Abstract
Considers a quantum 'conserving' approximation evaluation of the Van Hove dynamic liquid structure factor, S(q, omega ), in normal fluids. For a degenerate Fermi liquid the approximation simplifies to the generalised random phase approximation. Providing certain 'stability conditions' are obeyed, the 'conserving' S(q, omega ) will manifestly satisfy the f-sum rule, obey the detailed-balance condition and be positive for all values of omega . Also presents a detailed numerical evaluation of S(q, omega ) (and S(q)) for a model boson system and obtains S(q, omega ) in terms of the sum of a large number of weighted delta functions of omega which are then widened-out until they overlap sufficiently to produce a smooth curve. The results qualitatively resemble the behaviour of many monatomic fluids in so far as well-defined Rayleigh and Brillouin peaks are observed for low q and that for high q, the impulse approximation result is obtained.
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