  We present a method for calculating the spectrum of extended solids within
reduced density matrix functional theory. An application of this method to the
strongly correlated transition metal oxide series demonstrates that (i) an
insulating state is found in the absence of magnetic order and, in addition,
(ii) the interplay between the change transfer and Mott-Hubbard correlation is
correctly described. In this respect we find that while NiO has a strong charge
transfer character to the electronic gap, with substantial hybridization
between $t_{2g}$ and oxygen-$p$ states in the lower Hubbard band, for MnO this
is almost entirely absent. As a validation of our method we also calculate the
spectra for a variety of weakly correlated materials, finding good agreement
with experiment and other techniques.

Strongly Correlated Electrons;Materials Science