(284b) Novel, Efficient, and Accurate Methods for Calculating Pressure in Polymer Lattice Simulations

Pressure calculation in polymer lattice simulations is an important but nontrivial subject. The three existing methods—thermodynamic integration, repulsive-wall, and hydrostatic equilibrium methods—all have their limitations and cannot be used to accurately calculate the pressure at all polymer volume fractions φ. Here we propose two novel methods. In the first method, we combine the Wang-Landau--Optimized Ensemble (WL-OE) sampling with the chain insertion/deletion to accurately calculate the pressure over a range of polymer volume fraction in a single simulation run, which is very efficient at low to intermediate φ and exhibits negligible finite-size effects. In the second method, we introduce a repulsive plane with bridging bonds, which is similar to the repulsive-wall method but eliminates its confinement effects, and estimate the density of states in terms of the number of bridging bonds and the contact number, respectively, from two independent WL-OE simulations, and thus the pressure. This works well at all φ, especially at high φ where all the methods involving chain insertion/deletion fail. Finally, combining both our methods further gives the complete thermodynamics (e.g., chain chemical potential, isothermal compressibility, etc.) over the entire range of φ (from 0 to nearly 1) with negligible finite-size effects.