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The variables involved in the GII equation are discrepancy factor (di) and N, which is the number of atoms in the formula unit. During the optimization process the octahedral tilt angle is stepped incrementally and the individual A-X and M-X bond distances, discrepancy factors and global instability index are calculated at each step. After the first optimization process, the procedure is repeated using smaller tilt angle increments in order to minimize the GII. The stability of perovskite compositions with different atoms, symmetry, tilt systems and structure can be evaluated by comparing the GII. The GII value is typically <0.1 valence units (v.u.) for unstrained structures and as large as 0.2 v.u. in a structure with lattice induced strains. Crystal structures with a GII greater than 0.2 v.u. are typically found to be unstable, and reports of such structures are usually found to be incorrect. SPuDS 8
