  The two-dimensional O(3) model at a vacuum angle theta=pi is investigated.
This model has a severe sign problem. By a Wolff cluster algorithm an integer
or half-integer topological charge is assigned to each cluster. The meron
clusters (clusters with half-integer topological charge) are used to construct
an improved estimator for the correlation function of two spins at theta=pi.
Only configurations with 0 and 2 merons contribute to this correlation
function.
  An algorithm, that generates configurations with only 0 and 2 merons, is
constructed and numerical simulations at theta=pi are performed.
  The numerical results indicate the presence of long range correlations at
theta=pi.
