  A slave rotor-Hartree Fock formalism is presented for studying the properties
of the p-d model describing perovskite transition metal oxides, and a flexible
and efficient numerical formalism is developed for its solution. The
methodology is shown to yield, within an unified formulation, the significant
aspects of the rare earth nickelate phase diagram, including the paramagnetic
metal state observed for the LaNiO3 and the correct ground-state magnetic order
of insulating compounds. It is then used to elucidate ground state changes
occurring as morphology is varied from bulk to strained and un-strained
thin-film form. For ultrathin films, epitaxial strain and charge-transfer to
the apical out-of-plane oxygen sites are shown to have significant impact on
the phase diagram.
