This chapter describes the factors involved in deciding which of the 230 space groups is correct for a particular crystal structure, based on crystallographic experiments and chemical knowledge. Chiral materials can belong to only certain space groups; optical microscope observations can provide some information on crystal systems. The main evidence comes from the unit cell metric symmetry, diffraction pattern Laue class, systematic absences, and statistical tests on the distribution of diffraction intensities. The calculation of unit cell contents, based on measured or approximate density and unit cell volume, can provide information on possible molecular symmetry when special positions are occupied, and on the possible presence of solvent molecules. Space group determination is complicated by crystal twinning. An introduction is given to the information in International Tables for Crystallography, Volume A.
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