CALCULATION OF SOLID-FLUID PHASE-EQUILIBRIA FOR SYSTEMS OF CHAIN MOLECULES

We study the first order solid-fluid phase transition of a system of s emi-flexible Lennard-Jones chains using molecular dynamics simulations . Thermodynamic integration methods are used to calculate the free ene rgy of the solid and fluid phases. The solid phase free energy per cha in can be calculated to an accuracy of +/-0.03k(B)T with relative ease . The Gibbs-Duhem integration technique is used to trace out the compl ete melting curve, scarring with a single point on the curve obtained from the foe energy calculations. For the short chains studied here, w e find that increasing the chain length stabilizes the solid phase; i. e., it raises the melting temperature at fixed pressure, and lowers th e density at the transition at fixed temperature. Gibbs-Duhem integrat ion was used also to investigate the effects of chain stiffness on the transition. We find that increasing the stiffness also acts to stabil ize the solid phase. At fixed temperature, the transition is shifted t o lower pressure and lower density with increasing chain stiffness. Fu rther, we find that the density gap between solid and fluid broadens w ith increasing chain stiffness. (C) 1998 American Institute of Physics . [S0021-9606(98)50825-2].