Expectation values of p1xp2 as a measure of electron correlation in two-electron atoms

We examine expectation values of p1xp2 in ground states of He and H-, the singly and doubly excited states of He, and the ground and low-lying valence states of the alkaline-earth-metal atoms. A simple commutator relation derived by Froese Fischer and Smentek-Mielczarek [J. Phys. B 16, 3479 (1983)] can be used to explain the sign and relative magnitudes of in the ground states of He and H- and singly excited states of He. In the doubly excited states of He and in the ground and low-lying valence states of the alkaline-earth metals, the molecular model, which treats the electron--core-electron system as a floppy, linear triatomic ''molecule,'' correctly predicts signs and general trends for . These results represent an important test of the molecular model. The ability of this model to make intuitive predictions about quantities like is a strong confirmation of its validity and utility