We present electronic structure calculations in combination with local and nonlocal many-body correlation effects for the half-metallic ferromagnet $\mathrm{Cr}{\mathrm{O}}_2$. Both zero temperature results from a variational cluster approach as well as finite-temperature dynamical mean field theory results support the existence of low-energy nonquasiparticle states, which turn out to be essential for a correct quantitative description of depolarization effects.