Wavefunction illustrations extracted from b for h the ground state. The size of each orbital (3 d for the central Ni site and 3 p for the six neighboring S sites) is proportional to its hole occupation. The color represents the expectation value of the spin operator along the z axis <{hat{S}}_{z}> , again calculated separately for the Ni and S states. Therefore, the change in spin state and the partial transfer of holes involved in the exciton transition is encoded in the change in color and size of orbitals, respectively. We represent the ground state by only the down-spin configuration, omitting the up-spin and spin-zero elements of the triplet.
