Interactions between neutral molecules and Ca²⁺: An assessment of theoretical procedures

The performance of a selection of theoretical procedures in describing the binding of Ca²⁺ to ammonia and formaldehyde has been assessed. Geometries and vibrational frequencies were obtained using the density functional theory procedures, B3-LYP and G96-LYP, as well as with CCSD(T) with a variety of basis sets, with the CCSD(T)/cc-pWCVQZ-optimized structures being used as a reference. Binding energies, including the consideration of basis set superposition errors, were additionally obtained with variants of the G3, W1C, and W2C methods, with the W2C values providing benchmark values in this case. We find that Ca-X (X = N, O) bond lengths for the [Ca-NH₃]²⁺ and [Ca-OCH₂]²⁺ complexes show some sensitivity both to the size of the basis set and to the theoretical procedure employed. In general, the Ca-X lengths decrease as the basis set is systematically enlarged. The B3-LYP and G96-LYP methods yield Ca-X distances that are slightly shorter than the CCSD(T) values obtained with the same basis set. As a consequence of these two factors, B3-LYP/cc-pWCVTZ and B3-LYP/6-311+G(3df,2p) give very reasonable geometries for the two complexes. The B3-LYP/cc-pWCVTZ and G96-LYP/cc-pWCVQZ approaches provide a good compromise between accuracy and computational cost in the calculation of binding energies. Ca²⁺ is predicted to bind more strongly to formaldehyde than to ammonia, in contrast to the ordering of proton and Li⁺ affinities for the two molecules.