Expanded ensemble Monte Carlo calculations of free energy for closed, stretched and confined lattice polymers

We apply the expanded ensemble Monte Carlo method to calculate free energy (entropy) of the athermal self-avoiding lattice polymers with certain restr ictions (ring, stretched and confined chains). A two-stage simulation proce dure was created to take these restrictions into account. Data for entropy and the stretching force were obtained. N-dependencies for entropy demonstr ate the convergence of specific values to the infinite limit. The test comp arison of MC results with corresponding exact values for short chains shows a good coincidence of the data. The developed method can easily be applied to various polymer systems with restrictions. (C) 1999 Elsevier Science B. V. All rights reserved.