  Cluster number counts can constrain the properties of dark energy if and only
if the evolution in the relationship between observable quantities and the
cluster mass can be calibrated. Next generation surveys with ~10000 clusters
will have sufficient statistics to enable some degree of self-calibration. The
excess variance of counts due to the clustering of clusters provides such an
opportunity and can be measured from the survey without additional
observational cost. It can minimize the degradation in dark energy constraints
due to an unknown power law evolution in the mass-observable relation improving
constraints on the dark energy equation of state by a factor of 2 or more to
sigma(w)=0.06 for a deep 4000 deg2 survey.
