  We propose the zero-point of the cluster-cluster correlation function as a
sensitive test for the shape of the power spectrum of initial fluctuations. It
is now possible to go beyond the power law description to measure the point at
which the correlation function becomes zero. Four independent measurements
indicate that the zero point, $r_0$, should be in the range (40-60)$h^{-1}$Mpc.
The large value of $r_0$ at which the zero-point occurs rules out conventional
CDM models independently of the assumed amplitude. Models with $\Omega < 0.25$
should be rejected because they predict too large $r_0$. If the age of the
Universe is assumed to be larger than 15 Gyr, models with either $\Omega<0.5$
or $h> 0.55$ are rejected. We present the results of numerical simulations of
clusters in an $\Omega=1$ cosmological model with a mixture of cold plus hot
dark matter (CHDM). The correlation function we determined for the simulated
clusters has a zero-point, $r_0=55h^{-1}$Mpc that accurately matches the zero
point of the observed function.
