Accurately solving the electronic Schrödinger equation of atoms and molecules using explicitly correlated $(r12$-) multireference configuration interaction. III. Electron affinities of first-row atoms

The computation of electron affinities of atoms and molecules is one of the most demanding tasks in quantum chemistry. This is because the electronic structures of neutral systems compared to their respective anions are qualitatively different and thus errors in the computed correlation energies, in general, do not cancel. Correlation energies obtained from traditional configuration interaction (CI) expansions, however, are known to converge notoriously slowly due to the presence of interelectronic cusps in the exact wave function. We compute the electron affinities of the first-row atoms using the recently proposed (explicitly correlated) $r12$-[multireference configuration interaction (single double) MR-CI(SD)] and $r12$-MR-ACPF (averaged coupled-pair functional) methods which take care of the interelectronic cusps by means of terms being linear in the interelectronic distances $(r12).$ The reference spaces and basis sets (which are further augmented with diffuse functions) are taken from our former study on neutral atoms and their respective positive ions [J. Chem. Phys. 109, 9795 (1998)]. The performance of MR-ACPF is validated by comparison with full CI. The computed electron affinities (corrected for relativistic effects and nuclear motion) deviate from experiment by: $−0.4$ (H), $+0.3$ (Li), $+5$ (B, within experimental uncertainty), $−0.6$ (C), −15 (O), and $−16 meV$ (F). Without relying on fortuitous error compensations, the electron affinities of B, C, O, and F can presently not be obtained in such an accuracy with traditional CI methods without extrapolation to the basis set limit.