Dependence of the Intermolecular Electrostatic Interaction Energy on the Level of Theory and the Basis Set

As electrostatic forces play a prominent role in the process of folding and binding of biological macromolecules, an examination of the method dependence of the electrostatic interaction energy is of great importance. An extensive analysis of the basis set and method dependence of electrostatic interaction energies (Ees) in molecular systems using six test dimers of α-glycine is presented. A number of Hartree-Fock, Kohn-Sham, Møller-Plesset, configuration interaction (CI), quadratic CI, and coupled cluster calculations were performed using several double-, triple-, and quadruple-ζ-quality Gaussian- and Slater-type (Kohn-Sham calculations only) basis sets. The main factor affecting Ees was found to be the inclusion of diffuse functions in the basis set expansions. Møller-Plesset (even at second order), quadratic CI, and coupled cluster calculations produce the most consistent results. Hartree-Fock and CI methods usually overestimate the Ees, while the Kohn-Sham approach tends to underestimate the magnitude of the electrostatic interaction. The combination of the transferable-pseudoatom databank and the exact potential and multipole moment method reproduces Kohn-Sham B3LYP/6-31G** results on which it is based, confirming the excellent transferability of the pseudoatom densities within the systems studied. However, because Kohn-Sham calculations with double-ζ-quality basis sets show considerable deviations from advanced correlated methods, further development of the databank using electron densities from such methods is highly desirable.