The actual calculation of such energy levels is quite complex and, for a given atom, has to include the following parameters: the valence state, the symmetry of the ion's environment (i.e. the coordination polyhedron of charges which produces the "crystal field," also sometimes called the "ligand field") including any distortions from ideal tetrahedral, octahedral, etc.; and the strength of the crystal field, i.e. the nature and strength of the bonding. The result of such a calculation is an energy level scheme such as that of ruby, (about 1 % Cr3+ substituted for A13+ in the distorted octahedral sites of A12O3 with very strong bonding resulting in an Al-O distance of 1.91 A) shown in Figure 2a.