Configuration Interaction in Two- and Three-Electron Atoms

Abstract

The ground states of the first four members of the helium isoelectronic series and the 2²S and 2²P states of lithium are calculated using a configuration‐interaction expansion in a complete denumerable set of single‐particle functions, with one adjustable scale parameter. The best energies for the two‐electron systems, obtained with 120‐term expansions, are E(H⁻) = −0.52748, E(He) = −2.90335, E(Li⁺) = −7.27945, and E(Be^{+ +}) = −13.65504, in units of e²/a₀. The energies for all but He are lower than any heretofore obtained with a configuration‐interaction approach. The dependence of energy on scale factor is found to be very pronounced, in contrast to the corresponding behavior for wavefunctions which contain the interparticle coordinates explicitly. The best energies for the lithium states, obtained with 208‐term expansions, are E(2²S) = −7.47369 and E(2²P) = −7.40366. The 2²S energy is not as good as has been obtained with either expansions in terms of interparticle coordinates or configuration interaction with many nonlinear parameters. The 2²P energy is of approximately the same accuracy but is lower than any previously published.