  We show that charge ordering (more precisely, two-sublattice bond
disproportionation) in the rare earth nickelate perovskites is intimately
related to a negative charge transfer energy. By adding an additional potential
on the Ni d states we are able to vary the charge tranfer energy and compute
relaxed structures within an ab-initio framework. We show that the difference
in Ni-O bond lengths and the value of the ordered state magnetic moment
correlate with the charge transfer energy and that the transition to the
bond-disproportionated state occurs when the effective charge transfer energy
becomes negative.
