Ab initio study in the island of inversion within the two-major-shell valence space

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 the 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 sdf 7/2 p 3/2 space and, for the first time, to the full sdfp shell, we investigate low-lying spectra, E 2 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 ( E 2 ; 0 1 + → 2 1 + ) 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.