# Problem with Angular velocity in 2D simulation

Dear Users,

I am simulating a spherical colloids​ system in 2D. Basically I am using a Sphere atom_style with nvt/sphere integration with disc command( I am doing it in 2D). I have applied an additional force on each particle to make it a self propelled discs as well. I am trying to account for the rotational motion here. But the issue is that when I am printing the ‘omega’ ( by using dump command) it is giving zero value through out the simulation. What can be the cause for this? How can I resolve this problem…

-Rahul S

There is not enough information here to give meaningful advice.

What should be causing a torque and are you using the suitable time integration fix?

In all your previous posts there was no indication of any of this.

Axel

Actually I am simulating a colloid particle in 2D. The atom_style used is sphere for colloids. I am adding a additional force as cos(theta) and sin(theta) along X and Y directions (where ‘theta’ in the fix is defined as omega*timestep used) using a modified version of the fix linearforce script. So basically there will be a rotation due to this additional force. I am using the fix nvt/sphere which will update the angular velocity and I am using this omega to get ‘theta’ as above. The theta will be calculated for each atoms and then the additional force is added by using fix_addforce which will add a constant force and this force is multiplied by the cos(theta) and sin(theta) terms to get the X and Y components of force in the new fix. So basically the particle will have a self propulsion. as result it will produce some kind of rotation. But I have tried to calculate omega using compute omega command and also tried to print omega directly by using the dump command. The value of omega I am getting is zero. So I am confused like what is the problem with this…

Actually I am simulating a colloid particle in 2D. The atom_style used is sphere for colloids. I am adding a additional force as cos(theta) and sin(theta) along X and Y directions (where 'theta' in the fix is defined as omega*timestep used) using a modified version of the fix linearforce script. So basically there will be a rotation due to this additional force. I am using the fix nvt/sphere which will update the angular velocity and I am using this omega to get 'theta' as above. The theta will be calculated for each atoms and then the additional force is added by using fix_addforce which will add a constant force and this force is multiplied by the cos(theta) and sin(theta) terms to get the X and Y components of force in the new fix. So basically the particle will have a self propulsion. as result it will produce some kind of rotation. But I have tried to calculate omega using compute omega command and also tried to print omega directly by using the dump command. The value of omega I am getting is zero. So I am confused like what is the problem with this...

if it is only your custom fix that would add torque, and that is not
having an effect, then the cause *must* be somewhere in your fix
modifications. that makes it your personal problem and - in essense -
a code debugging problem. nobody will be doing this for you and it is
completely impossible with just a vague description of what you have
done.

axel.

Actually I am simulating a colloid particle in 2D. The atom_style used is sphere for colloids. I am adding a additional force as cos(theta) and sin(theta) along X and Y directions (where

... and to clarify: pair_style colloid does *not* require atom style
sphere. it works just fine with atom style atomic. by just changing
the atom style you are not changing the interactions.

axel.