Challenges for first-principles nuclear structure: ${}^{11}$Li and ${}^{29}$F

Ab initio calculations of atomic nuclei have had many successes in recent years. Nonetheless, important challenges that resist even brute-force calculation remain. As archetypal examples of these challenges, we consider ${}^{11}$Li and ${}^{29}$F, well known halo nuclides situated on islands of inversion. The deformed intruder levels, which are primarily two-particle, two-hole neutron excitations with respect to naive spherical shell model configurations, are slow, with respect to increases of the model space, to take their rightful place among, and potentially mix with, the lowest levels. We suggest these systems prototype the challenges for other important intruder states, and can serve as useful testbeds for potential approaches.