Configuration-interaction study of atoms. II. Electron affinities of B, C, N, O, and F

Extensive configuration-interaction calculations have been carried out to determine electron affinities of the first-row atoms, B to F. Calculated electron affinities in eV with the observed values in parentheses are 0. 15, 1. 11 (1. 25), -0. 52, 1. 13 (1. 461), and 3. 12 (3. 448), for B, C, N, O, and F, respectively. Our best estimates, based on an empirical extrapolation which makes use of the known C, O, and F affinities, for B and N electron affinities are 0. 24 and -0. 19 eV, respectively. Detailed analysis of the results in terms of contributions from various classes of configurations shows that the K-shell and KL-intershell correlation-energy contributions to these electron affinities are negligibly small, and that about 15% of the total correlation contribution comes from the triple and higher electron-excitation configurations. Symmetry-adapted pair-correlation calculations have been carried out for O, O^-, F, and F^- to study convergence patterns for the correlation energy and electron affinity with respect to the orbital basis used to construct the configurations. Such pair-correlation calculations are known to overcompute the correlation energy. The excess energies, which correspond to pair-pair interaction energies neglected in the pair-correlation calculations, were roughly constant over a range of orbital basis sizes, with magnitudes about twice the energy contributions from the configurations of triple and higher excitations omitted in the pair-correlation calculations. Accordingly, this approximation should overestimate the electron affinity if a complete orbital basis is used.