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Spectrum for non-magnetic Mott insulators from power functional within Reduced Density Matrix Functional Theory

A fully first principles theory capable of treating strongly correlated solids remains the outstanding challenge of modern day materials science. This is exemplified by the transition metal oxides, prototypical Mott insulators, that remain insulating even in the absence of long range magnetic order. Capturing the non-magnetic insulating state of these materials presents a difficult challenge for any modern electronic structure theory. In this paper we demonstrate that reduced density matrix functional theory, in conjunction with the power functional, can successfully treat the non-magnetic insulating state of the transition metal oxides NiO and MnO. We show that the electronic spectrum retains a gap even in the absence of spin order. We further discuss the detailed way in which RDMFT performs for Mott insulators and band insulators, finding that for the latter occupation number minimization alone is required, but for the former full minimization over both occupation numbers and natural orbitals is essential.