Low-lying intruder $T=0\mathrm{}$ states in ${}^8\mathrm{Be}$ have been posited and challenged. To address this issue, we performed ab initio shell model calculations in model spaces consisting of up to $10ħ\Omega$ excitations above the unperturbed ground state with the basis state dimensions reaching $1.87\times {10}^8.$ To gain predictive power we derive and use effective interactions from realistic nucleon-nucleon $\mathrm{}\left(\mathrm{NN}\right)\mathrm{}$ potentials in a way that guarantees convergence to the exact solution with increasing model space. Our $0ħ\Omega$ dominated states show good stability when the model space size increases. At the same time, we observe a rapid drop in excitation energy of the $2ħ\Omega$ dominated $T=0\mathrm{}$ states. In the $10ħ\Omega$ space the intruder $0^{+}0$ state falls below 18 MeV of excitation and, also, below the lowest $0^{+}1$ state. Our extrapolations suggest that this state may stabilize around 12 MeV. We hypothesize that these states might be the broad resonance intruder states needed in R-matrix analysis of $\alpha -\alpha$ elastic scattering. In addition, we present our predictions for the $A=8\mathrm{}$ binding energies with the CD-Bonn $\mathrm{NN}$ potential.

• Received 11 July 2001

DOI:https://doi.org/10.1103/PhysRevC.64.051301