M05-2X/6-31G*:PM3MM] and [B3LYP/6-31G*:PM3] ONIOM2 methods have been used to investigate the vitamin A propionate/b cyclodextrin complex with 1:2 stoichiometry. Both methods give almost the same lowest energy minimum. The minimum energy structure of the complex is found in good agreement with experimental data. In this configuration, the major structure of propionate of vitamin A (PVA) is embedded inside the two cavities of bCD while the propionate group is kept outside. However, the three methyl groups of PVA are positioned in the free space between both bCD molecules. The driving forces for complexation are dominated by Van der Waals interactions between PVA and the bCD molecules assisted with multiple hydrogen bond interactions between the two cyclodextrin molecules. These interactions were investigated using the natural bond orbital approach.

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