THE USE OF MIDBOND FUNCTIONS FOR ABINITIO CALCULATIONS OF THE ASYMMETRIC POTENTIALS OF HE-NE AND HE-AR

The van der Waals interaction potentials of the asymmetric pairs of noble-gas atoms, He-Ne and He-Ar, are calculated by complete fourth-order Moller-Plesset perturbation theory (MP4) with an extensive investigation of midbond functions. Calculations with several extended basis sets, with and without midbond functions, consistently demonstrate the effective role of midbond functions in accurate calculations of intermolecular energies and potentials. A moderately polarized basis set, augmented with a simple set of midbond functions (3s3p2d), is capable of recovering 92%-95% of the experimental well depth and reproducing the potential minimum within 0.1 a0. We attribute the remaining errors mainly to the deficiency of the MP4 theory. The present study shows that the use of midbond functions effectively recovers the intersystem correlation energy, which is normally obtained in a purely nucleus-centered basis set by use of diffuse low (first and second) polarization functions and high polarization functions. We therefore propose the use of nucleus-centered compact polarization functions and midbond functions to maintain the balanced treatment of the intra- and intersystem correlations for efficient and reliable calculations. The study also shows that the calculated interaction energy is highly insensitive to the displacement of midbond functions and thus the geometric midpoint of the van der Waals bond is a convenient and suitable choice for the position of midbond functions.