think the correct solution would be not to put the wall on the border
of the system, but to leave some safety margin to the border to catch
the particles that jump over the wall.

Best regards
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