  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.
