Re: [ESPResSo-devel] Nsquare cellsystem and folded positions

[Axel Arnold]

On Monday 13 August 2007 14:42, Olaf Lenz wrote:
> Hello!
>
> I've noticed a strange problem concerning the "nsquare cellsystem" and
> the folded positions.
> Attached, you'll find a simple script that sets up a simple system with
> the nsquare cellsystem, creates a single particle and lets it diffuse
> for 10000 timesteps.
> After this, it prints out the folded position of the particle.
> Unfortunately, the position is not actually folded, but it lies outside
> the system boundaries.
>
> Can somebody locate the problem in the Espresso code? I do not see, how
> the position folding and the cellsystem are connected in any way...

Hi all,

just to make that clear: in fact, the name "folded position" is misleading. 
This is the position as folded as the cellsystem needs it.

In case of the nsquare cellsystem, that means the absolute position, because 
it doesn't fold at all. Wouldn't make much sense. But also 
domaindecomposition does not always fold. Just let a particle move out of the 
box by less than the skin, e.g. with the following example:

setmd box_l 5 5 5
part 0 pos 0 0 0 v -0.1 0 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]
}

You will see positions up to 0.5 out of the box. Only after going past skin/2, 
domaindecomposition refolds (at least we have skin/2; some quite famous code 
had skin there ;-) ).

Cheers,
Axel

-- 
Dr. Axel Arnold
FOM Institute for Atomic and Molecular Physics
Kruislaan 407                                   Phone: +31 20 6081 275
1098 SJ Amsterdam, The Netherlands              E-mail: address@hidden