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[ESPResSo-users] Inside Constraint Forces

From: Salvador H-V
Subject: [ESPResSo-users] Inside Constraint Forces
Date: Wed, 11 Nov 2015 13:27:05 -0600

Dear All,

Sorry to bother you (again) with the constraints topic, but I am facing some unexpected  troubles with the rhomboid constraint.

I want to put my particles inside a parallelepiped channel along the x-direction. 
For that, I was using  four wall constraints:

% Planes in z parallel to xy-plane
constraint wall normal  0  0 -1 dist [expr { -($bl*0.5 + $ancho*0.5) }] type 1
constraint wall normal  0  0  1 dist [expr {   $bl*0.5 - $ancho*0.5  }] type 1

% Planes in y parallel to yz-plane
constraint wall normal 0  -1  0 dist [expr {-($bl*0.5 + $ancho*0.5) }] type 1
constraint wall normal 0  +1  0 dist [expr {  $bl*0.5 - $ancho*0.5  }] type 1

When I run the simulations, the constraint force inside the channel is similar to
#  z
constraint: 0.0 0.0 230301.3951708223
constraint: 0.0 0.0 -222545.7573551011

# y
constraint: 0.0 227151.0143805159 0.0
constraint: 0.0 -223949.8044180491 0.0

which I believe are reasonable values. Then I found the rhomboid constraint, thus, 
I decided to move and use this suited channel

In my script I have something like this:
part $i pos $posx $posy $posz type 0 fix 0 0 0
inter 0 0 lennard-jones $lj_ener $lj_sigma $lj_rcut $cshift $lj_roff

inter 0 1 lj-gen $lj_ener $lj_sigma $lj_rcut $cshift $lj_roff $e1 $e2 $b1 $b2
constraint rhomboid corner 0 $esq $esq a $bl 0 0 b 0 $factor 0 c 0 0 $factor direction -1 type 1

In contrast with the cyIinder and wall constraints, I can visualize correctly the rhomboid channel, but during the simulations the constraint force obtained with the command  [constraint force 0] is only along the z-direction. Something like this,
constraint force: 0.0 0.0 -4726.833698873771

I was expecting something similar to what I obtained with the cylinder constraint:
constraint force: 4.614819468504899e-10 -4030.584976737123 3630.226083974555

Does any body has experiencing a similar problem?  In fact, my particles inside the rhomboid channel exhibit overlapping with the channel after few integrate steps .

Any suggestion would be greatly appreciated.

Thanks a lot in advance



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