Now I am running a bending test by applying a bending deformation on the sample ends. I am wondering how to calculate the bending moment response on the sample ends.
How about dump the atoms at both ends. Compare their coordinates
in the bending direction. Calculate the difference.
What you said is a good method to calculate the bending angle (displacement), while I would like to calculate the bending moment (force) as a response to the applied bending angle.
On an arbitrary plane with arbitrary forces, as a general example, all the forces can be expressed by equivalent a force and a moment. The force is associated with the normal and shear stresses, which are pressure on the surface in LAMMPS. However, I am still wondering how to calculate the moment from the atom forces.
You can also dump the forces on any atoms. Or you can sum the
forces in LAMMPS on any group of atoms. Or multiple groups