New medium-size basis sets to evaluate the dispersion interaction of hydrocarbon molecules

Medium-size basis sets were proposed to evaluate efficiently the dispersion interactions of hydrocarbon molecules. The aug(d,p)-6-311G** basis set was prepared by the augmentation of the diffuse d and p functions (alpha(d)(C) = 0.1565 and alpha(p)(H) = 0.1875) to the 6-311G** basis set. The aug(df,pd)-6-311G** basis set was prepared by the further augmentation of the diffuse f and d functions (alpha(f)(C) = 0.2 and alpha(d)(H) = 0.25) to the aug(d,p)-6-311G** basis set. The calculated MP2 and CCSD(T) intermolecular interaction energies of methane, ethane, propane, ethylene, acetylene, and benzene dimers with these basis sets were compared with those calculated with Sadlej's basis set and Dunning's correlation-consistent basis sets. Although the aug(d,p)-6-311G** basis set was more compact than Sadlej's basis set, this basis set was more effective to evaluate the, dispersion energy. The aug(df,pd)-6-311G** basis set was considerably smaller than Dunning's cc-pVQZ and cc-pV5Z basis sets. The calculated interaction energies with the aug(df,pd)-6-311G** basis set were close to those calculated with the nearly BSSE-free cc-pVQZ and cc-pV5Z basis sets.