FREE-ENERGY CALCULATIONS BY EXPANDED ENSEMBLE METHOD FOR LATTICE AND CONTINUOUS POLYMERS

Expanded ensemble Monte Carlo method suggested earlier in J. Chem. Phy s. (1992, 96, 1776) was applied to calculate free energy (entropy) of a lattice polymer (self-avoiding random walk on a simple cubic lattice ) with respect to the standard reference system-a pure random walk wit h no restrictions (in the latter case the number of conformations is k nown exactly). An effective scheme for the initial choice of balancing factors is proposed with the following iteration procedure for their optimization. Calculations were performed for chain lengths N = 5, 8, 25, 50, and 100. Comparison of simulation results with the exact data available for short chains (N = 5, 8) indicate a high accuracy of the simulation method. For the continuous polymer model (several bead and spring chains in the periodic box of a fixed volume) the expanded ense mble Monte Carlo method was used to calculate free energy with respect to the ideal gas as a reference system. Configurations were changed w ith the aid of the constant temperature molecular dynamics procedure, while changes in expansion parameter were made within the usual Monte Carlo scheme. For several cases (a single chain of N = 32 monomers in the periodic box, two chains, and four chains), the balancing factors were optimized and free energies were calculated in a wide range of te mperatures.