Quaternions question

Hello there,

I am working on 2D simulation with rigid particles and extracting the results with quaternions.

I expect the quatk value, which is related to z-axis movement, should be zero or very small values, however the number is not ignorable.

compute q all rigid/local rigid1 quatw quati quatj quatk x y z
dump 1 all local 1000 dump.pour2d.rigid.details index c_q[1] c_q[2] c_q[3] c_q[4] c_q[5] c_q[6] c_q[7]

Can you please give me any comments on it?

data.square (109.5 KB)
in.square (1.4 KB)

I don’t think the quaternions are in the laboratory frame and thus their values are ok. What matters is that omega and the angular momentum are (near) zero for rotation about the x- and y-axis.

Thank you for your reply.

The reason why I am extracting quaternions is to read the morphological features of each rigid particle.
I don’t find the details of quaternions used in Lammps documents. As you said, if those are in other frame, could you provide me any link or details to understand the rotation?

If not available, is there other way to extract the rotation information of rigid particles?

The obvious place to look for such details would be the source code.