Accurate theoretical determination of the structure of aromatic complexes is complicated: the phenol dimer and phenol...methanol cases.

The structure of the phenol dimer and phenol...methanol complexes was determined by gradient optimization using the Hartree-Fock (HF), MP2, DFT, and RI-DFT-D methods with various basis sets. Theoretical rotational constants were compared with experimental values and the following conclusions were made: (1) HF and DFT methods fail to predict cluster geometries; (2) MP2 with a medium basis set yields reliable cluster geometries but only because of a compensation for errors; (3) when the AO basis set is enlarged, the geometry becomes incorrect, and the theoretical geometry becomes reliable only when the higher correlation energy contributions (CCSD(T)) are included; and (4) the RI-DFT-D procedure covering the dispersion energy provides excellent geometries.