Re: [ESPResSo-users] How to adjust reflecting constraints in version 4.0.1

[Jean-Noël Grad]

Hi,

The feature was removed due to bugs and energy conservation issues. A 
brief discussion can be found in these two threads:
https://github.com/espressomd/espresso/pull/1952#issuecomment-395213683
https://github.com/espressomd/espresso/pull/2010
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I'll open a ticket to clear the docs from outdated files.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,
JN

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
>
> http://espressomd.org/html/doc/inter.html
>
> 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
> range."
>
> 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,
> David.
>
>
>
>
> 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:
> > http://espressomd.org/html/doc/constraints.html and there is a sample
> > script /samples/constraints.py 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?
> >
> > David
> >
> > 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,
> > > David.