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## Re: [ESPResSo-devel] accessibility of p1->r.p and p1->l.i on ghost cells

 From: Ivan Cimrak Subject: Re: [ESPResSo-devel] accessibility of p1->r.p and p1->l.i on ghost cells Date: Sun, 25 Jan 2015 14:05:39 +0100

```Hi Axel,

> Yes, the image is not set for ghost particles. Even worse, p1->r.p is shifted
> so that the Euclidian distance can be used for particles in the primary cell.
> That is, ghost particles on the left of the simulation box have their x
> coordinate shifted by -box_l[0], those to the right by +box_l[0]. But their
> p1->l.i is not updated when doing this shifting, so you would get wrong
> unfolded coordinates.

Ok, so the image is not set for ghost particles, I get it. But for two
particles with bonded interaction I can get correct unfolded
positions, right? When evaluating bonded forces, one gets always the
first particle which is not a ghost particle. Am I correct? And
unfolding this non-ghost particle give us its unfolded position.
Afterwards, using relative positions to other neighbors of bonded
interaction (which might be ghost particles) we get unfolded position
of all neighbors from bonded interaction...

Imagine
set box_l 10 10 10
part 0 pos 9 5 5
part 1 pos 9.5 5 5
inter 0 fene 1.0 1.0
part 0 bond 0 1

Let us move particles p0 and p1 to the right so that x coord of p0 is
e.g. 9.8 and x-coord of p1 is 10.2. When evaluating fene forces, we
get p0 from inside simulation box and we can unfold position of p0.
Using get_mi_vector and p0->r.p and p1->r.p we get unfolded position
also for p1, right?

Thanks,
Ivan

>
>
>> If yes, this practically means that I can not unfold the position of
>> particles from ghost cells? (Because I do not know its img needed for
>> unfold_position).
>
> Exactly, as I explained, there are even more reasons why there is no way to
> get the unfolded position of a ghost particle.
>
> Best,
> Axel
>
> ------------------------------------------------
> JP Dr. Axel Arnold
> ICP, Universität Stuttgart
> Allmandring 3
> 70569 Stuttgart, Germany
> Phone: +49 711 685 67609
>

--
Ivan Cimrak
FRI Zilinska Univerzita v Ziline
personal web: http://www.kst.fri.uniza.sk/~icimrak/
cell-in-fluid group: http://cell-in-fluid.fri.uniza.sk

```