  We perform a spectral decomposition of the dynamical correlation function of
the spin $1/2$ XXZ model into an infinite sum of products of form factors.
Beneath the four-particle threshold in momentum space the only non-zero
contributions to this sum are the two-particle term and the trivial vacuum
term. We calculate the two-particle term by making use of the integral
expressions for form factors provided recently by the Kyoto school. We evaluate
the necessary integrals by expanding to twelfth order in $q$. We show plots of
$S(w,k)$, for $k=0$ and $\pi$ at various values of the anisotropy parameter,
and for fixed anisotropy at various $k$ around $0$ and $\pi$.
