Basis set validation for polyatomic cation-water interactions

Abstract

A study of the effects of counterpoise (CP) corrections in polyatomic cation-water interactions is added to the systematic analysis performed in the past of the basis set superposition error (BSSE) in neutral and anionic adducts. The interaction with water of the ammonium cation and its methyl and -CHO derivatives is considered due to the need to model accurately this functional group, which is common in biological molecules. The basis sets employed are the STO-3G and MINI-1 minimal basis sets and the 3-21G, 4-31G and 6-31G** extended ones. In addition, the 6-311++G (2d,p) and 6-311++G (3d, 2p) basis sets have been used for the smallest system, at the SCF and MP2 levels, both without and with CP correction. These basis sets give an equilibrium distance slightly larger and an interaction energy less favourable than the 6-31G** basis set at the corresponding level, while the inclusion of correlation corrections produces a stronger H-bond at a shorter distance. The results confirm the previous hint of a lower incidence of BSSEs in medium size cationic systems, at a different extent for the various basis sets. While the STO-3G basis set is sharply affected by BSSEs in both the equilibrium distance and the interaction energy, the MINI-1 basis set shows a small BSSE, though its trend is not completely satisfactory because the charge transfer component has an anomalous behaviour with respect to our reference basis set. The 4-31G basis set is the only one able to hold comparison with the 6-31G**, even though the interaction energy produced is slightly overestimated. The 3-21G basis set, when corrected, almost parallels the 4-31G one in this set of compounds. The reliability of the CPED correction is checked and discussed.