We present an ab initio study of nuclear structure in the island of inversion around neutron number N=20, using multishell effective Hamiltonians derived from valence-space in-medium similarity renormalization group approach combined with the quantum-number projected generator coordinate method. By progressively expanding the valence space from the sd shell to the intermediate sdf₇/₂p₃/₂ space and, for the first time, to the full sdfp shell, we investigate low-lying spectra, E2 transition strengths, deformation properties, and neutron occupancies in even-even Ne, Mg, and Si isotopes around N=20. Our results show that enlarging the valence space significantly improves the description of quadrupole collectivity, yielding better agreement with experimental data for key observables such as the lowered 2^+ energies and the enhanced B (E2;0^+₁2₁^+) values. The analysis reveals the critical role of cross-shell multi-particle multi-hole excitations in breaking the N=20 shell closure and establishing intruder-dominated ground states. It also demonstrates the ability of the VS-IMSRG+PGCM framework to capture both dynamical (short range) and static (long range) correlations across multiple major-oscillator shells.
Cao et al. (Fri,) studied this question.