Molecular dynamics simulation is used to model the microscopic structure (free volume in particular) and thermodynamic properties of a silicon-containing rubbery polymer that exhibits promising membrane properties for hydrocarbon separation. For this purpose, an accurate united atom force field is developed based on density functional theory calculations. Furthermore, molecular simulation is used to model the solubility and diffusivity of various n-alkanes in the polymer. Overall, the agreement between experimental data and simulation results is very satisfactory. (c) 2004 Elsevier B.V. All rights reserved.