[ESPResSo-devel] Cellsystem, folded position, constraints (was: Nsquare cellsystem and folded positions)

[Ulf Schiller]

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Hi,

Axel Arnold wrote:
> On Monday 13 August 2007 16:26, Olaf Lenz wrote:
>>Axel Arnold wrote:
>>>Why not????? For forces, energy etc. the distances are correctly folded,
>>>as far as I can remember.
>>Hmm, apparently you are not of the same opinion as Hanjo in that
>>respect. However, my tests showed that at least for constraints, the
>>force computation does not work correctly. I'll provide an example
>>script soon.
> 
> Ok, to clarify that: The pair forces use the minimum image distance, both in 
> domdec and nsquare. For the constraint forces, for some reason we use the 
> internally folded coordinates. That can indeed cause a problem, but actually 
> for both cellsystems. E.g. consider an excluded sphere close to the boundary. 
> Again, the example:
> 
> setmd box_l 5 5 5
> part 0 pos 0 0 0 v -0.1 0 0
> 
> inter 0 0 lennard-jones 1.0 1.0 1.12246 0.25 0
> 
> constraint sphere center 4 0 0 radius 1 direction outside type 0
> 
> setmd time_step 0.01
> thermostat off
> setmd skin 1
> for {set i 0} {$i < 100} {incr i} {
>     integrate 10
>     puts [part 0 pr folded]
> }
> 
> The particles goes unperturbed right into the sphere, then gets folded, and 
> Espresso crashes :). And that is domaindecomposition.
> 
> So, we have a bug treating constraints, but that actually applies to all 
> cellsystems.

Another issue about constraints is whether they have periodic images.
E.g. in case of walls, a common tutorial example is that one can
simulate channel flow by putting a single wall into a system with
periodic boundary conditions. This is not the case in ESPResSo. This
might be a source of confusion (actually it was already).

Cheers,
Ulf

- --
Ulf D. Schiller   *   Room 1.404   *   Phone +49 6131 379-481
Max Planck Institute for Polymer Research
Theory Group
D-55128 Mainz, Germany                   50° 0' N, 008° 16' E
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