Halos epitomize the fascinating interplay between weak binding, shell evolution, and deformation effects, especially in nuclei near the drip line. In this Letter, we apply the state-of-the-art ab initio valence-space in-medium similarity renormalization group approach to predict potential candidates for one- and two-neutron halo in the intermediate-mass region. Notably, we use spectroscopic factors (SF) and two-nucleon amplitudes (TNA) as criteria for suggesting one- and two-neutron halo candidates, respectively. This approach is not only theoretically sound but also amenable to experimental validation. Our research focuses on Mg, Al, Si, P, and S neutron-drip-line nuclei, offering systematic predictions of neutron halo candidates in terms of separation energies, SF (TNA), and average occupation. The calculation suggests the ground states of ${}^{40,42,44,46}\mathrm{Al}$, ${}^{41,43,45,47}\mathrm{Si}$, ${}^{46,48}\mathrm{P}$, and ${}^{47,49}\mathrm{S}$ are promising candidates for one-neutron halos, while ${}^{40,42,44,46}\mathrm{Mg}$, ${}^{45,47}\mathrm{Al}$, ${}^{46,48}\mathrm{Si}$, ${}^{49}\mathrm{P}$, and ${}^{50}\mathrm{S}$ may harbor two-neutron halos. In addition, the relative mean-square neutron radius between halo nuclei and inner core is calculated for suggested potential neutron halos. Finally, the relations of halo formations and shell evolution are discussed.

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• Binding energy & masses
• Nuclear forces
• Nuclear many-body theory
• Nuclear structure & decays
• Nucleon distribution
• 39 ≤ A ≤ 58
• Ab initio calculations