PROTON AFFINITIES OF NH3, H2O, AND HF AND THEIR ANIONS - A QUEST FOR THE BASIS-SET LIMIT USING THE DUNNING AUGMENTED CORRELATION-CONSISTENT BASIS-SETS

Proton affinities of NH3, H2O, HF, NH2-, OH- and F- have been computed using many-body Moller-Plesset perturbation theory with the Dunning correlation-consistent polarized split-valence basis sets (cc-pVXZ, where X = D for double-, T for triple-, Q for quadruple-, and 5 for quintuple-split) and these same basis sets augmented with diffuse functions (aug-cc-pVXZ) with one set of diffuse functions for each ''1'' value in the original basis. Diffuse functions on non-hydrogen atoms are required to lower computed proton affinities and reduce the basis set superposition error, but diffuse functions on hydrogen atoms are not necessary. Computed proton affinities with the diffuse-truncated basis sets (aug'-cc-pVXZ) appear to converge, so that aug'-cc-pVQZ (and aug'-cc-pV5Z) proton affinities may be regarded as basis-set limit values. The aug'-cc-pVTZ basis set without the counterpoise correction is recommended for future studies of proton affinities in larger molecules. Computed MP4/aug'-cc-pVTZ and coupled-cluster CCSD+T(CCSD)/aug'-cc-pVTZ proton affinities are compared with experimental data.