  The spectrum of 12Be exhibits exotic features, e.g., an intruder ground state
and shape coexistence, normally associated with the breakdown of a shell
closure. While previous phenomenological treatments indicated the ground state
has substantial contributions from intruder configurations, it is only with
advances in computational abilities and improved interactions that this
intruder mixing is observed in ab initio no-core shell model (NCSM)
predictions. In this work, we extract electromagnetic observables and symmetry
decompositions from the NCSM wave functions to demonstrate that the low-lying
positive parity spectrum can be explained in terms of mixing of rotational
bands with very different intrinsic structure coexisting within the low-lying
spectrum. These observed bands exhibit an approximate SU(3) symmetry and are
qualitatively consistent with Elliott model predictions.
