The thermodynamic properties and microscopic structure of PDMS are examined using Molecular Dynamics. A force field is developed that focuses on the accurate representation of polymer melt density over a wide temperature range at ambient pressure. The force field is also used for melt density prediction at high pressures. Model predictions and experimental data are in good agreement in all cases. Subsequently, the solubility of n-alkanes, n-perfluoroalkanes, noble and light gases in PDMS at various temperatures is calculated using the Widom test particle insertion technique. Accurate force fields are used for the solute molecules. Results agree very well with available experimental data, with the exception of CF4. MD runs at 300 K are further used for the calculation of diffusion coefficients of light gases and n-alkanes in PDMS. Calculations for the lighter gases are in good agreement with limited experimental data available from the literature while for the alkanes the deviation is higher.