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Solid-fluid phase behavior in molecular systems is determined not only by packing considerations (as occurs in simple fluids, in which freezing can be understood in terms of the frezzing of hard spheres), but also accouting for shape, polarity and flexibility. Although it is generally assumed that flexibility is not crucial in determining the essential phase behavior of many chain-like fluids, this microscopic effect can proundly affect the phase equilibria of a given system. For instance, rigid non-spherical molecules can exhibit liquid crystalline phase behavior, which is never observed in fully flexible systems. In addition, vapor-liquid critical points of chain-like molecules with different degree of flexibility are different.

One of the most successful and studied molecular model accounting for chain-like systems is one in which the molecules are modelled as chains formed by connected spherical segments. In these models the pair potential between the monomers (either in the same or in different chains) that form the chains is given by a spherical potential. Such models incorporates two essential features: the excluded volume of the chains, and the connectivity between segments. Chain molecules of tangent segments can be considered: (1) fully flexible, in which bending and torsional potentials are not considered; (2) rigid, in which connected spherical segments have fixed bond angles and internal degrees of fredom; and (3) semi-flexible, in which bending and torsional potential are explicitly considered.

Very recently our group (in collaboration with the Statistical Thermodynamics of Molecular Fluids Group of Prof. Carlos Vega de las Heras, Universidad Complutense, Madrid, Spain, and the Multiphase Fluid Systems Group of Dr. Amparo Galindo, Imperial College London, UK) has shown that the Wertheim's thermodynamic perturbation theory approach (which constitutes the foundamental basis of the Statistical Associating Fluid Theory or SAFT formalism) can also be applied to the solid phase. This fundamental advance allows to predict fluid-solid equilibrium by using the SAFT formalism for both fluid and solid phases. One major advances on this field are summarized below:

• Extension of the Soft-SAFT equation of state to deal with solid-liquid phase behavior.
• Determination of the phase behavior (solid-liquid, vapor-liquid and vapor-solid) of fully flexible Lennard-Jones chains.

• Global phase behavior of the two-center Lennard-Jones model.

• Determination of the global phase behavior of fully flexible hard-sphere chains under the mean-field approximation.
• Global phase beahviour of linear rigid hard-sphere chain under the mean-field approximation.
• Global phase behaviour (solid-liquid, vapor-liquid and vapor-solid) of linear rigid Lennard-Jones chains.