# Zero the Angular Momentum

Hi All,

In order to better understand MD algorithms, I am writing an MD engine. An area that I am a bit stumped is how to zero the angular momentum of a collection of particles. I understand how to zero the linear momentum but not angular. I’ve looked at the lammps code but I’m still a bit confused. Is anyone able to give me the basic idea of zero’ing the angular momentum?

Thanks,
Sara

Hi All,

In order to better understand MD algorithms, I am writing an MD engine. An
area that I am a bit stumped is how to zero the angular momentum of a
collection of particles. I understand how to zero the linear momentum but
not angular. I've looked at the lammps code but I'm still a bit confused.
Is anyone able to give me the basic idea of zero'ing the angular momentum?

like i mentioned in a previous e-mail with a similar intent: this
mailing list is not a classroom. if you have a specific problem with
lammps or its implementation, feel free to ask a specific question.
"can somebody give me the basic idea?" is not it. that is something
you need to discuss with your adviser/teacher or pick up from suitable
text books.

why don't you just pick up a good text book about classical mechanics?
for your intents and purposes, a collection of particles is nothing
else than a rigid body. the rest should be straightforward and on the
level of high-school physics (well, at least that is how i remember
it. it has been a while since then).

axel.

In the Velocity class (velocity.cpp) there is a

zero_rotation() method which does it for a group of
atoms.

The basic idea is that you pick a center point,
typically the center-of-mass. Then compute the
current angular momentum around it and convert
to omega (angular velocity). Typically the net omega should

be tiny if the velocities of the atoms were created
randomly. Then you can subtract that omega off

of each atom, which is a component of its velocity
perpendicular to its vector to the center of mass point.

Steve