MRD-CI potential surfaces using balanced basis sets. I. First-row diatomic hydrides

Abstract

Multireference configuration-interaction (MRD-CI) methods are used to calculate binding energies, potential curves, and vibrational energy levels for the diatomic hydrides H2, BH, CH, NH, OH, and FH. The curves are calculated out to the dissociation limit. It is shown how the use of bond functions (located midway between the nuclei) play an essential role in augmenting the standard double-zeta plus polarization basis set. This provides a relatively small but balanced basis set which describes equally well both atomic and molecular regions. The potential curves are accurately fitted by generalized Morse functions over a range of distance which spans 90% of the well depth. Computed binding energies are within 0.1 eV of experimental values, except for the experimentally uncertain NH, which is within 0.2 eV of an estimated value. Potential curves and vibrational levels for H2, OH, and FH agree well with spectroscopic (and RKR) values. Vibrational levels and potential curves for the other hydrides are compared to theoretical values since few experimental data are known, with less good but still reasonable agreement. The selection/extrapolation and multireference features of the MRD-CI method provide good energy continuity at modest computational expense over the whole range of internuclear distances.