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Free energy calculations of small molecules in dense amorphous polymers. Effect of the initial guess configuration in molecular dynamics studies

The excess free energy of small molecules in the amorphous polymers poly(ethylene) and poly(dimethylsiloxane) was calculated, using the test-particle-insertion method. The method was applied to polymer configurations obtained from molecular dynamics simulations with differently prepared initial guess configurations. It was found that the calculated solubility coefficients strongly depend on the quality of the initial guess configuration. Slow compression of dilute systems, during which process only the repulsive parts of the nonbonded Lennard-Jones potentials are taken into account, yields polymer melts which are better relaxed, and which offer lower solubilities for guest molecules compared with polymer melts generated at the experimental density or prepared by compressing boxes with soft-core nonbonded potentials. For the last two methods initial stresses relax by straining the internal modes (bond angles, torsion angles) of the chains