  $\tilde{S}\!=\!1/2$ triangular-lattice $f$-electron materials define a
dynamic research area in condensed matter magnetism. In various Yb 4$f^{13}$
triangular-lattice compounds, for example, spin-liquid ground states seem to be
realized. Using {\it ab initio} quantum chemical methods, we here investigate
how correlation effects involving the 4$f$ electrons affect the on-site $f$-$f$
excitation spectrum in NaYbSe$_2$. The system is well suited for such a study
since unambiguous inelastic neutron scattering data are available for the
Yb$^{3+}$ $f$-$f$ transitions. The excitation energies obtained by
configuration-interaction calculations with single and double substitutions
agree within 3-4 meV with experimental values, which provides a not so
expensive alternative to fitting experimental data at the model-Hamiltonian
level in order to analyze $f$-center multiplet structures.

Strongly Correlated Electrons