  Using Langevin dynamics complemented by Wang-Landau Monte Carlo simulations,
we study the phase behavior of single and multiple semiflexible polymer chains
in solution under poor-solvent conditions. In the case of a single chain, we
obtain the full phase diagram in the temperature-bending rigidity (stiffness)
plane and we provide connections with a classical mean field result on a
lattice as well as with past results on the same model. At low bending rigidity
and upon cooling, we find a second order coil-globule transition, followed by a
subsequent first order globule-crystal transition at lower temperatures. The
obtained crystals have the shape of a twisted rod whose length increases with
the increase of the stiffness of the chain. Above a critical value of the
stiffness, we also find a direct first order globule-crystal transition, with
the crystal having the form of a twisted toroid. Close to the triple point, we
find a region with isoenergetic structures with frequent switching from rods to
toroids, with the toroid eventually becoming the only observed stable phase at
higher stiffness. The model is then extended to many thermally equilibrated
chains in a box and the analogous phase diagram is deduced where the chains are
observed to first fold into a globule bundle at low stiffness upon cooling, and
then rearrange into a nematic bundle via a nucleation process involving an
isotropic-nematic transition. As in the single chain counterpart, above a
critical stiffness the chains are observed to undergo a direct transition from
a gas of isotropically distributed chains to a nematic bundle as the
temperature decreases in agreement with recent suggestions from mean field
theory. The consequences of these findings for self-assembly of biopolymers in
solutions are discussed.
