Binding energies of hydrogen-bonded complexes from extrapolation with localized basis sets

By incorporating effective basis sets containing diffuse functions only in the interaction region of hydrogen-bonded complexes into the simple extrapolation scheme suitable for such basis sets, an accurate estimation of the MP2 basis set limit hydrogen-bonding energies of formic acid tetramer, formamide tetramer, alanine-water, phenol-water, and guanine-cytosine base pair is made with all estimates falling within 0.1-0.3 kcalmol of the reference basis set limits. The basis sets for extrapolation are composed of the cc-pVDZ and cc-pVDZ plus highest polarization functions from the cc-pVTZ set, or cc-pVDZ and cc-pVTZ set, all of which are augmented by the diffuse functions of the atoms in the interaction region of hydrogen bond. In contrast to the extrapolated estimates by this method, density functional theory binding energies with B3LYP, B971, and TPSS methods yield the differences from the reference values as large as 3.9 kcalmol with much higher computational cost in most cases, signifying the efficacy of the employed extrapolation scheme for study of large hydrogen-bonded complexes.