  Electron transfer with changing occupation in the 4f subshell poses a
considerable challenge for quantitative predictions in quantum chemistry. Using
the example of cerium oxide, we identify the main deficiencies of common
parameter-dependent one-electron approaches, such as density functional theory
(DFT) with a Hubbard correction, or hybrid functionals. As a response, we
present the first benchmark of ab initio many-electron theory for electron
transfer energies and lattice parameters under periodic boundary conditions. We
show that the direct random phase approximation clearly outperforms all DFT
variations. From this foundation, we, then, systematically improve even
further. Periodic second-order M{\o}ller-Plesset perturbation theory meanwhile
manages to recover standard hybrid functional values. Using these approaches to
eliminate parameter bias allows for highly accurate benchmarks of strongly
correlated materials, the reliable assessment of various density functionals,
and functional fitting via machine-learning.
