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

From: Axel Arnold
Subject: Re: [ESPResSo-devel] accessibility of p1->r.p and p1->l.i on ghost cells
Date: Sat, 24 Jan 2015 21:44:47 +0100

> On 24.01.2015, at 17:14, Ivan Cimrak <address@hidden> wrote:
> Hi,
> I have a question regarding computation of unfolded position. In the
> code, this unfolded position is computed by unfold_position(pos,img),
> where pos is obtained from p1->r.p and img is obtained from p1->l.i.
> In Espresso code documentation it is written, that p1->l is a
> structure holding "Information on a particle that is needed only on
> the node the particle belongs to". Does it mean, that for particles
> from ghost cells I can not access p1->l structure?

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.

> 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.


JP Dr. Axel Arnold
ICP, Universit├Ąt Stuttgart
Allmandring 3
70569 Stuttgart, Germany
Email: address@hidden
Phone: +49 711 685 67609

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