Self-consistent polymer reference interaction site model (PRISM) calculations and molecular dynamics (MD) simulations were performed for a blend of isotactic polypropylene (iPP) and polyethylene (PE) at various compositions and chain lengths. United atom models were used for both components. Both PRISM theory and MD simulations predicted that the change in intramolecular energy going from the melt to the blend for each species was a significant positive contribution to the total mixing energy. Furthermore, this intramolecular heat of mixing was dominated by the torsional and van der Waals contributions. Constant pressure MD simulations indicated that, with the united atom parameters used, the iPP/PE blend had essentially zero volume change of mixing. MD simulations were used to compute the partial structure factors for the blend and together with the standard random phase approximation analysis were used to estimate the small angle neutron scattering (SANS) chi parameter. Good agreement was found between estimates from PRISM and MD of both the enthalpic and SANS chi parameters. (C) 2003 American Institute of Physics.