Molecular modeling of flexible molecules. Vapor–liquid and fluid–solid equilibria

We consider recent applications of Wertheim’s first order perturbation theory (TPT1) to the description of the critical properties and the freezing transition of chain molecules. Firstly we consider an extension of TPT1 which allows one to describe the equation of state of atomistic molecular models which incorporate fine chemical details such as overlap between the sites, fixed bond angles and torsional potentials. The theory is applied to the description of the critical properties of all isomers ranging from butane to octane and good qualitative agreement is found. We then show how TPT1 may be applied to the description of the freezing transition of chain molecules. We apply the theory to chains of tangent hard spheres and Lennard–Jones chains and find good agreement for the equation of state and free energies of the fluid and solid phases. Fluid–solid coexistence properties predicted by the theory are in close agreement with simulation results. It is shown that for hard sphere and Lennard–Jones dimers the stable solid structure is a disordered one, with the atoms forming a close packed arrangement, but with the bonds distributed randomly within the solid. 2004 Elsevier B.V. All rights reserved.