# Rigid body in PBC simulation box

Dear LAMMPSers,

I am trying to simulate a rigid cylinder in a PBC simulation box, and using ‘fix rigit/nvt’ command. I created the cylinder with axis along y direction, but the cylinders images (of different periodic boxes) move separately as rigid bodies; what I am trying to do is to make all the cylinder periodic images as one rigid body and move together. Is there a way to achieve that?

Best Regards,
Z. Q.

Dear LAMMPSers,

I am trying to simulate a rigid cylinder in a PBC simulation box, and using
'fix rigit/nvt' command. I created the cylinder with axis along y direction,
but the cylinders images (of different periodic boxes) move separately as

How is this possible? You are asking the periodic images of one real
cylinder to move separately? These images are identical replications
of the real body, and they certainly can not move separately.

rigid bodies; what I am trying to do is to make all the cylinder periodic
images as one rigid body and move together. Is there a way to achieve that?

If all periodic images move, then the real one moves. If everything
moves together, than it is no different from not moving at all -
simply due to translational invariant.

Ray

Hi Ray,

Thanks for your reply, I probably didn’t make my point clear enough.

Obviously, the images and the real one always move together due to periodic boundary condition; but what I meant is that the real one and images are not fixed together and move by itself individually (i.e. the distance between an image and the real one changes with time). In another word, I am trying to make the rigid body to be ‘bulk’ in space (infinitely long cylinder along y-axis) instead of finite size. Does this make sense? It could be much easier to discuss in person with a piece of paper…

Best Regards,
Z. Q.

Dear LAMMPSers,

I am trying to simulate a rigid cylinder in a PBC simulation box, and using
‘fix rigit/nvt’ command. I created the cylinder with axis along y direction,
but the cylinders images (of different periodic boxes) move separately as

How is this possible? You are asking the periodic images of one real
cylinder to move separately? These images are identical replications
of the real body, and they certainly can not move separately.

rigid bodies; what I am trying to do is to make all the cylinder periodic
images as one rigid body and move together. Is there a way to achieve that?

If all periodic images move, then the real one moves. If everything
moves together, than it is no different from not moving at all -
simply due to translational invariant.

Ray

I am afraid this still does not make sense.

[...]

Obviously, the images and the real one always move together due to periodic
boundary condition; but what I meant is that the real one and images are not
fixed together and move by itself individually (i.e. the distance between an

As you said in the previous sentence, the real body and its periodic
images must move together. How can they move by themselves
individually?

image and the real one changes with time). In another word, I am trying to

The only way to change the distance between an image and the real body
is by changing the periodic box dimension.

make the rigid body to be 'bulk' in space (infinitely long cylinder along
y-axis) instead of finite size. Does this make sense? It could be much

I don't know what "bulk in space" means. Maybe somebody else on the
mailing list know what it means. Anyhow, a tube that extends along
the periodic boundary is considered infinitely long.

easier to discuss in person with a piece of paper...

True, but we are now in the Information Age.

Ray

Hi Ray,

Thanks for your reply, I probably didn't make my point clear enough.

Obviously, the images and the real one always move together due to periodic
boundary condition; but what I meant is that the real one and images are not
fixed together and move by itself individually (i.e. the distance between an
image and the real one changes with time). In another word, I am trying to
make the rigid body to be 'bulk' in space (infinitely long cylinder along
y-axis) instead of finite size. Does this make sense? It could be much

if you want that, why is there space between the images? and how can it change?
sounds that your input configuration is just wrong.

easier to discuss in person with a piece of paper...

well, a rigid, periodically continued body cannot rotate (except
around the infinite axis) so what would be the point of time integrate
it, especially with a thermostat?

axel.

Hi Axel,

Thanks for your input, it could because my configuration is just wrong; I will rethink about the simulation setup.

Best Regards,
Z. Q.

Hi Ray,

Thanks for your reply, I probably didn’t make my point clear enough.

Obviously, the images and the real one always move together due to periodic
boundary condition; but what I meant is that the real one and images are not
fixed together and move by itself individually (i.e. the distance between an
image and the real one changes with time). In another word, I am trying to
make the rigid body to be ‘bulk’ in space (infinitely long cylinder along
y-axis) instead of finite size. Does this make sense? It could be much

if you want that, why is there space between the images? and how can it change?
sounds that your input configuration is just wrong.

easier to discuss in person with a piece of paper…

well, a rigid, periodically continued body cannot rotate (except
around the infinite axis) so what would be the point of time integrate
it, especially with a thermostat?

axel.