The structure of 10 Be is investigated using a microscopic ␣ϩ␣ϩnϩn model based on the molecular orbit ͑MO͒ model. The low-lying states are characterized by several configurations of valence neutrons, which are constructed as combinations of three basic orbits. The model space employed is extended from the traditional MO models, and the orbits are expressed as linear combinations of local Gaussians. Their positions are determined variationally. Using this model, we reanalyze the structure of 9 Be and show that this extension enables us to use the original two-body spin-orbit interaction determined from a scattering phase-shift analysis of ␣-n. In 10 Be, all of the observed positive-parity bands and the negative-parity bands are described within the model. The 0 ϩ ground state of 10 Be is described by a dominant (3/2 Ϫ ) 2 configuration. The state has a rather large binding energy ͑8.38 MeV from the ␣ϩ␣ϩnϩn threshold experimentally͒, and the mechanism leading to binding, such as a pairing effect and reduction of the kinetic energy between two clusters, is discussed in detail. In spite of this large binding, the ␣-␣ clustering in the ground state persists due to a coupling effect between the 6 Heϩ␣ configuration and the 5 Heϩ 5 He configuration, which provides a smooth potential for the valence neutrons. The second 0 ϩ state of 10 Be has a large ␣-␣ structure with a (1/2 ϩ ) 2configuration. An enlargement of the ␣-␣ distance due to two-valence neutrons along the ␣-␣ axis makes their wave function smooth and reduces the kinetic energy drastically. Furthermore, the contribution of the spinorbit interaction due to coupling between the S z ϭ0 and the S z ϭ1 configurations is important. We also show the mediation effect of two valence neutrons between two ␣ clusters.

The moleculelike structure of the C isotopes (Aϭ12, 14, 16) is investigated using a microscopic ␣ϩ␣ ϩ␣ϩnϩnϩ••• model. The valence neutrons are classified based on the molecular-orbit model, and both orbit and orbit are introduced around three ␣ clusters. The valence neutrons which occupy the orbit increase the binding energy and stabilize the linear chain of 3␣ against the breathinglike breakup. However, 14 C with the orbit does not show a clear energy minimum against the bendinglike path. The combination of the valence neutrons in the and orbits is promising to stabilize the linear-chain state against the breathing and bending modes, and it is found that the excited states of 16 C with the (3/2 Ϫ ) 2 (1/2 Ϫ ) 2 configuration for the four valence neutrons is one of the most promising candidates for such a structure.

t+t clustering in He isotopes is investigated by using two theoretical approaches. A role of the t+t cluster component in the ground state is examined with AMD triple-S, allowing the wider configuration space containing simultaneously the "t+t+valence neutrons" structure and "4 He +valence neutrons" structure. We understand the importance of the t + t component even for the ground state. Further, t + t resonances are investigated with RGM type approach. We obtained many t + t states as resonances near to t + t threshold.

The generalized two-center cluster model, which can treat static structures and dynamical reactions in excited states, is applied to the light neutron-rich system, 12Be=alpha+alpha+4N. We discuss the change of the neutrons' configuration around two alpha cores from the covalent structure to the ionic one. We show that, in the unbound region above particle-decay thresholds, the ionic configurations appear as the molecular resonances of alpha+8He, 6He+6He, and 5He+7He. A new type of superdeformation is possible, and we find here a covalent superdeformation with a hybrid configuration of both the covalent and ionic structures. The excitation of these exotic structures through the two-neutron transfer reaction is also discussed.

The structure of the second 0 + state of 10 Be is investigated using a microscopic α+α+n+n model based on the molecular-orbit (MO) model. The second 0 + state, which has dominantly the (1/2 + ) 2 configuration, is shown to have a particularly enlarged α-α structure. The kinetic energy of the two valence neutrons occupying along the α-α axis is reduced remarkably due to the strong α clustering and, simultaneously, the spin-orbit interaction unexpectedly plays important role to make the energy of this state much lower. The mixing of states with different spin structure is shown to be important in negative-parity states. The experimentally observed small-level spacing between 1 − and 2 − (∼300 keV) is found to be an evidence of this spin-mixing effect. 12 Be is also investigated using α+α+4n model, in which four valence neutrons are considered to occupy the (3/2 − ) 2 (1/2 + ) 2 configuration. The energy surface of 12 Be is shown to exhibit similar characteristics, that the remarkable α clustering and the contribution of the spin-orbit interaction make the binding of the state with (3/2 − ) 2 (1/2 + ) 2 configuration properly stronger in comparison with the closed p-shell (3/2 − ) 2 (1/2 − ) 2 configuration.