Strongly separated pairs of core electrons in computed ground states of small molecules

We have performed full configuration interaction computations of the ground states of the molecules Be, BeH₂, Li, LiH, B, and BH and verified that the core electrons constitute "separated electron pairs." These separated pairs of core electrons have nontrivial structure; the core pair does not simply occupy a single spatial orbital. Our method of establishing the presence of separated electron pairs is direct and conclusive. We do not fit a separated pair model; we work with the wavefunctions of interest directly. To establish that a given group of spin-orbitals contains a quasi-separated pair, we verify by direct computation that the quantum state of the electrons that occupy those spin-orbitals is nearly a pure 2-electron state.