Re: [ESPResSo-devel] A bug in Espresso

[Ulf Schiller]

Hi Ivan,

On 06/29/2012 01:37 PM, Ivan Cimrak wrote:
> Hello,
>
> I have been using Espresso quite some time. Together with Markus
> Gusenbauer from FH Sankt Poelten we have done some extensions. During
> my tests I found out that the version 3.0.2 does not compute the same
> results as the 3.1.0 does.
>
> I did the following test:
> 2 particles bonded with harmonic interaction. I put them in the
> lbfluid, I set the velocity of the fluid on the left side of the
> channel  and I look at the positions of both particles and their
> distance. I compiled two versions from your sources available at
> http://espressomd.org/wordpress/download/. In both cases I compiled
> only the LB and LB_BOUNDARIES in. With these two versions I obtained
> different results.
>
> I attach two output files.
>
> Do you have any idea why is it so?

I had a quick look at your data and at first glance it seems there is 
only a numerical difference. Do you think the results are physically 
incorrect? The difference seems to be that the coupling between 
particles and fluid sets in at different time steps (i=11 in 310.dat and 
i=12 in 302.dat). This could be due to round-off differences in the two 
versions of ESPResSo. The results should converge for dt->0.

Hope this helps,
Ulf

> TCL code:
> set vmd "n"
> setmd time_step 0.1;
> setmd skin 0.2; 
> thermostat off;
>
> set pipeX 50; set pipeY 20; set pipeZ 20;
> setmd box_l $pipeX $pipeY $pipeZ;
>
> part 0 pos 10 10 10 type 1;
> part 1 pos 15 10 10 type 1;
> inter 0 harmonic 0.1 5.0;
> part 0 bond 0 1;
>
> cellsystem domain_decomposition -no_verlet_list
> lbfluid grid 1.0 dens 1.0 visc 1.0 tau 0.1 friction 1.0
>
> set upscale 5.;
> if { $vmd == "y" } { prepare_vmd_connection simEspresso 3000 1
> $upscale; exec sleep 1; imd positions $upscale; }
>
> set fp [open "output.dat" "w"]
> set i 0
> while { $i<  500 } {
>         puts "$i";
>      if { $vmd == "y"} { imd positions $upscale; }
>
>         for { set ii 0 } { $ii<  1} { incr ii } { for { set j 0 } { $j<
> $pipeY } { incr j } { for { set k 0 } { $k<  $pipeZ } { incr k } {
>                                         lbnode $ii $j $k set u 0.5 0.0 0.0;     
> } } }
>         set aa [part 0 print pos]; set coraa [expr [lindex $aa 0]];
>         set bb [part 1 print pos]; set corbb [expr [lindex $bb 0]];
>         
>         puts $fp "$i:   $coraa, $corbb, [expr $corbb - $coraa]";
>      integrate 1;
>      incr i
> }
> close $fp;
>
>
> -------------------------
> Dr. Ivan Cimrak
> Dep. Software Technologies, University of Zilina, Slovakia
> web: www.kst.fri.uniza.sk/~icimrak

--
Dr. Ulf D. Schiller                        Building 04.16, Room 3006
Institute of Complex Systems (ICS-2)       Phone:   +49 2461 61-6144
Forschungszentrum Jülich, Germany          Fax:     +49 2461 61-3180

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