BASIS-SET EFFECTS ON COMPUTED ACID-BASE INTERACTION ENERGIES USING THE DUNNING CORRELATION-CONSISTENT POLARIZED SPLIT-VALENCE BASIS-SETS

Ab initio calculations have been performed at correlated levels of the ory to investigate the convergence of the binding energies of hydrogen -bonded neutral, positive ion and negative ion complexes with systemat ic expansion of the basis set. For this study, the Dunning correlation -consistent polarized split-valence basis sets augmented with diffuse functions only on nonhydrogen atoms (aug'-cc-pVXZ, where X = D for dou ble-split, T for triple-split, and Q for quadruple-split) have been us ed. Computed hydrogen bond energies of all complexes converge with inc reasing basis set size, with satisfactory convergence occurring at aug '-cc-pVTZ. Binding energies are not sensitive to the presence of diffu se f functions in the aug'-cc-pVTZ basis set, and these f functions ma y be omitted. The presence or absence of diffuse d functions has littl e effect on the computed binding energies of neutral and positive ion complexes. However, these functions are important for describing the b inding energies of the negative ion complexes. In contrast, quantitati vely accurate proton affinities for corresponding neutral and anionic monomeric bases require the presence of both diffuse f and diffuse d f unctions in the aug'-cc-pVTZ basis set. For neutral and positive ion h ydrogen-bonded complexes, MP4/aug'-cc-pVTZ binding energies are within 0.1 kcal mol-1 of CCSD+T(CCSD) binding energies, but may be as much a s 1 kcal mol-1 smaller for the negative ion complexes.