COVALENT, IONIC, AND ATOMIC STRUCTURES IN 12Be

The generalized two-center cluster model (GTCM), which can treat various single particle configurations in general two center systems, is applied to the light neutron-rich system, 12Be= α+α+4N. We discuss the change of the neutrons' configuration around two α-cores as a variation of an excitation energy. We show that the covalent, ionic and atomic configurations appear in the unbound region above the α+8Heg.s. particle-decay threshold, and they coexist within a quite small energy interval.