  In this work we explore the performance of a recently derived many-body
effective energy theory for the calculation of photoemission spectra in the
regime of strong electron correlation. We apply the theory to paramagnetic MnO,
FeO, CoO, and NiO, which are typical examples of strongly correlated materials
and, therefore, a challenge for standard theories. We show that our methods
open a correlation gap in all the oxides studied without breaking the symmetry.
Although the materials seem similar, we show that an analysis of the occupation
numbers reveals that the nature of the gap is not the same for these materials.
Overall the results are very promising, although improvements are clearly
required, since the band gap is overestimated for all the systems studied. We
indicate some possible strategies to further develop the theory.
