how lammps calculates forces on groups

Dear Users

If I apply a velocity to move a group of atoms (CNTLeft) at the left end of a CNT (say 3 rings) (and fix the other end).

Will the force given by, say compute reduce sum fx on this group, include the forces exerted by the right end (remaining part) of the CNT. If it does then the net force outputted should be zero. If it doesn’t, I should get the tensile force (the force that the other side exerts on the left side). I am wondering which is the case?

Many thanks


You don’t say how you are applying a velocity.

If you aren’t doing anything to the forces on the
atoms in the group (e.g. fix setforce), then
they will have the force due to all the other
atoms acting on them. You can also look

at fix store/force which will archive the forces due
to other atom before

you (possibly) modify them. So you can do a compute
reduce on those forces if you prefer.