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In the S=½ linear Heisenberg antiferromagnet (HB AF)

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although investigated by various theoretical approaches -many important questions concerning the statics and the dynamics have remained open. Recent low-temperature neutron-scattering experiments on CuCl22N(C5H5) (CPC), which is a good realization of an S=½ HB AF chain, provided new important information on the dynamics of the system, such as lineshapes and the behaviour in a magnetic field /1/. The important quantity for direct comparison with experiments of this kind is the dynamic spin-correlation function in (q,ω) -space. It is the Fourier transform of < Sz (l. t) Sz (l', o) > and for T = o it can be written as

Gzz(q,ω) = Σλ Mλδ(ω + EO - Eλ), Mλ = 2π|< o||Sz(q)||λ >|2