Ab initio quantum mechanical calculations have been performed to establish the potentials for alkyl-substituted polyhedral oligomeric silsesquioxane (POSS) monomers RxH8-x(SiO1.5)8. More specifically, we have examined the unsubstituted POSS (SiO1.5H) 8 cage as well as linear and cyclic alkyl-substituted cages where one of the terminating hydrogen atoms is replaced by a hydrocarbon group, that is, R1H7(SiO1.5)8. The results for the minimum-energy configurations indicate that the presence of the linear hydrocarbon chains and cyclic intermediates have very little effect on the structure of the POSS cage. Although the POSS monomeric cage does influence the partial charges of the first few carbon atoms covalently bound to the POSS monomer, its effect on the structural properties of the alkyl chain is small. Differences arise, however, for cyclic alkyl substitutents bound to the POSS cage due to the repulsive interactions between the POSS cage and bulkier cyclic intermediates that result upon rotation of the Si - C - C - C dihedral angles. The interatomic potentials for these rotational, or torsional, terms need to be modified slightly in order to appropriately simulate sterically hindered substitutents on the cage. Our results suggest that combining an atomistic force field independently developed to describe silsesquioxanes with an independent atomistic model developed to describe hydrocarbon chains can be used in classical molecular simulation studies of most alkyl-silsesquioxanes. This avoids the need to develop specific force fields for each substituted POSS cage studied and opens up the possibility of using molecular simulation to probe the thermodynamic and structural properties of these unique nanoscale building blocks.