I'm doing MD calculations and I am having trouble providing a satisfactory explanation for the following to my profs. I am trying to calculate c44 for a diamond cubic crystal (Si), and to do that I have to calculate energy vs shear strain. For each value of the shear strain, I have to do relaxation (eg. conjugate gradient) because forces appear on the atoms. What I am asked to do is to explain why there are forces on the atoms. "A shear strain reduces the symmetry of the crystal" is deemed to be an insufficient reason for this. (Background: my profs have little background in solid state physics ). Right now I am thinking that I will prove it by calculations i.e. i consider the coordination polyhedron for any Si atom (which is a tetrahedron), and show that shearing this will result in a net force on the atom. Is there any other way that I can convince them? eg. analogous examples using bathroom tiles?