Space Group Symmetry
The arrangements of molecules in a unit cell are described by space group symmetry. These symmetry groups include translational symmetry elements (screw axes and glide planes) as well as the non-translational elements already dealt with in point groups. The space group label begins with an upper case letter indicating the lattice type (P, A, B, C, I, F) followed by the characteristic symmetry elements. (To this point, only P type cells have been discussed, the P standing for “primitive”, i.e., cells with lattice points only at the corners of the unit cell; the other lattice types will be surveyed later). Thus, the triclinic crystal class has just two space groups: P1, which has no symmetry elements, and , which has only a center of symmetry.