Mass systematics for A

Further evidence for the presence of an anomaly in binding energies for the "island of inversion" centered at Z =11, N =21 is obtained by comparison of shell-model calculations to experiment. The calculations were done with a shell-model interaction that is applicable to nuclei with active valence nucleons in both the (1s,Od) and (Of, lp) major shells. This interaction is described in detail as are its predictions for binding energies and energy spectra of Z =8-20, N =18—25 nuclei. These calculations provide the background for the exploration of the "island of inversion. " The extent of the "island" and the magnitude of the anomaly is explored by calculating the binding energies of