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Eight complexes of various aromatic molecules with water have been studied theoretically at the local Møller-Plesset 2nd order theory (LMP2)/aug-ccpVTZ(-f)//LMP2/6-31ϩG* level of theory. Two types of complexes can be formed, depending on the electronic structure of aromatic molecules. Donor hydrocarbons form A-type complexes, while aromatics bearing electron-withdrawing substituents form Btype complexes. A-type complexes are stabilized due to -H interactions with the OH bond pointing to the aromatic molecule plane, while B-type complexes have geometry with the oxygen atom pointing to the aromatic molecule plane stabilized by the interaction of highest occupied molecular orbital (HOMO) of water molecule with * orbitals of the aromatics. It has been found that a (OHOMO-lowest unoccupied molecular orbital (LUMO)/2 value of aromatic molecule, which can be called "molecular electronegativity," is useful to predict the type of complex formed by aromatic molecule and water. Aromatic hydrocarbons with "molecular electronegativity" of Ͻ0.15 tend to form A-type complexes, while aromatic molecules with "molecular electronegativity" of Ͻ0.15 a.u. form B-type complexes. The binding energy of water-aromatic complexes undergoes a minimum in the area of switching from A-type to B type complexes, which can be rationalize in terms of frontier orbital interactions.

Benzene, pyrimidine, and naphthalene dimers have been studied at canonical MP2, LMP2, and MP4(SDTQ) levels of theory. It has been shown that the LMP2 method is superior to canonical MP2 due to reduced BSSE. Thus, basis set limit-extrapolated LMP2 binding energies of T-shaped and parallel displaced (PD) benzene dimers are nearly the same, as has been found in higher level ab initio calculations. MP4(SDTQ) binding energies calculated at LMP2 optimized geometries are always more negative than those calculated for MP2-optimized geometries. MP4(SDTQ)/CC-pVDZ-corrected complete basis set-extrapolated LMP2 binding energies of T-shaped and PD dimers of benzene and naphthalene were found to be -2.80, -2.59 and -4.39, -6.29 kcal/mol, respectively.

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