An explanation for the Rule of Four in Inorganic Materials

Recently, Gazzarrini et al. observed a ‘rule of four’, where the number of atoms in a primitive unit cell for inorganic materials taken from large databases has been observed to favour multiples of four.1 The number of atoms in a primitive cell is given by the product of the number of atoms in a formula unit (nF) and the number of formula units per primitive cell (Z). Here it is shown the rule of four can be explained by taking into account the most probable values of nF and Z in inorganic materials datasets.