THERMODYNAMIC PERTURBATION-THEORY - LENNARD-JONES CHAINS

The compressibility factors for freely-jointed Lennard-Jones chains ar e determined in the context of Wertheim's first-order thermodynamic pe rturbation theory (TPT1). In the TPT1 treatment, nonbonded Lennard-Jon es spheres are used as the reference system. The compressibility facto rs of the chain system, at temperature T, segment density rho*, and c hain length m, are obtained based on the compressibility factors of th e Lennard-Jones spheres, and the density derivative of the radial dist ribution function, partial derivative ln g(LIS)(sigma;rho,T*)/partial derivative rho, evaluated at the same temperature T* and segment den sity rho. The exact values for these two quantities, obtained from Mo nte Carlo simulations, are used in our calculations. The TPT1 predicti ons are found in agreement with Monte Carlo simulation results for the 8-mer, 16-mer, and 32-mer Lennard-Jones chains over a wide range of d ensities and temperatures. When values of the density derivative of th e reference Lennard-Jones sphere system are estimated from the approxi mate Weeks-Chandler-Andersen perturbation theory, however, the TPT1 pr edictions do not match the simulation results.