  Atomic nuclei can exhibit shape coexistence and multi-reference physics that
enters in their ground states, and to accurately capture the ensuing
correlations and entanglement is challenging. We address this problem by
applying single-reference coupled-cluster theory based on spherical and
deformed reference states and the tailored coupled-cluster method. The latter
combines configuration interaction to capture static correlations with
coupled-cluster theory for dynamic correlations. We compute the atomic nuclei
$^{12}$C, $^{28}$Si, and $^{56}$Ni and find that the tailored coupled-cluster
method and the single-reference approach based on a deformed Hartree-Fock state
yield the most accurate results.
