Past exam of the condensed matter physics major option course of the University of Oxford / 2018 / 1 / b / ii

To obtain a crystal's point group, discard the translation parts of its space-group operations. A screw becomes a rotation and a glide becomes a mirror. Thus the tetragonal group's $4/n$, $2_1/m$ and $2/m$ operations giveFor the orthorhombic structure the three perpendicular twofold/glide or screw/mirror directions giveThe primitive Bravais lattice symbol does not itself add a point operation; the fractional translations matter for the space group but disappear from this point-group reduction.