Ab initio vs literature stiffness values for Ga: a caveat about crystal settings

The structure of orthorhombic gallium was first determined in 1933 in the space group Cmca. The recent crystal structure literature also consistently uses this setting. The ab initio stiffness coefficients calculated from this crystal structure are: C₁₁=88, C₁₂=29, C₁₃=34, C₂₂=116, C₂₃=24, C₃₃=98, C₄₄=37, C₅₅=38, C₆₆=27 GPa. Comparison with experimental stiffness literature values averaged from three studies, followed by an extended search of the literature showed that the Abma setting was used in all three independent experiments but not clearly reported. This finding stresses the need for proper reporting of the Cartesian system for both computed and measured properties. Printing the cell data, space-group name, fractional atomic coordinates for independent atoms and a reference to the set of rules used to fix the Cartesian axes constitutes sufficient information to fix the crystal and its setting in the Cartesian axes for all symmetries and for all properties.