Mid-gap excitations in Anderson polyoxometalates

The excitation spectrum of a series of paramagnetic ions in Anderson polyoxometalates ([XMo6O24H6]n−, X = Cr3+, Fe2+,3+, Co2+,3+) is studied. We compare multireference second-order perturbation theory (CASPT2) results with density functional theory calculations. Standard DFT calculations tend to stabilize the low-spin states too much with respect to the high-spin states, whereas time-dependent DFT give a more balanced description of both cases and correctly reproduces the relative energies of the different spin states.