The gas-phase interaction energies of methane and neopentane dimers are calculated at various intermolecular distances and geometries using several molecular mechanics and semiempirical parameter sets. For comparisons, a set ลฝ . of reference calculations are also performed using the 6แ311G 2 d, 2 p basis set ลฝ . with the inclusion of second-order MollerแPlesset energies MP2 and basis set superposition corrections. These calculations are further used to examine the mechanism by which the AM1 and PM3 methods account for dispersion ลฝ . interactions in molecular systems. While no specific parameter s are included in semiempirical energy functions to capture such effects, the results indicate that both methods produce favorable interaction energies at near contact distances for the dimer systems. AM1 energies, however, show much closer agreement with the reference calculations, indicating potential deficiencies in the PM3 parameter set. Although the source of the dispersion energy could be traced to the attractive Gaussians of the core repulsion function in the AM1 Hamiltonian, a similar link could not be established for PM3. In contrast, PM3 dispersion energies apparently stem from a collection of contributions implicitly included during parameter optimization, providing no clear mechanism for correction or adjustment. Based on the analysis presented, an approach is also suggested for improving the AM1 parameter set.