The MP2 intermolecular interaction energies of the title complexes were calculated with the Dunning's correlation consistent basis sets (cc-pVXZ, X=D, T, Q, and 5) and the interaction energies at the basis set limit were estimated. The second-order Møller-Plesset (MP2) interaction energies greatly depend on the basis sets used, while the Hartree-Fock (HF) energies do not. Small basis sets considerably underestimate the attractive interaction. The coupled cluster single double triple [CCSD(T)] interaction energies are close to the MP2 ones. The expected CCSD(T) interaction energies of the H2O-MeOH, H2O-Me2O, H2O-H2CO, MeOH-MeOH, and HCOOH-HCOOH complexes at the basis set limit are -4.90, -5.51, -5.17, -5.45, and -13.93 kcal/mol, respectively, while the HF/cc-pV5Z energies are -3.15, -2.58, -3.60, -2.69, and -11.29 kcal/ mol, respectively. The HF calculations greatly underestimate the attractive energies and fail to predict the order of the bonding energies in these complexes. These results show that a large basis set and the consideration of an appropriate electron correlation correction are essential to study interactions of hydrogen bonding complexes by ab initio molecular orbital calculation.
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