Coulomb shifts for intra- and intershell electron pairs in LiH

The influence of electron correlation on the interparticle momentum distribution f ( p12), where p12=‖p1−p2‖, is examined for the individual intra- and intershell electron pairs in LiH. These changes Δ f ( p12) vs p12, known as Coulomb shifts, are compared with a similar and earlier analysis of correlation effects in momentum space for the united-atom Be. Selected expectation values are also reported at both the correlated and Hartree–Fock levels of approximation. The use of momentum space for such a study is particularly advantageous for a molecule since, unlike an analysis in position space, the calculation is reduced in complexity by being effectively single-centered. Consequently, atomic-based concepts such as ‘‘radial’’ and ‘‘angular’’ correlation remain applicable. For this initial example of a many-electron molecule, the Coulomb shifts for LiH emphasized the ionic components of the two intrashells, as well as their reduced interdependence by comparison with Be. Correlation produced an increase in momentum, especially for the more loosely bound electrons. This corresponds to a contraction of the charge density in the outer regions of the molecule when viewed in position space.