Effects of basis set superposition error on optimized geometries and complexation energies of organo-alkali metal cation complexes.

Theoretical studies were performed to study the binding of alkali metal cations, X(+) (X = Li, Na, K), to poly(ethylene oxide) (PEO, I), poly(ethylene amine) (PEA, II), and poly(ethylene N-methylamine) (PEMA, III) by the Hartree-Fock (HF) and B3LYP methods using the 6-31G(d) and 6-311+G(d,p) basis sets. Two types of complex were considered in this study: a singly coordinated system (SCS) and a doubly coordinated system (DCS). Complexation energies were calculated both without and with basis set superposition error (BSSE). Because of the strong charge-dipole interactions, the complexation energies were largely negative and decreased in the order Li(+) > Na(+) > K(+). Three possible counterpoise (CP) approaches were examined in detail. In the case of the function CP (fCP) correction, the complexation energies exhibited an unusual trend because of the deformation of the subunits. This problem was solved by including geometry relaxation in the CP-corrected (GCP) interaction energies. The effects on the structures and vibrational frequencies were small when the complexes were reoptimized on the CP-corrected potential energy surface (PES).