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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
this:
#  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

and
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

Salvador

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