Crossing of single-particle energy levels resulting from neutron excess in the sd shell

Abstract
It has been shown that the large discrepancies between calculation and experiment in the binding energies of neutron-rich isotopes of sodium and magnesium can be removed by extending the model space to include the f7/2 shell. It is shown that the physical explanation of this result lies in the severe distortion of the underlying single-particle spectrum by the large neutron excess.