how lammps calculates forces on groups

Steve,

Thanks for your reply.
I have a CNT. I hold a few rings of CNT atoms at the right end (groupRight) using fix setforce 0 0 0. I then apply fix move linear to some rings of atoms on the left end (groupLeft)
Will “compute groupRight reduce sum fx” and “compute groupLeft reduce sum fx” both give the tensile force?
I’m just looking for the tensile force in the CNT.

Thanks very much

Daniel

Steve,

Thanks for your reply.
I have a CNT. I hold a few rings of CNT atoms at the right end
(groupRight) using fix setforce 0 0 0. I then apply fix move linear to some
rings of atoms on the left end (groupLeft)
Will "compute groupRight reduce sum fx" and "compute groupLeft reduce sum
fx" both give the tensile force?

like the documentation says, compute reduce sum will sum up the per
atom property across all atoms in that group. whether this is the
force you are looking for depends on your overall setup. you can
usually resolve this with some simple classical mechanical
considerations on a piece of paper.