  We present a theory describing the local electronic properties of the
perovskite rare earth nickelates--materials which have negative charge transfer
energies, strong O $2p$ -- Ni $3d$ covalence, and breathing mode lattice
distortions at the origin of highly studied metal-insulator and
antiferromagnetic ordering transitions. Utilizing a full orbital, full
correlation double cluster approach, we find strong charge fluctuations in
agreement with a bond disproportionation interpretation. The unique double
cluster formulation permits the inclusion of necessary orbital degeneracies and
Coulomb interactions to calculate resonant x-ray spectral responses, with which
we find excellent agreement with well-established experimental results. This
previously absent, crucial link between theory and experiment provides
validation of the recently proposed bond disproportionation theory, and
provides an analysis methodology for spectroscopic studies of engineered phases
of nickelates and other high valence transition metal compounds.
