  The alpha cluster states of 16C are investigated by using the antisymmetrized
molecular dynamics. It is shown that two different types of alpha cluster
states exist: triangular and linear-chain states. The former has an approximate
isosceles triangular configuration of alpha particles surrounded by four
valence neutrons occupying sd-shell, while the latter has the linearly aligned
alpha particles with two sd-shell neutrons and two pf-shell neutrons. It is
found that the structure of the linear-chain state is qualitatively understood
in terms of the 3/2 pi- and 1/2 sigma- molecular orbit as predicted by
molecular-orbital model, but there exists non-negligible Be+alpha+2n
correlation. The band-head energies of the triangular and linear-chain
rotational bands are 8.0 and 15.5 MeV, and the latter is close to the He+Be
threshold energy. It is also shown that the linear-chain state becomes the
yrast sstate at J=10 with excitation energy 27.8 MeV owing to its very large
moment-of-inertia comparable with hyperdeformation.
