  Using density functional theory based calculations, we show that the
correlated mixed-valent compound SmO is a 3D strongly topological semi-metal as
a result of a 4$f$-5$d$ band inversion at the X point. The [001] surface Bloch
spectral density reveals two weakly interacting Dirac cones that are
quasi-degenerate at the M_bar-point and another single Dirac cone at the
Gamma_bar-point. We also show that the topological non-triviality in SmO is
very robust and prevails for a wide range of lattice parameters, making it an
ideal candidate to investigate topological nontrivial correlated flat bands in
thin-film form. Moreover, the electron filling is tunable by strain. In
addition, we find conditions for which the inversion is of the 4f-6s type,
making SmO to be a rather unique system. The similarities of the crystal
symmetry and the lattice constant of SmO to the well studied ferromagnetic
semiconductor EuO, makes SmO/EuO thin film interfaces an excellent contender
towards realizing the quantum anomalous Hall effect in a strongly correlated
electron system.
