Abstract 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 (n F ) 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 n F and Z in inorganic materials datasets.
Robert G. Palgrave (2026) studied this question.