  Ce compounds feature a remarkable diversity of electronic properties, which
motivated extensive investigations over the last decades. Inelastic neutron
scattering represents an important tool for understanding their underlying
electronic structures but in certain cases a straightforward interpretation of
the measured spectra is hampered by the presence of strong vibronic couplings.
The latter may give rise to extra spectral features, which complicates the
mapping of experimental data onto standard multiplet diagrams. To benchmark the
performance of embedded-cluster quantum chemical computational schemes for the
case of $4f$ systems, we here address the Ce 4$f^1$ multiplet structure of
NaCeO$_2$, an antiferromagnet with $D_{2d}$ magnetic-site symmetry for which
neutron scattering measurements indicate only weak vibronic effects. Very good
agreement with the experimental results is found in the computations, which
validates our computational approach and confirms NaCeO$_2$ as a 4$f$ magnet in
the intermediate coupling regime with equally strong 4$f$-shell spin-orbit and
crystal-field interactions.
