  We present a comprehensive theoretical study of the electronic structures of
the Yb atom and the Yb$_2$ molecule, respectively, focusing on their ground and
lowest-lying electronically excited states. Our study includes various
state-of-the-art quantum chemistry methods such as CCSD, CCSD(T), CASPT2
(including spin--orbit coupling), and EOM-CCSD as well as some recently
developed pCCD-based approaches and their extensions to target excited states.
Specifically, we scan the lowest-lying potential energy surfaces of the
\ce{Yb2} dimer and provide a reliable benchmark set of spectroscopic parameters
including optimal bond lengths, vibrational frequencies, potential energy
depths, and adiabatic excitation energies. Our in-depth analysis unravels the
complex nature of the electronic spectrum of \ce{Yb2}, which is difficult to
model accurately by any conventional quantum chemistry method. Finally, we
scrutinize the bi-excited character of the first $^1\Sigma_g^+$ excited state
and its evolution along the potential energy surface.
