Ab initio nuclear shape coexistence and emergence of island of inversion around $N=20

We extend a nuclear ab initio framework based on chiral two- and three-nucleon interactions to investigate shape coexistence and the degradation of the $N=20$ magic number in both even-even and odd-even neutron-rich nuclei. The quantum-number projected generator coordinate method, combined with the in-medium similarity renormalization group (IMSRG), is employed to compute their low-lying states. This approach successfully reproduces the coexistence of weakly and strongly deformed states at comparable energies, providing a natural explanation for the emergence of the $N=20$ island of inversion through the IMSRG evolution of the chiral Hamiltonian. The results highlight the essential roles of both dynamic and static collective correlations in reproducing the ordering of nuclear states with distinct shapes. This work represents a significant advance in the application of ab initio methods to the intricate phenomenon of shape coexistence in medium-mass nuclei.