Multipair excitations and sum rules in interacting-electron systems

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Abstract
Multipair excitations in the interacting-electron gas at metallic densities are quantitatively investigated at large momentum transfers with use of the microscopic Green-Neilson-Szymański (GNS) theory. The effect of single-pair and multipair excitations on the dynamic response function χ(q,ω) are contrasted, and the relative strengths of these excitations are compared. It is shown that the multiple-peak structures in the dynamic structure factor S(q,ω) as calculated in the GNS theory are associated with pure multipair effects. The essential role multipair effects play in determining the high-energy-transfer tail of S(q,ω) is stressed, and the way this affects the frequency moment sum rules is examined. Other properties of these sum rules, when they are generated within approximate but conserving theories, are discussed.