Cation–ligand interactions: Reproduction of extended basis set Ab initio SCF computations by the SIBFA 2 additive procedure

In the framework of the additive SIBFA 2 procedure, the intermolecular interaction energy is computed as a sum of five terms: ΔE = E_MTP + E_rep + E_pol + E_CT + E_disp. In order to assess the accuracy of the procedure to compute cation–ligand interactions, the interaction of alkali (Na⁺, K⁺) and alkaline-earth (Mg²⁺, Ca²⁺) cations with two representative ligands H₂O and HCOO^? has been studied and the results compared with those of ab initio SCF extended basis set computations. The additive procedure reproduces very satisfactorily the results of ab initio computations as concerns the numerical values of the interaction energies and the equilibrium cation–ligand distances, as well as the evolution of the energy components. A detailed study of these components at different distances helps, in particular, to delineate the relative weights of the charge-transfer and polarization contributions within the second-order energy.