The geometries and interaction energies of the heterodimeric capsule [tetrakis(4-hydroxyhphenyl)-cavitand 1 and tetra(4-pyridyl)-cavitand 2] complexes with methyl p-acetoxybenzoate 3, methyl p-ethoxybenzeoate 4, and p-ethoxyiodobenzene 5 were studied by ab initio molecular orbital calculations. The optimized structures and charge distributions of the complexes suggest that the electrostatic interactions of oxygen atoms in the guest molecules with the hydrogen atoms of aromatic rings and methylene-bridge rim in the heterodimeric capsule stabilize the complexes. The calculated relative energies of the two orientational isomers of the complexes well reproduce the experimentally observed orientational selectivity of the guest molecules. The calculated stabilization energies for the major orientational isomers of the heterodimeric capsule (1.2) complexes with guest molecules (3, 4, and 5) are -21.6, -19.6, and -19.4 kcal/mol, respectively. Those for the minor orientational isomers are 1.5, 3.5, and 3.7 kcal/mol smaller (less negative), respectively. The magnitude of the calculated energy differences agrees well with the order of the experimental population of the major orientational isomer (3 < 4 approximately 5). The large electron correlation contributions to the attraction (-27.0 to -31.8 kcal/mol) show that the dispersion interactions are the major source of the attraction in the complexes, while the electrostatic interactions (-4.9 to -12.5 kcal/mol) are also an important source of the attraction. Although the electrostatic interactions are weaker than the dispersion interactions, the highly orientation dependent electrostatic interactions mainly determine the orientation of the unsymmetrical guest molecules in the complexes. The electrostatic interactions in the major orientational isomer are 2.6-3.9 kcal/mol larger (more negative) than those in the minor orientational isomer, while the differences of other energy terms are small (less than 1.1 kcal/mol). The interaction energies calculated for model complexes show that the CH/pi interactions are not playing important roles in controlling the orientation of the guest molecules in the complexes.