Configuration Interaction in Simple Atomic Systems

The ground-state wave functions of the helium, lithium, and beryllium atoms were approximated by a superposition of configurations with expansion lengths ranging from 35 for helium to 55 for beryllium. The discrepancies in the total energy are 0.014 ev for helium, 0.026 ev for lithium, and 0.17 ev for beryllium. A 19-configuration function was also applied to the lowest ${}_{}{}^3{}_{}{}^{}S$ state of helium, with a resulting accuracy of 0.0005 ev. The calculations were also made on all the isoelectronic series of ions through $Z=8\mathrm{}$, the discrepancy remaining of the same order of magnitude but increasing with increasing $Z$. A lower bound to the electron affinity of lithium is set at 0.4773 ev, with the most probable value, obtained by extrapolating the isoelectronic series, being placed at 0.62 ev.