The effects of basis set quality and configuration mixing in ab initio calculations of the ionization potentials of the nitrogen molecule

Abstract

Systematic configuration interaction calculations of the adiabatic ionization potentials of N2(1Σg+) to the X 2Σg+, A 2Πu, and B 2Σu+ states of N2+ give best results of 15.62, 16.41,and 18.93 eV compared with observed values of 15.576, 16.694, and 18.746 eV. The computed values are near the complete basis limit for a wave function containing single and double excitations from the Hartree-Fock configuration with the Langhoff-Davidson correction for unlinked cluster quadruple excitations. The critical dependence of ionization potentials on basis is demonstrated. Vertical ionization potentials at near the N2 ground state equilibrium internuclear separation are compared for a variety of basis sets using different computational methods. In particular, an important comparison is made with the EOM calculation of Herman, Yeager, Freed, and McKoy using a "double-zeta + polarization" basis identical to one used in this study. We conclude that agreement to within ∼0.2 eV can be expected for ionization potentials between good state of the art EOM and CI calculations. Potential curves computed from our best wave functions are tabulated since they should stand as reference data for some time; spectroscopic constants obtained from them are presented. © 1980 American Institute of Physics.