A recently developed atomistic force field for poly (dimethylsiloxane) (PDMS) is used to calculate the permeability properties of the polymer to light gases and to n-alkanes. The torsional potential barrier is re-estimated compared to the original force field. Chain dynamics and chain sizes are affected by this change while thermodynamic properties of the melt and of PDMS mixtures remain unaffected. The diffusion coefficients of penetrants to PDMS are calculated at different temperatures based on long molecular dynamics simulations. Subsequently, the permeability coefficients are estimated and ideal mixture selectivities are evaluated for binary hydrocarbon mixtures. In all cases, agreement with literature experimental data ranges from good to excellent. Calculations for mixed penetrant permeation reveal that, in the presence of a second penetrant species, solubility and diffusion coefficients increase, in agreement with recent experimental evidence.