Properties of electron momentum distributions: A study of correlation effects in some two-electron systems and an examination of directional compton profiles for the lithium halides.

In Part 1, the history and current situation concerning the determination of electron momentum distributions is surveyed, mainly from a theoretical point of view. Two aspects of momentum space are outlined with a view to introducing the specific problems considered in Parts 2 and 3. In Part 2.1, the concept of a Coulomb 'hole' ? f(P12) in momentum space is investigated, where p12 is the momentum difference [ p1-p2]. Results are presented for the iso-electronic systems H , He and Li+. Using the natural expansion representation of the appropriate configuration-interaction wave functions, the structure of ? f(P12) is assessed in terms of radial and angular correlation effects. The two-particle radial momentum distribution and several one- and two- particle expectation quantities are also examined.;In Part 2.2, the study of correlation effects is extended to include the simplest hetero-nuclear molecular ion HeH+ for various internuclear separations R. By introducing a distribution function ? g(p12,p1,o1) it is possible to examine the effects of correlation with respect to the magnitude and orientation of the momentum p, of the 'test' electron. Various 'radial' and 'angular' correlation coefficients are determined via several one- and two- particle expectation quantities. The findings in Part 2 demonstrate that radial and angular correlation create opposite effects on momentum distributions. The shape and formation of ? f(P12) example, is therefore considerably more complex than that found for its counterpart in position space.;In Part 3, directional Compton profiles are calculated for the crystalline lithium halides LiF, LiCl, LiBr and Lil using two theoretical techniques: a tight-binding approximation and the 'molecular simulated crystal' (MSC) model --- a formalism in which the unit cell in the crystal lattice is simulated by non-interacting diatomic molecules. Poor agreement is found between the profiles obtained within the two formalisms; the disagreement is attributed to an over-estimation of electron momentum in the non-bonding directions for the isolated molecules used in the MSC model. Within the tight-binding model, profiles are presented for a first nearest neighbour interaction and for the 'infinite' lattice.