The low-lying level structure of ${}^{31}\mathrm{Mg}$ has been investigated by the antisymmetrized molecular dynamics (AMD) plus generator coordinate method (GCM) with the Gogny D1S force. It is shown that the $N=20$ magic number is broken and the ground state has a pure neutron 2p3h configuration. The coexistence of many-particle, many-hole states at very low excitation energy is discussed.

• Received 1 February 2007

DOI:https://doi.org/10.1103/PhysRevC.75.041302