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Re: [ESPResSo-users] How to adjust reflecting constraints in version 4.0

From: Jean-Noël Grad
Subject: Re: [ESPResSo-users] How to adjust reflecting constraints in version 4.0.1
Date: Mon, 18 Feb 2019 14:55:50 +0100
User-agent: Mozilla/5.0 (X11; Linux i686; rv:60.0) Gecko/20100101 Thunderbird/60.4.0


The feature was removed due to bugs and energy conservation issues. A brief discussion can be found in these two threads:
I am unaware of any plan to re-implement a fixed version of this feature in the short term. You could probably achieve the desired effect with an LJ potential between the shape and particles, using a small sigma to limit the "curving" of trajectories near the shape surface, and using cutoff=sigma to only get the repulsive part of LJ (this is basically what the WCA potential does, which can be enabled during compilation).

The reason we couldn't find the keywords "reflection" and "reflecting" in the documentation is that the url you provided points to an old page which is not referenced by the Sphinx search engine anymore. This page should have been erased a year ago. We apologize for the inconvenience. I'll open a ticket to clear the docs from outdated files.

Best regards,

On 2/18/19 10:29 AM, address@hidden wrote:
Hey David!

Thanks for your answer!
Exactly, what I'm looking for is such a reflect_particle() function
like the one mentioned by you within shaope based constraints.
In the docs it is mentioned at

under point 4.10.1:

"In variants wall; sphere; cylinder; rhomboid and stomacyte it is also possible to specify a flag indicating if the constraints should be reflecting. The flags can equal  1 or 2. The flag 1 corresponds to a reflection process where the normal component of the velocity is reflected and the tangential component remains unchanged. If the flag is 2, also the tangential component is turned around, so that a bounce back motion is performed. The second variant is useful for boundaries of DPD. The reflection property is only activated if an interaction is defined between a particular particle and the constraint! This will usually be a lennard-jones interaction with ϵ=0, but finite interaction

So what I'm trying to do is to implement a cylindrical (shape based) constraint in which the particles are
reflected by the constraint as described above.

However, I haven't found a way to specify a flag as indicated above for shape based constraints.
How would you use the reflect_particle() function which you mentioned?

Best regards,

Zitat von David Sean <address@hidden>:

Hi David,

I am trying to understand what you mean by 'reflecting'. I tried
searching the docs for 'reflect' but could not find it mentioned in
the context of a constraint. Constraints are described here: and there is a sample
script /samples/ that you can look at.

There used to be a reflect_particle() function for shape-based
constraints, did you mean that? Can you describe what you are trying
do, i.e, is this for particles, or an LB fluid... or both?


Le mer. 13 févr. 2019 à 08:07, <address@hidden> a écrit :

Hello everybody!

In my simulation with  ESPResSO 4.0.1 I'm trying to implement a
cylindrical constraint which should be reflecting.
I saw in the docs that this should be possible for the variants wall,
sphere, cylinder, rhomboid and stomacyte.
However, it seems there is no option to include the reflecting flag,
either to "espressomd.shapes.Cylinder" nor to
"espressomd.constraints.ShapeBasedConstraint". In both cases I receive
a "unknown parameter"-error.
Can anyone help me here and explain where to specify the flag?
I also have an activated Lennard-Jones interaction defined between the
particles and the constraint.
I would really appreciate your help.

Best regards,

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